FreeBSD/Linux Kernel Cross Reference
sys/kernel/workqueue.c

     /*
      * kernel/workqueue.c - generic async execution with shared worker pool
      *
      * Copyright (C) 2002           Ingo Molnar
      *
      *   Derived from the taskqueue/keventd code by:
      *     David Woodhouse <dwmw2@infradead.org>
      *     Andrew Morton
      *     Kai Petzke <wpp@marie.physik.tu-berlin.de>
     *     Theodore Ts'o <tytso@mit.edu>
     *
     * Made to use alloc_percpu by Christoph Lameter.
     *
     * Copyright (C) 2010           SUSE Linux Products GmbH
     * Copyright (C) 2010           Tejun Heo <tj@kernel.org>
     *
     * This is the generic async execution mechanism.  Work items as are
     * executed in process context.  The worker pool is shared and
     * automatically managed.  There is one worker pool for each CPU and
     * one extra for works which are better served by workers which are
     * not bound to any specific CPU.
     *
     * Please read Documentation/workqueue.txt for details.
     */
    
    #include <linux/export.h>
    #include <linux/kernel.h>
    #include <linux/sched.h>
    #include <linux/init.h>
    #include <linux/signal.h>
    #include <linux/completion.h>
    #include <linux/workqueue.h>
    #include <linux/slab.h>
    #include <linux/cpu.h>
    #include <linux/notifier.h>
    #include <linux/kthread.h>
    #include <linux/hardirq.h>
    #include <linux/mempolicy.h>
    #include <linux/freezer.h>
    #include <linux/kallsyms.h>
    #include <linux/debug_locks.h>
    #include <linux/lockdep.h>
    #include <linux/idr.h>
    
    #include "workqueue_sched.h"
    
    enum {
            /*
             * global_cwq flags
             *
             * A bound gcwq is either associated or disassociated with its CPU.
             * While associated (!DISASSOCIATED), all workers are bound to the
             * CPU and none has %WORKER_UNBOUND set and concurrency management
             * is in effect.
             *
             * While DISASSOCIATED, the cpu may be offline and all workers have
             * %WORKER_UNBOUND set and concurrency management disabled, and may
             * be executing on any CPU.  The gcwq behaves as an unbound one.
             *
             * Note that DISASSOCIATED can be flipped only while holding
             * assoc_mutex of all pools on the gcwq to avoid changing binding
             * state while create_worker() is in progress.
             */
            GCWQ_DISASSOCIATED      = 1 << 0,       /* cpu can't serve workers */
            GCWQ_FREEZING           = 1 << 1,       /* freeze in progress */
    
            /* pool flags */
            POOL_MANAGE_WORKERS     = 1 << 0,       /* need to manage workers */
            POOL_MANAGING_WORKERS   = 1 << 1,       /* managing workers */
    
            /* worker flags */
            WORKER_STARTED          = 1 << 0,       /* started */
            WORKER_DIE              = 1 << 1,       /* die die die */
            WORKER_IDLE             = 1 << 2,       /* is idle */
            WORKER_PREP             = 1 << 3,       /* preparing to run works */
            WORKER_CPU_INTENSIVE    = 1 << 6,       /* cpu intensive */
            WORKER_UNBOUND          = 1 << 7,       /* worker is unbound */
    
            WORKER_NOT_RUNNING      = WORKER_PREP | WORKER_UNBOUND |
                                      WORKER_CPU_INTENSIVE,
    
            NR_WORKER_POOLS         = 2,            /* # worker pools per gcwq */
    
            BUSY_WORKER_HASH_ORDER  = 6,            /* 64 pointers */
            BUSY_WORKER_HASH_SIZE   = 1 << BUSY_WORKER_HASH_ORDER,
            BUSY_WORKER_HASH_MASK   = BUSY_WORKER_HASH_SIZE - 1,
    
            MAX_IDLE_WORKERS_RATIO  = 4,            /* 1/4 of busy can be idle */
            IDLE_WORKER_TIMEOUT     = 300 * HZ,     /* keep idle ones for 5 mins */
    
            MAYDAY_INITIAL_TIMEOUT  = HZ / 100 >= 2 ? HZ / 100 : 2,
                                                    /* call for help after 10ms
                                                       (min two ticks) */
            MAYDAY_INTERVAL         = HZ / 10,      /* and then every 100ms */
            CREATE_COOLDOWN         = HZ,           /* time to breath after fail */
    
            /*
             * Rescue workers are used only on emergencies and shared by
             * all cpus.  Give -20.
            */
           RESCUER_NICE_LEVEL      = -20,
           HIGHPRI_NICE_LEVEL      = -20,
   };
   
   /*
    * Structure fields follow one of the following exclusion rules.
    *
    * I: Modifiable by initialization/destruction paths and read-only for
    *    everyone else.
    *
    * P: Preemption protected.  Disabling preemption is enough and should
    *    only be modified and accessed from the local cpu.
    *
    * L: gcwq->lock protected.  Access with gcwq->lock held.
    *
    * X: During normal operation, modification requires gcwq->lock and
    *    should be done only from local cpu.  Either disabling preemption
    *    on local cpu or grabbing gcwq->lock is enough for read access.
    *    If GCWQ_DISASSOCIATED is set, it's identical to L.
    *
    * F: wq->flush_mutex protected.
    *
    * W: workqueue_lock protected.
    */
   
   struct global_cwq;
   struct worker_pool;
   
   /*
    * The poor guys doing the actual heavy lifting.  All on-duty workers
    * are either serving the manager role, on idle list or on busy hash.
    */
   struct worker {
           /* on idle list while idle, on busy hash table while busy */
           union {
                   struct list_head        entry;  /* L: while idle */
                   struct hlist_node       hentry; /* L: while busy */
           };
   
           struct work_struct      *current_work;  /* L: work being processed */
           struct cpu_workqueue_struct *current_cwq; /* L: current_work's cwq */
           struct list_head        scheduled;      /* L: scheduled works */
           struct task_struct      *task;          /* I: worker task */
           struct worker_pool      *pool;          /* I: the associated pool */
           /* 64 bytes boundary on 64bit, 32 on 32bit */
           unsigned long           last_active;    /* L: last active timestamp */
           unsigned int            flags;          /* X: flags */
           int                     id;             /* I: worker id */
   
           /* for rebinding worker to CPU */
           struct work_struct      rebind_work;    /* L: for busy worker */
   };
   
   struct worker_pool {
           struct global_cwq       *gcwq;          /* I: the owning gcwq */
           unsigned int            flags;          /* X: flags */
   
           struct list_head        worklist;       /* L: list of pending works */
           int                     nr_workers;     /* L: total number of workers */
   
           /* nr_idle includes the ones off idle_list for rebinding */
           int                     nr_idle;        /* L: currently idle ones */
   
           struct list_head        idle_list;      /* X: list of idle workers */
           struct timer_list       idle_timer;     /* L: worker idle timeout */
           struct timer_list       mayday_timer;   /* L: SOS timer for workers */
   
           struct mutex            assoc_mutex;    /* protect GCWQ_DISASSOCIATED */
           struct ida              worker_ida;     /* L: for worker IDs */
   };
   
   /*
    * Global per-cpu workqueue.  There's one and only one for each cpu
    * and all works are queued and processed here regardless of their
    * target workqueues.
    */
   struct global_cwq {
           spinlock_t              lock;           /* the gcwq lock */
           unsigned int            cpu;            /* I: the associated cpu */
           unsigned int            flags;          /* L: GCWQ_* flags */
   
           /* workers are chained either in busy_hash or pool idle_list */
           struct hlist_head       busy_hash[BUSY_WORKER_HASH_SIZE];
                                                   /* L: hash of busy workers */
   
           struct worker_pool      pools[NR_WORKER_POOLS];
                                                   /* normal and highpri pools */
   } ____cacheline_aligned_in_smp;
   
   /*
    * The per-CPU workqueue.  The lower WORK_STRUCT_FLAG_BITS of
    * work_struct->data are used for flags and thus cwqs need to be
    * aligned at two's power of the number of flag bits.
    */
   struct cpu_workqueue_struct {
           struct worker_pool      *pool;          /* I: the associated pool */
           struct workqueue_struct *wq;            /* I: the owning workqueue */
           int                     work_color;     /* L: current color */
           int                     flush_color;    /* L: flushing color */
           int                     nr_in_flight[WORK_NR_COLORS];
                                                   /* L: nr of in_flight works */
           int                     nr_active;      /* L: nr of active works */
           int                     max_active;     /* L: max active works */
           struct list_head        delayed_works;  /* L: delayed works */
   };
   
   /*
    * Structure used to wait for workqueue flush.
    */
   struct wq_flusher {
           struct list_head        list;           /* F: list of flushers */
           int                     flush_color;    /* F: flush color waiting for */
           struct completion       done;           /* flush completion */
   };
   
   /*
    * All cpumasks are assumed to be always set on UP and thus can't be
    * used to determine whether there's something to be done.
    */
   #ifdef CONFIG_SMP
   typedef cpumask_var_t mayday_mask_t;
   #define mayday_test_and_set_cpu(cpu, mask)      \
           cpumask_test_and_set_cpu((cpu), (mask))
   #define mayday_clear_cpu(cpu, mask)             cpumask_clear_cpu((cpu), (mask))
   #define for_each_mayday_cpu(cpu, mask)          for_each_cpu((cpu), (mask))
   #define alloc_mayday_mask(maskp, gfp)           zalloc_cpumask_var((maskp), (gfp))
   #define free_mayday_mask(mask)                  free_cpumask_var((mask))
   #else
   typedef unsigned long mayday_mask_t;
   #define mayday_test_and_set_cpu(cpu, mask)      test_and_set_bit(0, &(mask))
   #define mayday_clear_cpu(cpu, mask)             clear_bit(0, &(mask))
   #define for_each_mayday_cpu(cpu, mask)          if ((cpu) = 0, (mask))
   #define alloc_mayday_mask(maskp, gfp)           true
   #define free_mayday_mask(mask)                  do { } while (0)
   #endif
   
   /*
    * The externally visible workqueue abstraction is an array of
    * per-CPU workqueues:
    */
   struct workqueue_struct {
           unsigned int            flags;          /* W: WQ_* flags */
           union {
                   struct cpu_workqueue_struct __percpu    *pcpu;
                   struct cpu_workqueue_struct             *single;
                   unsigned long                           v;
           } cpu_wq;                               /* I: cwq's */
           struct list_head        list;           /* W: list of all workqueues */
   
           struct mutex            flush_mutex;    /* protects wq flushing */
           int                     work_color;     /* F: current work color */
           int                     flush_color;    /* F: current flush color */
           atomic_t                nr_cwqs_to_flush; /* flush in progress */
           struct wq_flusher       *first_flusher; /* F: first flusher */
           struct list_head        flusher_queue;  /* F: flush waiters */
           struct list_head        flusher_overflow; /* F: flush overflow list */
   
           mayday_mask_t           mayday_mask;    /* cpus requesting rescue */
           struct worker           *rescuer;       /* I: rescue worker */
   
           int                     nr_drainers;    /* W: drain in progress */
           int                     saved_max_active; /* W: saved cwq max_active */
   #ifdef CONFIG_LOCKDEP
           struct lockdep_map      lockdep_map;
   #endif
           char                    name[];         /* I: workqueue name */
   };
   
   struct workqueue_struct *system_wq __read_mostly;
   EXPORT_SYMBOL_GPL(system_wq);
   struct workqueue_struct *system_highpri_wq __read_mostly;
   EXPORT_SYMBOL_GPL(system_highpri_wq);
   struct workqueue_struct *system_long_wq __read_mostly;
   EXPORT_SYMBOL_GPL(system_long_wq);
   struct workqueue_struct *system_unbound_wq __read_mostly;
   EXPORT_SYMBOL_GPL(system_unbound_wq);
   struct workqueue_struct *system_freezable_wq __read_mostly;
   EXPORT_SYMBOL_GPL(system_freezable_wq);
   
   #define CREATE_TRACE_POINTS
   #include <trace/events/workqueue.h>
   
   #define for_each_worker_pool(pool, gcwq)                                \
           for ((pool) = &(gcwq)->pools[0];                                \
                (pool) < &(gcwq)->pools[NR_WORKER_POOLS]; (pool)++)
   
   #define for_each_busy_worker(worker, i, pos, gcwq)                      \
           for (i = 0; i < BUSY_WORKER_HASH_SIZE; i++)                     \
                   hlist_for_each_entry(worker, pos, &gcwq->busy_hash[i], hentry)
   
   static inline int __next_gcwq_cpu(int cpu, const struct cpumask *mask,
                                     unsigned int sw)
   {
           if (cpu < nr_cpu_ids) {
                   if (sw & 1) {
                           cpu = cpumask_next(cpu, mask);
                           if (cpu < nr_cpu_ids)
                                   return cpu;
                   }
                   if (sw & 2)
                           return WORK_CPU_UNBOUND;
           }
           return WORK_CPU_NONE;
   }
   
   static inline int __next_wq_cpu(int cpu, const struct cpumask *mask,
                                   struct workqueue_struct *wq)
   {
           return __next_gcwq_cpu(cpu, mask, !(wq->flags & WQ_UNBOUND) ? 1 : 2);
   }
   
   /*
    * CPU iterators
    *
    * An extra gcwq is defined for an invalid cpu number
    * (WORK_CPU_UNBOUND) to host workqueues which are not bound to any
    * specific CPU.  The following iterators are similar to
    * for_each_*_cpu() iterators but also considers the unbound gcwq.
    *
    * for_each_gcwq_cpu()          : possible CPUs + WORK_CPU_UNBOUND
    * for_each_online_gcwq_cpu()   : online CPUs + WORK_CPU_UNBOUND
    * for_each_cwq_cpu()           : possible CPUs for bound workqueues,
    *                                WORK_CPU_UNBOUND for unbound workqueues
    */
   #define for_each_gcwq_cpu(cpu)                                          \
           for ((cpu) = __next_gcwq_cpu(-1, cpu_possible_mask, 3);         \
                (cpu) < WORK_CPU_NONE;                                     \
                (cpu) = __next_gcwq_cpu((cpu), cpu_possible_mask, 3))
   
   #define for_each_online_gcwq_cpu(cpu)                                   \
           for ((cpu) = __next_gcwq_cpu(-1, cpu_online_mask, 3);           \
                (cpu) < WORK_CPU_NONE;                                     \
                (cpu) = __next_gcwq_cpu((cpu), cpu_online_mask, 3))
   
   #define for_each_cwq_cpu(cpu, wq)                                       \
           for ((cpu) = __next_wq_cpu(-1, cpu_possible_mask, (wq));        \
                (cpu) < WORK_CPU_NONE;                                     \
                (cpu) = __next_wq_cpu((cpu), cpu_possible_mask, (wq)))
   
   #ifdef CONFIG_DEBUG_OBJECTS_WORK
   
   static struct debug_obj_descr work_debug_descr;
   
   static void *work_debug_hint(void *addr)
   {
           return ((struct work_struct *) addr)->func;
   }
   
   /*
    * fixup_init is called when:
    * - an active object is initialized
    */
   static int work_fixup_init(void *addr, enum debug_obj_state state)
   {
           struct work_struct *work = addr;
   
           switch (state) {
           case ODEBUG_STATE_ACTIVE:
                   cancel_work_sync(work);
                   debug_object_init(work, &work_debug_descr);
                   return 1;
           default:
                   return 0;
           }
   }
   
   /*
    * fixup_activate is called when:
    * - an active object is activated
    * - an unknown object is activated (might be a statically initialized object)
    */
   static int work_fixup_activate(void *addr, enum debug_obj_state state)
   {
           struct work_struct *work = addr;
   
           switch (state) {
   
           case ODEBUG_STATE_NOTAVAILABLE:
                   /*
                    * This is not really a fixup. The work struct was
                    * statically initialized. We just make sure that it
                    * is tracked in the object tracker.
                    */
                   if (test_bit(WORK_STRUCT_STATIC_BIT, work_data_bits(work))) {
                           debug_object_init(work, &work_debug_descr);
                           debug_object_activate(work, &work_debug_descr);
                           return 0;
                   }
                   WARN_ON_ONCE(1);
                   return 0;
   
           case ODEBUG_STATE_ACTIVE:
                   WARN_ON(1);
   
           default:
                   return 0;
           }
   }
   
   /*
    * fixup_free is called when:
    * - an active object is freed
    */
   static int work_fixup_free(void *addr, enum debug_obj_state state)
   {
           struct work_struct *work = addr;
   
           switch (state) {
           case ODEBUG_STATE_ACTIVE:
                   cancel_work_sync(work);
                   debug_object_free(work, &work_debug_descr);
                   return 1;
           default:
                   return 0;
           }
   }
   
   static struct debug_obj_descr work_debug_descr = {
           .name           = "work_struct",
           .debug_hint     = work_debug_hint,
           .fixup_init     = work_fixup_init,
           .fixup_activate = work_fixup_activate,
           .fixup_free     = work_fixup_free,
   };
   
   static inline void debug_work_activate(struct work_struct *work)
   {
           debug_object_activate(work, &work_debug_descr);
   }
   
   static inline void debug_work_deactivate(struct work_struct *work)
   {
           debug_object_deactivate(work, &work_debug_descr);
   }
   
   void __init_work(struct work_struct *work, int onstack)
   {
           if (onstack)
                   debug_object_init_on_stack(work, &work_debug_descr);
           else
                   debug_object_init(work, &work_debug_descr);
   }
   EXPORT_SYMBOL_GPL(__init_work);
   
   void destroy_work_on_stack(struct work_struct *work)
   {
           debug_object_free(work, &work_debug_descr);
   }
   EXPORT_SYMBOL_GPL(destroy_work_on_stack);
   
   #else
   static inline void debug_work_activate(struct work_struct *work) { }
   static inline void debug_work_deactivate(struct work_struct *work) { }
   #endif
   
   /* Serializes the accesses to the list of workqueues. */
   static DEFINE_SPINLOCK(workqueue_lock);
   static LIST_HEAD(workqueues);
   static bool workqueue_freezing;         /* W: have wqs started freezing? */
   
   /*
    * The almighty global cpu workqueues.  nr_running is the only field
    * which is expected to be used frequently by other cpus via
    * try_to_wake_up().  Put it in a separate cacheline.
    */
   static DEFINE_PER_CPU(struct global_cwq, global_cwq);
   static DEFINE_PER_CPU_SHARED_ALIGNED(atomic_t, pool_nr_running[NR_WORKER_POOLS]);
   
   /*
    * Global cpu workqueue and nr_running counter for unbound gcwq.  The
    * gcwq is always online, has GCWQ_DISASSOCIATED set, and all its
    * workers have WORKER_UNBOUND set.
    */
   static struct global_cwq unbound_global_cwq;
   static atomic_t unbound_pool_nr_running[NR_WORKER_POOLS] = {
           [0 ... NR_WORKER_POOLS - 1]     = ATOMIC_INIT(0),       /* always 0 */
   };
   
   static int worker_thread(void *__worker);
   
   static int worker_pool_pri(struct worker_pool *pool)
   {
           return pool - pool->gcwq->pools;
   }
   
   static struct global_cwq *get_gcwq(unsigned int cpu)
   {
           if (cpu != WORK_CPU_UNBOUND)
                   return &per_cpu(global_cwq, cpu);
           else
                   return &unbound_global_cwq;
   }
   
   static atomic_t *get_pool_nr_running(struct worker_pool *pool)
   {
           int cpu = pool->gcwq->cpu;
           int idx = worker_pool_pri(pool);
   
           if (cpu != WORK_CPU_UNBOUND)
                   return &per_cpu(pool_nr_running, cpu)[idx];
           else
                   return &unbound_pool_nr_running[idx];
   }
   
   static struct cpu_workqueue_struct *get_cwq(unsigned int cpu,
                                               struct workqueue_struct *wq)
   {
           if (!(wq->flags & WQ_UNBOUND)) {
                   if (likely(cpu < nr_cpu_ids))
                           return per_cpu_ptr(wq->cpu_wq.pcpu, cpu);
           } else if (likely(cpu == WORK_CPU_UNBOUND))
                   return wq->cpu_wq.single;
           return NULL;
   }
   
   static unsigned int work_color_to_flags(int color)
   {
           return color << WORK_STRUCT_COLOR_SHIFT;
   }
   
   static int get_work_color(struct work_struct *work)
   {
           return (*work_data_bits(work) >> WORK_STRUCT_COLOR_SHIFT) &
                   ((1 << WORK_STRUCT_COLOR_BITS) - 1);
   }
   
   static int work_next_color(int color)
   {
           return (color + 1) % WORK_NR_COLORS;
   }
   
   /*
    * While queued, %WORK_STRUCT_CWQ is set and non flag bits of a work's data
    * contain the pointer to the queued cwq.  Once execution starts, the flag
    * is cleared and the high bits contain OFFQ flags and CPU number.
    *
    * set_work_cwq(), set_work_cpu_and_clear_pending(), mark_work_canceling()
    * and clear_work_data() can be used to set the cwq, cpu or clear
    * work->data.  These functions should only be called while the work is
    * owned - ie. while the PENDING bit is set.
    *
    * get_work_[g]cwq() can be used to obtain the gcwq or cwq corresponding to
    * a work.  gcwq is available once the work has been queued anywhere after
    * initialization until it is sync canceled.  cwq is available only while
    * the work item is queued.
    *
    * %WORK_OFFQ_CANCELING is used to mark a work item which is being
    * canceled.  While being canceled, a work item may have its PENDING set
    * but stay off timer and worklist for arbitrarily long and nobody should
    * try to steal the PENDING bit.
    */
   static inline void set_work_data(struct work_struct *work, unsigned long data,
                                    unsigned long flags)
   {
           BUG_ON(!work_pending(work));
           atomic_long_set(&work->data, data | flags | work_static(work));
   }
   
   static void set_work_cwq(struct work_struct *work,
                            struct cpu_workqueue_struct *cwq,
                            unsigned long extra_flags)
   {
           set_work_data(work, (unsigned long)cwq,
                         WORK_STRUCT_PENDING | WORK_STRUCT_CWQ | extra_flags);
   }
   
   static void set_work_cpu_and_clear_pending(struct work_struct *work,
                                              unsigned int cpu)
   {
           /*
            * The following wmb is paired with the implied mb in
            * test_and_set_bit(PENDING) and ensures all updates to @work made
            * here are visible to and precede any updates by the next PENDING
            * owner.
            */
           smp_wmb();
           set_work_data(work, (unsigned long)cpu << WORK_OFFQ_CPU_SHIFT, 0);
   }
   
   static void clear_work_data(struct work_struct *work)
   {
           smp_wmb();      /* see set_work_cpu_and_clear_pending() */
           set_work_data(work, WORK_STRUCT_NO_CPU, 0);
   }
   
   static struct cpu_workqueue_struct *get_work_cwq(struct work_struct *work)
   {
           unsigned long data = atomic_long_read(&work->data);
   
           if (data & WORK_STRUCT_CWQ)
                   return (void *)(data & WORK_STRUCT_WQ_DATA_MASK);
           else
                   return NULL;
   }
   
   static struct global_cwq *get_work_gcwq(struct work_struct *work)
   {
           unsigned long data = atomic_long_read(&work->data);
           unsigned int cpu;
   
           if (data & WORK_STRUCT_CWQ)
                   return ((struct cpu_workqueue_struct *)
                           (data & WORK_STRUCT_WQ_DATA_MASK))->pool->gcwq;
   
           cpu = data >> WORK_OFFQ_CPU_SHIFT;
           if (cpu == WORK_CPU_NONE)
                   return NULL;
   
           BUG_ON(cpu >= nr_cpu_ids && cpu != WORK_CPU_UNBOUND);
           return get_gcwq(cpu);
   }
   
   static void mark_work_canceling(struct work_struct *work)
   {
           struct global_cwq *gcwq = get_work_gcwq(work);
           unsigned long cpu = gcwq ? gcwq->cpu : WORK_CPU_NONE;
   
           set_work_data(work, (cpu << WORK_OFFQ_CPU_SHIFT) | WORK_OFFQ_CANCELING,
                         WORK_STRUCT_PENDING);
   }
   
   static bool work_is_canceling(struct work_struct *work)
   {
           unsigned long data = atomic_long_read(&work->data);
   
           return !(data & WORK_STRUCT_CWQ) && (data & WORK_OFFQ_CANCELING);
   }
   
   /*
    * Policy functions.  These define the policies on how the global worker
    * pools are managed.  Unless noted otherwise, these functions assume that
    * they're being called with gcwq->lock held.
    */
   
   static bool __need_more_worker(struct worker_pool *pool)
   {
           return !atomic_read(get_pool_nr_running(pool));
   }
   
   /*
    * Need to wake up a worker?  Called from anything but currently
    * running workers.
    *
    * Note that, because unbound workers never contribute to nr_running, this
    * function will always return %true for unbound gcwq as long as the
    * worklist isn't empty.
    */
   static bool need_more_worker(struct worker_pool *pool)
   {
           return !list_empty(&pool->worklist) && __need_more_worker(pool);
   }
   
   /* Can I start working?  Called from busy but !running workers. */
   static bool may_start_working(struct worker_pool *pool)
   {
           return pool->nr_idle;
   }
   
   /* Do I need to keep working?  Called from currently running workers. */
   static bool keep_working(struct worker_pool *pool)
   {
           atomic_t *nr_running = get_pool_nr_running(pool);
   
           return !list_empty(&pool->worklist) && atomic_read(nr_running) <= 1;
   }
   
   /* Do we need a new worker?  Called from manager. */
   static bool need_to_create_worker(struct worker_pool *pool)
   {
           return need_more_worker(pool) && !may_start_working(pool);
   }
   
   /* Do I need to be the manager? */
   static bool need_to_manage_workers(struct worker_pool *pool)
   {
           return need_to_create_worker(pool) ||
                   (pool->flags & POOL_MANAGE_WORKERS);
   }
   
   /* Do we have too many workers and should some go away? */
   static bool too_many_workers(struct worker_pool *pool)
   {
           bool managing = pool->flags & POOL_MANAGING_WORKERS;
           int nr_idle = pool->nr_idle + managing; /* manager is considered idle */
           int nr_busy = pool->nr_workers - nr_idle;
   
           /*
            * nr_idle and idle_list may disagree if idle rebinding is in
            * progress.  Never return %true if idle_list is empty.
            */
           if (list_empty(&pool->idle_list))
                   return false;
   
           return nr_idle > 2 && (nr_idle - 2) * MAX_IDLE_WORKERS_RATIO >= nr_busy;
   }
   
   /*
    * Wake up functions.
    */
   
   /* Return the first worker.  Safe with preemption disabled */
   static struct worker *first_worker(struct worker_pool *pool)
   {
           if (unlikely(list_empty(&pool->idle_list)))
                   return NULL;
   
           return list_first_entry(&pool->idle_list, struct worker, entry);
   }
   
   /**
    * wake_up_worker - wake up an idle worker
    * @pool: worker pool to wake worker from
    *
    * Wake up the first idle worker of @pool.
    *
    * CONTEXT:
    * spin_lock_irq(gcwq->lock).
    */
   static void wake_up_worker(struct worker_pool *pool)
   {
           struct worker *worker = first_worker(pool);
   
           if (likely(worker))
                   wake_up_process(worker->task);
   }
   
   /**
    * wq_worker_waking_up - a worker is waking up
    * @task: task waking up
    * @cpu: CPU @task is waking up to
    *
    * This function is called during try_to_wake_up() when a worker is
    * being awoken.
    *
    * CONTEXT:
    * spin_lock_irq(rq->lock)
    */
   void wq_worker_waking_up(struct task_struct *task, unsigned int cpu)
   {
           struct worker *worker = kthread_data(task);
   
           if (!(worker->flags & WORKER_NOT_RUNNING)) {
                   WARN_ON_ONCE(worker->pool->gcwq->cpu != cpu);
                   atomic_inc(get_pool_nr_running(worker->pool));
           }
   }
   
   /**
    * wq_worker_sleeping - a worker is going to sleep
    * @task: task going to sleep
    * @cpu: CPU in question, must be the current CPU number
    *
    * This function is called during schedule() when a busy worker is
    * going to sleep.  Worker on the same cpu can be woken up by
    * returning pointer to its task.
    *
    * CONTEXT:
    * spin_lock_irq(rq->lock)
    *
    * RETURNS:
    * Worker task on @cpu to wake up, %NULL if none.
    */
   struct task_struct *wq_worker_sleeping(struct task_struct *task,
                                          unsigned int cpu)
   {
           struct worker *worker = kthread_data(task), *to_wakeup = NULL;
           struct worker_pool *pool = worker->pool;
           atomic_t *nr_running = get_pool_nr_running(pool);
   
           if (worker->flags & WORKER_NOT_RUNNING)
                   return NULL;
   
           /* this can only happen on the local cpu */
           BUG_ON(cpu != raw_smp_processor_id());
   
           /*
            * The counterpart of the following dec_and_test, implied mb,
            * worklist not empty test sequence is in insert_work().
            * Please read comment there.
            *
            * NOT_RUNNING is clear.  This means that we're bound to and
            * running on the local cpu w/ rq lock held and preemption
            * disabled, which in turn means that none else could be
            * manipulating idle_list, so dereferencing idle_list without gcwq
            * lock is safe.
            */
           if (atomic_dec_and_test(nr_running) && !list_empty(&pool->worklist))
                   to_wakeup = first_worker(pool);
           return to_wakeup ? to_wakeup->task : NULL;
   }
   
   /**
    * worker_set_flags - set worker flags and adjust nr_running accordingly
    * @worker: self
    * @flags: flags to set
    * @wakeup: wakeup an idle worker if necessary
    *
    * Set @flags in @worker->flags and adjust nr_running accordingly.  If
    * nr_running becomes zero and @wakeup is %true, an idle worker is
    * woken up.
    *
    * CONTEXT:
    * spin_lock_irq(gcwq->lock)
    */
   static inline void worker_set_flags(struct worker *worker, unsigned int flags,
                                       bool wakeup)
   {
           struct worker_pool *pool = worker->pool;
   
           WARN_ON_ONCE(worker->task != current);
   
           /*
            * If transitioning into NOT_RUNNING, adjust nr_running and
            * wake up an idle worker as necessary if requested by
            * @wakeup.
            */
           if ((flags & WORKER_NOT_RUNNING) &&
               !(worker->flags & WORKER_NOT_RUNNING)) {
                   atomic_t *nr_running = get_pool_nr_running(pool);
   
                   if (wakeup) {
                           if (atomic_dec_and_test(nr_running) &&
                               !list_empty(&pool->worklist))
                                   wake_up_worker(pool);
                   } else
                           atomic_dec(nr_running);
           }
   
           worker->flags |= flags;
   }
   
   /**
    * worker_clr_flags - clear worker flags and adjust nr_running accordingly
    * @worker: self
    * @flags: flags to clear
    *
    * Clear @flags in @worker->flags and adjust nr_running accordingly.
    *
    * CONTEXT:
    * spin_lock_irq(gcwq->lock)
    */
   static inline void worker_clr_flags(struct worker *worker, unsigned int flags)
   {
           struct worker_pool *pool = worker->pool;
           unsigned int oflags = worker->flags;
   
           WARN_ON_ONCE(worker->task != current);
   
           worker->flags &= ~flags;
   
           /*
            * If transitioning out of NOT_RUNNING, increment nr_running.  Note
            * that the nested NOT_RUNNING is not a noop.  NOT_RUNNING is mask
            * of multiple flags, not a single flag.
            */
           if ((flags & WORKER_NOT_RUNNING) && (oflags & WORKER_NOT_RUNNING))
                   if (!(worker->flags & WORKER_NOT_RUNNING))
                           atomic_inc(get_pool_nr_running(pool));
   }
   
   /**
    * busy_worker_head - return the busy hash head for a work
    * @gcwq: gcwq of interest
    * @work: work to be hashed
    *
    * Return hash head of @gcwq for @work.
    *
    * CONTEXT:
    * spin_lock_irq(gcwq->lock).
    *
    * RETURNS:
    * Pointer to the hash head.
    */
   static struct hlist_head *busy_worker_head(struct global_cwq *gcwq,
                                              struct work_struct *work)
   {
           const int base_shift = ilog2(sizeof(struct work_struct));
           unsigned long v = (unsigned long)work;
   
           /* simple shift and fold hash, do we need something better? */
           v >>= base_shift;
           v += v >> BUSY_WORKER_HASH_ORDER;
           v &= BUSY_WORKER_HASH_MASK;
   
           return &gcwq->busy_hash[v];
   }
   
   /**
    * __find_worker_executing_work - find worker which is executing a work
    * @gcwq: gcwq of interest
    * @bwh: hash head as returned by busy_worker_head()
    * @work: work to find worker for
    *
    * Find a worker which is executing @work on @gcwq.  @bwh should be
    * the hash head obtained by calling busy_worker_head() with the same
    * work.
    *
    * CONTEXT:
    * spin_lock_irq(gcwq->lock).
    *
    * RETURNS:
    * Pointer to worker which is executing @work if found, NULL
    * otherwise.
    */
   static struct worker *__find_worker_executing_work(struct global_cwq *gcwq,
                                                      struct hlist_head *bwh,
                                                      struct work_struct *work)
   {
           struct worker *worker;
           struct hlist_node *tmp;
   
           hlist_for_each_entry(worker, tmp, bwh, hentry)
                   if (worker->current_work == work)
                           return worker;
           return NULL;
   }
   
   /**
    * find_worker_executing_work - find worker which is executing a work
    * @gcwq: gcwq of interest
    * @work: work to find worker for
    *
    * Find a worker which is executing @work on @gcwq.  This function is
    * identical to __find_worker_executing_work() except that this
    * function calculates @bwh itself.
    *
    * CONTEXT:
    * spin_lock_irq(gcwq->lock).
    *
    * RETURNS:
    * Pointer to worker which is executing @work if found, NULL
    * otherwise.
    */
   static struct worker *find_worker_executing_work(struct global_cwq *gcwq,
                                                    struct work_struct *work)
   {
           return __find_worker_executing_work(gcwq, busy_worker_head(gcwq, work),
                                               work);
   }
   
   /**
    * move_linked_works - move linked works to a list
    * @work: start of series of works to be scheduled
    * @head: target list to append @work to
    * @nextp: out paramter for nested worklist walking
    *
    * Schedule linked works starting from @work to @head.  Work series to
    * be scheduled starts at @work and includes any consecutive work with
    * WORK_STRUCT_LINKED set in its predecessor.
    *
    * If @nextp is not NULL, it's updated to point to the next work of
    * the last scheduled work.  This allows move_linked_works() to be
    * nested inside outer list_for_each_entry_safe().
    *
    * CONTEXT:
    * spin_lock_irq(gcwq->lock).
    */
   static void move_linked_works(struct work_struct *work, struct list_head *head,
                                 struct work_struct **nextp)
   {
           struct work_struct *n;
   
           /*
            * Linked worklist will always end before the end of the list,
            * use NULL for list head.
            */
           list_for_each_entry_safe_from(work, n, NULL, entry) {
                   list_move_tail(&work->entry, head);
                   if (!(*work_data_bits(work) & WORK_STRUCT_LINKED))
                           break;
           }
   
           /*
            * If we're already inside safe list traversal and have moved
            * multiple works to the scheduled queue, the next position
            * needs to be updated.
            */
           if (nextp)
                   *nextp = n;
   }
   
   static void cwq_activate_delayed_work(struct work_struct *work)
   {
           struct cpu_workqueue_struct *cwq = get_work_cwq(work);
   
           trace_workqueue_activate_work(work);
           move_linked_works(work, &cwq->pool->worklist, NULL);
           __clear_bit(WORK_STRUCT_DELAYED_BIT, work_data_bits(work));
           cwq->nr_active++;
   }
   
   static void cwq_activate_first_delayed(struct cpu_workqueue_struct *cwq)
   {
           struct work_struct *work = list_first_entry(&cwq->delayed_works,
                                                       struct work_struct, entry);
   
           cwq_activate_delayed_work(work);
   }
   
  /**
   * cwq_dec_nr_in_flight - decrement cwq's nr_in_flight
   * @cwq: cwq of interest
   * @color: color of work which left the queue
   *
   * A work either has completed or is removed from pending queue,
   * decrement nr_in_flight of its cwq and handle workqueue flushing.
   *
   * CONTEXT:
   * spin_lock_irq(gcwq->lock).
   */
  static void cwq_dec_nr_in_flight(struct cpu_workqueue_struct *cwq, int color)
  {
          /* ignore uncolored works */
          if (color == WORK_NO_COLOR)
                  return;
  
          cwq->nr_in_flight[color]--;
  
          cwq->nr_active--;
          if (!list_empty(&cwq->delayed_works)) {
                  /* one down, submit a delayed one */
                  if (cwq->nr_active < cwq->max_active)
                          cwq_activate_first_delayed(cwq);
          }
  
          /* is flush in progress and are we at the flushing tip? */
          if (likely(cwq->flush_color != color))
                  return;
  
          /* are there still in-flight works? */
          if (cwq->nr_in_flight[color])
                  return;
  
          /* this cwq is done, clear flush_color */
          cwq->flush_color = -1;
  
          /*
           * If this was the last cwq, wake up the first flusher.  It
           * will handle the rest.
           */
          if (atomic_dec_and_test(&cwq->wq->nr_cwqs_to_flush))
                  complete(&cwq->wq->first_flusher->done);
  }
  
  /**
   * try_to_grab_pending - steal work item from worklist and disable irq
   * @work: work item to steal
   * @is_dwork: @work is a delayed_work
   * @flags: place to store irq state
   *
   * Try to grab PENDING bit of @work.  This function can handle @work in any
   * stable state - idle, on timer or on worklist.  Return values are
   *
   *  1           if @work was pending and we successfully stole PENDING
   *  0           if @work was idle and we claimed PENDING
   *  -EAGAIN     if PENDING couldn't be grabbed at the moment, safe to busy-retry
   *  -ENOENT     if someone else is canceling @work, this state may persist
   *              for arbitrarily long
   *
   * On >= 0 return, the caller owns @work's PENDING bit.  To avoid getting
   * interrupted while holding PENDING and @work off queue, irq must be
   * disabled on entry.  This, combined with delayed_work->timer being
   * irqsafe, ensures that we return -EAGAIN for finite short period of time.
   *
   * On successful return, >= 0, irq is disabled and the caller is
   * responsible for releasing it using local_irq_restore(*@flags).
   *
   * This function is safe to call from any context including IRQ handler.
   */
  static int try_to_grab_pending(struct work_struct *work, bool is_dwork,
                                 unsigned long *flags)
  {
          struct global_cwq *gcwq;
  
          local_irq_save(*flags);
  
          /* try to steal the timer if it exists */
          if (is_dwork) {
                  struct delayed_work *dwork = to_delayed_work(work);
  
                  /*
                   * dwork->timer is irqsafe.  If del_timer() fails, it's
                   * guaranteed that the timer is not queued anywhere and not
                   * running on the local CPU.
                   */
                  if (likely(del_timer(&dwork->timer)))
                          return 1;
          }
  
          /* try to claim PENDING the normal way */
          if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work)))
                  return 0;
  
          /*
           * The queueing is in progress, or it is already queued. Try to
           * steal it from ->worklist without clearing WORK_STRUCT_PENDING.
           */
          gcwq = get_work_gcwq(work);
          if (!gcwq)
                  goto fail;
  
          spin_lock(&gcwq->lock);
          if (!list_empty(&work->entry)) {
                  /*
                   * This work is queued, but perhaps we locked the wrong gcwq.
                   * In that case we must see the new value after rmb(), see
                   * insert_work()->wmb().
                   */
                  smp_rmb();
                  if (gcwq == get_work_gcwq(work)) {
                          debug_work_deactivate(work);
  
                          /*
                           * A delayed work item cannot be grabbed directly
                           * because it might have linked NO_COLOR work items
                           * which, if left on the delayed_list, will confuse
                           * cwq->nr_active management later on and cause
                           * stall.  Make sure the work item is activated
                           * before grabbing.
                           */
                          if (*work_data_bits(work) & WORK_STRUCT_DELAYED)
                                  cwq_activate_delayed_work(work);
  
                          list_del_init(&work->entry);
                          cwq_dec_nr_in_flight(get_work_cwq(work),
                                  get_work_color(work));
  
                          spin_unlock(&gcwq->lock);
                          return 1;
                  }
          }
          spin_unlock(&gcwq->lock);
  fail:
          local_irq_restore(*flags);
          if (work_is_canceling(work))
                  return -ENOENT;
          cpu_relax();
          return -EAGAIN;
  }
  
  /**
   * insert_work - insert a work into gcwq
   * @cwq: cwq @work belongs to
   * @work: work to insert
   * @head: insertion point
   * @extra_flags: extra WORK_STRUCT_* flags to set
   *
   * Insert @work which belongs to @cwq into @gcwq after @head.
   * @extra_flags is or'd to work_struct flags.
   *
   * CONTEXT:
   * spin_lock_irq(gcwq->lock).
   */
  static void insert_work(struct cpu_workqueue_struct *cwq,
                          struct work_struct *work, struct list_head *head,
                          unsigned int extra_flags)
  {
          struct worker_pool *pool = cwq->pool;
  
          /* we own @work, set data and link */
          set_work_cwq(work, cwq, extra_flags);
  
          /*
           * Ensure that we get the right work->data if we see the
           * result of list_add() below, see try_to_grab_pending().
           */
          smp_wmb();
  
          list_add_tail(&work->entry, head);
  
          /*
           * Ensure either worker_sched_deactivated() sees the above
           * list_add_tail() or we see zero nr_running to avoid workers
           * lying around lazily while there are works to be processed.
           */
          smp_mb();
  
          if (__need_more_worker(pool))
                  wake_up_worker(pool);
  }
  
  /*
   * Test whether @work is being queued from another work executing on the
   * same workqueue.  This is rather expensive and should only be used from
   * cold paths.
   */
  static bool is_chained_work(struct workqueue_struct *wq)
  {
          unsigned long flags;
          unsigned int cpu;
  
          for_each_gcwq_cpu(cpu) {
                  struct global_cwq *gcwq = get_gcwq(cpu);
                  struct worker *worker;
                  struct hlist_node *pos;
                  int i;
  
                  spin_lock_irqsave(&gcwq->lock, flags);
                  for_each_busy_worker(worker, i, pos, gcwq) {
                          if (worker->task != current)
                                  continue;
                          spin_unlock_irqrestore(&gcwq->lock, flags);
                          /*
                           * I'm @worker, no locking necessary.  See if @work
                           * is headed to the same workqueue.
                           */
                          return worker->current_cwq->wq == wq;
                  }
                  spin_unlock_irqrestore(&gcwq->lock, flags);
          }
          return false;
  }
  
  static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,
                           struct work_struct *work)
  {
          struct global_cwq *gcwq;
          struct cpu_workqueue_struct *cwq;
          struct list_head *worklist;
          unsigned int work_flags;
          unsigned int req_cpu = cpu;
  
          /*
           * While a work item is PENDING && off queue, a task trying to
           * steal the PENDING will busy-loop waiting for it to either get
           * queued or lose PENDING.  Grabbing PENDING and queueing should
           * happen with IRQ disabled.
           */
          WARN_ON_ONCE(!irqs_disabled());
  
          debug_work_activate(work);
  
          /* if dying, only works from the same workqueue are allowed */
          if (unlikely(wq->flags & WQ_DRAINING) &&
              WARN_ON_ONCE(!is_chained_work(wq)))
                  return;
  
          /* determine gcwq to use */
          if (!(wq->flags & WQ_UNBOUND)) {
                  struct global_cwq *last_gcwq;
  
                  if (cpu == WORK_CPU_UNBOUND)
                          cpu = raw_smp_processor_id();
  
                  /*
                   * It's multi cpu.  If @work was previously on a different
                   * cpu, it might still be running there, in which case the
                   * work needs to be queued on that cpu to guarantee
                   * non-reentrancy.
                   */
                  gcwq = get_gcwq(cpu);
                  last_gcwq = get_work_gcwq(work);
  
                  if (last_gcwq && last_gcwq != gcwq) {
                          struct worker *worker;
  
                          spin_lock(&last_gcwq->lock);
  
                          worker = find_worker_executing_work(last_gcwq, work);
  
                          if (worker && worker->current_cwq->wq == wq)
                                  gcwq = last_gcwq;
                          else {
                                  /* meh... not running there, queue here */
                                  spin_unlock(&last_gcwq->lock);
                                  spin_lock(&gcwq->lock);
                          }
                  } else {
                          spin_lock(&gcwq->lock);
                  }
          } else {
                  gcwq = get_gcwq(WORK_CPU_UNBOUND);
                  spin_lock(&gcwq->lock);
          }
  
          /* gcwq determined, get cwq and queue */
          cwq = get_cwq(gcwq->cpu, wq);
          trace_workqueue_queue_work(req_cpu, cwq, work);
  
          if (WARN_ON(!list_empty(&work->entry))) {
                  spin_unlock(&gcwq->lock);
                  return;
          }
  
          cwq->nr_in_flight[cwq->work_color]++;
          work_flags = work_color_to_flags(cwq->work_color);
  
          if (likely(cwq->nr_active < cwq->max_active)) {
                  trace_workqueue_activate_work(work);
                  cwq->nr_active++;
                  worklist = &cwq->pool->worklist;
          } else {
                  work_flags |= WORK_STRUCT_DELAYED;
                  worklist = &cwq->delayed_works;
          }
  
          insert_work(cwq, work, worklist, work_flags);
  
          spin_unlock(&gcwq->lock);
  }
  
  /**
   * queue_work_on - queue work on specific cpu
   * @cpu: CPU number to execute work on
   * @wq: workqueue to use
   * @work: work to queue
   *
   * Returns %false if @work was already on a queue, %true otherwise.
   *
   * We queue the work to a specific CPU, the caller must ensure it
   * can't go away.
   */
  bool queue_work_on(int cpu, struct workqueue_struct *wq,
                     struct work_struct *work)
  {
          bool ret = false;
          unsigned long flags;
  
          local_irq_save(flags);
  
          if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) {
                  __queue_work(cpu, wq, work);
                  ret = true;
          }
  
          local_irq_restore(flags);
          return ret;
  }
  EXPORT_SYMBOL_GPL(queue_work_on);
  
  /**
   * queue_work - queue work on a workqueue
   * @wq: workqueue to use
   * @work: work to queue
   *
   * Returns %false if @work was already on a queue, %true otherwise.
   *
   * We queue the work to the CPU on which it was submitted, but if the CPU dies
   * it can be processed by another CPU.
   */
  bool queue_work(struct workqueue_struct *wq, struct work_struct *work)
  {
          return queue_work_on(WORK_CPU_UNBOUND, wq, work);
  }
  EXPORT_SYMBOL_GPL(queue_work);
  
  void delayed_work_timer_fn(unsigned long __data)
  {
          struct delayed_work *dwork = (struct delayed_work *)__data;
          struct cpu_workqueue_struct *cwq = get_work_cwq(&dwork->work);
  
          /* should have been called from irqsafe timer with irq already off */
          __queue_work(dwork->cpu, cwq->wq, &dwork->work);
  }
  EXPORT_SYMBOL_GPL(delayed_work_timer_fn);
  
  static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
                                  struct delayed_work *dwork, unsigned long delay)
  {
          struct timer_list *timer = &dwork->timer;
          struct work_struct *work = &dwork->work;
          unsigned int lcpu;
  
          WARN_ON_ONCE(timer->function != delayed_work_timer_fn ||
                       timer->data != (unsigned long)dwork);
          WARN_ON_ONCE(timer_pending(timer));
          WARN_ON_ONCE(!list_empty(&work->entry));
  
          /*
           * If @delay is 0, queue @dwork->work immediately.  This is for
           * both optimization and correctness.  The earliest @timer can
           * expire is on the closest next tick and delayed_work users depend
           * on that there's no such delay when @delay is 0.
           */
          if (!delay) {
                  __queue_work(cpu, wq, &dwork->work);
                  return;
          }
  
          timer_stats_timer_set_start_info(&dwork->timer);
  
          /*
           * This stores cwq for the moment, for the timer_fn.  Note that the
           * work's gcwq is preserved to allow reentrance detection for
           * delayed works.
           */
          if (!(wq->flags & WQ_UNBOUND)) {
                  struct global_cwq *gcwq = get_work_gcwq(work);
  
                  /*
                   * If we cannot get the last gcwq from @work directly,
                   * select the last CPU such that it avoids unnecessarily
                   * triggering non-reentrancy check in __queue_work().
                   */
                  lcpu = cpu;
                  if (gcwq)
                          lcpu = gcwq->cpu;
                  if (lcpu == WORK_CPU_UNBOUND)
                          lcpu = raw_smp_processor_id();
          } else {
                  lcpu = WORK_CPU_UNBOUND;
          }
  
          set_work_cwq(work, get_cwq(lcpu, wq), 0);
  
          dwork->cpu = cpu;
          timer->expires = jiffies + delay;
  
          if (unlikely(cpu != WORK_CPU_UNBOUND))
                  add_timer_on(timer, cpu);
          else
                  add_timer(timer);
  }
  
  /**
   * queue_delayed_work_on - queue work on specific CPU after delay
   * @cpu: CPU number to execute work on
   * @wq: workqueue to use
   * @dwork: work to queue
   * @delay: number of jiffies to wait before queueing
   *
   * Returns %false if @work was already on a queue, %true otherwise.  If
   * @delay is zero and @dwork is idle, it will be scheduled for immediate
   * execution.
   */
  bool queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
                             struct delayed_work *dwork, unsigned long delay)
  {
          struct work_struct *work = &dwork->work;
          bool ret = false;
          unsigned long flags;
  
          /* read the comment in __queue_work() */
          local_irq_save(flags);
  
          if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) {
                  __queue_delayed_work(cpu, wq, dwork, delay);
                  ret = true;
          }
  
          local_irq_restore(flags);
          return ret;
  }
  EXPORT_SYMBOL_GPL(queue_delayed_work_on);
  
  /**
   * queue_delayed_work - queue work on a workqueue after delay
   * @wq: workqueue to use
   * @dwork: delayable work to queue
   * @delay: number of jiffies to wait before queueing
   *
   * Equivalent to queue_delayed_work_on() but tries to use the local CPU.
   */
  bool queue_delayed_work(struct workqueue_struct *wq,
                          struct delayed_work *dwork, unsigned long delay)
  {
          return queue_delayed_work_on(WORK_CPU_UNBOUND, wq, dwork, delay);
  }
  EXPORT_SYMBOL_GPL(queue_delayed_work);
  
  /**
   * mod_delayed_work_on - modify delay of or queue a delayed work on specific CPU
   * @cpu: CPU number to execute work on
   * @wq: workqueue to use
   * @dwork: work to queue
   * @delay: number of jiffies to wait before queueing
   *
   * If @dwork is idle, equivalent to queue_delayed_work_on(); otherwise,
   * modify @dwork's timer so that it expires after @delay.  If @delay is
   * zero, @work is guaranteed to be scheduled immediately regardless of its
   * current state.
   *
   * Returns %false if @dwork was idle and queued, %true if @dwork was
   * pending and its timer was modified.
   *
   * This function is safe to call from any context including IRQ handler.
   * See try_to_grab_pending() for details.
   */
  bool mod_delayed_work_on(int cpu, struct workqueue_struct *wq,
                           struct delayed_work *dwork, unsigned long delay)
  {
          unsigned long flags;
          int ret;
  
          do {
                  ret = try_to_grab_pending(&dwork->work, true, &flags);
          } while (unlikely(ret == -EAGAIN));
  
          if (likely(ret >= 0)) {
                  __queue_delayed_work(cpu, wq, dwork, delay);
                  local_irq_restore(flags);
          }
  
          /* -ENOENT from try_to_grab_pending() becomes %true */
          return ret;
  }
  EXPORT_SYMBOL_GPL(mod_delayed_work_on);
  
  /**
   * mod_delayed_work - modify delay of or queue a delayed work
   * @wq: workqueue to use
   * @dwork: work to queue
   * @delay: number of jiffies to wait before queueing
   *
   * mod_delayed_work_on() on local CPU.
   */
  bool mod_delayed_work(struct workqueue_struct *wq, struct delayed_work *dwork,
                        unsigned long delay)
  {
          return mod_delayed_work_on(WORK_CPU_UNBOUND, wq, dwork, delay);
  }
  EXPORT_SYMBOL_GPL(mod_delayed_work);
  
  /**
   * worker_enter_idle - enter idle state
   * @worker: worker which is entering idle state
   *
   * @worker is entering idle state.  Update stats and idle timer if
   * necessary.
   *
   * LOCKING:
   * spin_lock_irq(gcwq->lock).
   */
  static void worker_enter_idle(struct worker *worker)
  {
          struct worker_pool *pool = worker->pool;
          struct global_cwq *gcwq = pool->gcwq;
  
          BUG_ON(worker->flags & WORKER_IDLE);
          BUG_ON(!list_empty(&worker->entry) &&
                 (worker->hentry.next || worker->hentry.pprev));
  
          /* can't use worker_set_flags(), also called from start_worker() */
          worker->flags |= WORKER_IDLE;
          pool->nr_idle++;
          worker->last_active = jiffies;
  
          /* idle_list is LIFO */
          list_add(&worker->entry, &pool->idle_list);
  
          if (too_many_workers(pool) && !timer_pending(&pool->idle_timer))
                  mod_timer(&pool->idle_timer, jiffies + IDLE_WORKER_TIMEOUT);
  
          /*
           * Sanity check nr_running.  Because gcwq_unbind_fn() releases
           * gcwq->lock between setting %WORKER_UNBOUND and zapping
           * nr_running, the warning may trigger spuriously.  Check iff
           * unbind is not in progress.
           */
          WARN_ON_ONCE(!(gcwq->flags & GCWQ_DISASSOCIATED) &&
                       pool->nr_workers == pool->nr_idle &&
                       atomic_read(get_pool_nr_running(pool)));
  }
  
  /**
   * worker_leave_idle - leave idle state
   * @worker: worker which is leaving idle state
   *
   * @worker is leaving idle state.  Update stats.
   *
   * LOCKING:
   * spin_lock_irq(gcwq->lock).
   */
  static void worker_leave_idle(struct worker *worker)
  {
          struct worker_pool *pool = worker->pool;
  
          BUG_ON(!(worker->flags & WORKER_IDLE));
          worker_clr_flags(worker, WORKER_IDLE);
          pool->nr_idle--;
          list_del_init(&worker->entry);
  }
  
  /**
   * worker_maybe_bind_and_lock - bind worker to its cpu if possible and lock gcwq
   * @worker: self
   *
   * Works which are scheduled while the cpu is online must at least be
   * scheduled to a worker which is bound to the cpu so that if they are
   * flushed from cpu callbacks while cpu is going down, they are
   * guaranteed to execute on the cpu.
   *
   * This function is to be used by rogue workers and rescuers to bind
   * themselves to the target cpu and may race with cpu going down or
   * coming online.  kthread_bind() can't be used because it may put the
   * worker to already dead cpu and set_cpus_allowed_ptr() can't be used
   * verbatim as it's best effort and blocking and gcwq may be
   * [dis]associated in the meantime.
   *
   * This function tries set_cpus_allowed() and locks gcwq and verifies the
   * binding against %GCWQ_DISASSOCIATED which is set during
   * %CPU_DOWN_PREPARE and cleared during %CPU_ONLINE, so if the worker
   * enters idle state or fetches works without dropping lock, it can
   * guarantee the scheduling requirement described in the first paragraph.
   *
   * CONTEXT:
   * Might sleep.  Called without any lock but returns with gcwq->lock
   * held.
   *
   * RETURNS:
   * %true if the associated gcwq is online (@worker is successfully
   * bound), %false if offline.
   */
  static bool worker_maybe_bind_and_lock(struct worker *worker)
  __acquires(&gcwq->lock)
  {
          struct global_cwq *gcwq = worker->pool->gcwq;
          struct task_struct *task = worker->task;
  
          while (true) {
                  /*
                   * The following call may fail, succeed or succeed
                   * without actually migrating the task to the cpu if
                   * it races with cpu hotunplug operation.  Verify
                   * against GCWQ_DISASSOCIATED.
                   */
                  if (!(gcwq->flags & GCWQ_DISASSOCIATED))
                          set_cpus_allowed_ptr(task, get_cpu_mask(gcwq->cpu));
  
                  spin_lock_irq(&gcwq->lock);
                  if (gcwq->flags & GCWQ_DISASSOCIATED)
                          return false;
                  if (task_cpu(task) == gcwq->cpu &&
                      cpumask_equal(&current->cpus_allowed,
                                    get_cpu_mask(gcwq->cpu)))
                          return true;
                  spin_unlock_irq(&gcwq->lock);
  
                  /*
                   * We've raced with CPU hot[un]plug.  Give it a breather
                   * and retry migration.  cond_resched() is required here;
                   * otherwise, we might deadlock against cpu_stop trying to
                   * bring down the CPU on non-preemptive kernel.
                   */
                  cpu_relax();
                  cond_resched();
          }
  }
  
  /*
   * Rebind an idle @worker to its CPU.  worker_thread() will test
   * list_empty(@worker->entry) before leaving idle and call this function.
   */
  static void idle_worker_rebind(struct worker *worker)
  {
          struct global_cwq *gcwq = worker->pool->gcwq;
  
          /* CPU may go down again inbetween, clear UNBOUND only on success */
          if (worker_maybe_bind_and_lock(worker))
                  worker_clr_flags(worker, WORKER_UNBOUND);
  
          /* rebind complete, become available again */
          list_add(&worker->entry, &worker->pool->idle_list);
          spin_unlock_irq(&gcwq->lock);
  }
  
  /*
   * Function for @worker->rebind.work used to rebind unbound busy workers to
   * the associated cpu which is coming back online.  This is scheduled by
   * cpu up but can race with other cpu hotplug operations and may be
   * executed twice without intervening cpu down.
   */
  static void busy_worker_rebind_fn(struct work_struct *work)
  {
          struct worker *worker = container_of(work, struct worker, rebind_work);
          struct global_cwq *gcwq = worker->pool->gcwq;
  
          if (worker_maybe_bind_and_lock(worker))
                  worker_clr_flags(worker, WORKER_UNBOUND);
  
          spin_unlock_irq(&gcwq->lock);
  }
  
  /**
   * rebind_workers - rebind all workers of a gcwq to the associated CPU
   * @gcwq: gcwq of interest
   *
   * @gcwq->cpu is coming online.  Rebind all workers to the CPU.  Rebinding
   * is different for idle and busy ones.
   *
   * Idle ones will be removed from the idle_list and woken up.  They will
   * add themselves back after completing rebind.  This ensures that the
   * idle_list doesn't contain any unbound workers when re-bound busy workers
   * try to perform local wake-ups for concurrency management.
   *
   * Busy workers can rebind after they finish their current work items.
   * Queueing the rebind work item at the head of the scheduled list is
   * enough.  Note that nr_running will be properly bumped as busy workers
   * rebind.
   *
   * On return, all non-manager workers are scheduled for rebind - see
   * manage_workers() for the manager special case.  Any idle worker
   * including the manager will not appear on @idle_list until rebind is
   * complete, making local wake-ups safe.
   */
  static void rebind_workers(struct global_cwq *gcwq)
  {
          struct worker_pool *pool;
          struct worker *worker, *n;
          struct hlist_node *pos;
          int i;
  
          lockdep_assert_held(&gcwq->lock);
  
          for_each_worker_pool(pool, gcwq)
                  lockdep_assert_held(&pool->assoc_mutex);
  
          /* dequeue and kick idle ones */
          for_each_worker_pool(pool, gcwq) {
                  list_for_each_entry_safe(worker, n, &pool->idle_list, entry) {
                          /*
                           * idle workers should be off @pool->idle_list
                           * until rebind is complete to avoid receiving
                           * premature local wake-ups.
                           */
                          list_del_init(&worker->entry);
  
                          /*
                           * worker_thread() will see the above dequeuing
                           * and call idle_worker_rebind().
                           */
                          wake_up_process(worker->task);
                  }
          }
  
          /* rebind busy workers */
          for_each_busy_worker(worker, i, pos, gcwq) {
                  struct work_struct *rebind_work = &worker->rebind_work;
                  struct workqueue_struct *wq;
  
                  if (test_and_set_bit(WORK_STRUCT_PENDING_BIT,
                                       work_data_bits(rebind_work)))
                          continue;
  
                  debug_work_activate(rebind_work);
  
                  /*
                   * wq doesn't really matter but let's keep @worker->pool
                   * and @cwq->pool consistent for sanity.
                   */
                  if (worker_pool_pri(worker->pool))
                          wq = system_highpri_wq;
                  else
                          wq = system_wq;
  
                  insert_work(get_cwq(gcwq->cpu, wq), rebind_work,
                          worker->scheduled.next,
                          work_color_to_flags(WORK_NO_COLOR));
          }
  }
  
  static struct worker *alloc_worker(void)
  {
          struct worker *worker;
  
          worker = kzalloc(sizeof(*worker), GFP_KERNEL);
          if (worker) {
                  INIT_LIST_HEAD(&worker->entry);
                  INIT_LIST_HEAD(&worker->scheduled);
                  INIT_WORK(&worker->rebind_work, busy_worker_rebind_fn);
                  /* on creation a worker is in !idle && prep state */
                  worker->flags = WORKER_PREP;
          }
          return worker;
  }
  
  /**
   * create_worker - create a new workqueue worker
   * @pool: pool the new worker will belong to
   *
   * Create a new worker which is bound to @pool.  The returned worker
   * can be started by calling start_worker() or destroyed using
   * destroy_worker().
   *
   * CONTEXT:
   * Might sleep.  Does GFP_KERNEL allocations.
   *
   * RETURNS:
   * Pointer to the newly created worker.
   */
  static struct worker *create_worker(struct worker_pool *pool)
  {
          struct global_cwq *gcwq = pool->gcwq;
          const char *pri = worker_pool_pri(pool) ? "H" : "";
          struct worker *worker = NULL;
          int id = -1;
  
          spin_lock_irq(&gcwq->lock);
          while (ida_get_new(&pool->worker_ida, &id)) {
                  spin_unlock_irq(&gcwq->lock);
                  if (!ida_pre_get(&pool->worker_ida, GFP_KERNEL))
                          goto fail;
                  spin_lock_irq(&gcwq->lock);
          }
          spin_unlock_irq(&gcwq->lock);
  
          worker = alloc_worker();
          if (!worker)
                  goto fail;
  
          worker->pool = pool;
          worker->id = id;
  
          if (gcwq->cpu != WORK_CPU_UNBOUND)
                  worker->task = kthread_create_on_node(worker_thread,
                                          worker, cpu_to_node(gcwq->cpu),
                                          "kworker/%u:%d%s", gcwq->cpu, id, pri);
          else
                  worker->task = kthread_create(worker_thread, worker,
                                                "kworker/u:%d%s", id, pri);
          if (IS_ERR(worker->task))
                  goto fail;
  
          if (worker_pool_pri(pool))
                  set_user_nice(worker->task, HIGHPRI_NICE_LEVEL);
  
          /*
           * Determine CPU binding of the new worker depending on
           * %GCWQ_DISASSOCIATED.  The caller is responsible for ensuring the
           * flag remains stable across this function.  See the comments
           * above the flag definition for details.
           *
           * As an unbound worker may later become a regular one if CPU comes
           * online, make sure every worker has %PF_THREAD_BOUND set.
           */
          if (!(gcwq->flags & GCWQ_DISASSOCIATED)) {
                  kthread_bind(worker->task, gcwq->cpu);
          } else {
                  worker->task->flags |= PF_THREAD_BOUND;
                  worker->flags |= WORKER_UNBOUND;
          }
  
          return worker;
  fail:
          if (id >= 0) {
                  spin_lock_irq(&gcwq->lock);
                  ida_remove(&pool->worker_ida, id);
                  spin_unlock_irq(&gcwq->lock);
          }
          kfree(worker);
          return NULL;
  }
  
  /**
   * start_worker - start a newly created worker
   * @worker: worker to start
   *
   * Make the gcwq aware of @worker and start it.
   *
   * CONTEXT:
   * spin_lock_irq(gcwq->lock).
   */
  static void start_worker(struct worker *worker)
  {
          worker->flags |= WORKER_STARTED;
          worker->pool->nr_workers++;
          worker_enter_idle(worker);
          wake_up_process(worker->task);
  }
  
  /**
   * destroy_worker - destroy a workqueue worker
   * @worker: worker to be destroyed
   *
   * Destroy @worker and adjust @gcwq stats accordingly.
   *
   * CONTEXT:
   * spin_lock_irq(gcwq->lock) which is released and regrabbed.
   */
  static void destroy_worker(struct worker *worker)
  {
          struct worker_pool *pool = worker->pool;
          struct global_cwq *gcwq = pool->gcwq;
          int id = worker->id;
  
          /* sanity check frenzy */
          BUG_ON(worker->current_work);
          BUG_ON(!list_empty(&worker->scheduled));
  
          if (worker->flags & WORKER_STARTED)
                  pool->nr_workers--;
          if (worker->flags & WORKER_IDLE)
                  pool->nr_idle--;
  
          list_del_init(&worker->entry);
          worker->flags |= WORKER_DIE;
  
          spin_unlock_irq(&gcwq->lock);
  
          kthread_stop(worker->task);
          kfree(worker);
  
          spin_lock_irq(&gcwq->lock);
          ida_remove(&pool->worker_ida, id);
  }
  
  static void idle_worker_timeout(unsigned long __pool)
  {
          struct worker_pool *pool = (void *)__pool;
          struct global_cwq *gcwq = pool->gcwq;
  
          spin_lock_irq(&gcwq->lock);
  
          if (too_many_workers(pool)) {
                  struct worker *worker;
                  unsigned long expires;
  
                  /* idle_list is kept in LIFO order, check the last one */
                  worker = list_entry(pool->idle_list.prev, struct worker, entry);
                  expires = worker->last_active + IDLE_WORKER_TIMEOUT;
  
                  if (time_before(jiffies, expires))
                          mod_timer(&pool->idle_timer, expires);
                  else {
                          /* it's been idle for too long, wake up manager */
                          pool->flags |= POOL_MANAGE_WORKERS;
                          wake_up_worker(pool);
                  }
          }
  
          spin_unlock_irq(&gcwq->lock);
  }
  
  static bool send_mayday(struct work_struct *work)
  {
          struct cpu_workqueue_struct *cwq = get_work_cwq(work);
          struct workqueue_struct *wq = cwq->wq;
          unsigned int cpu;
  
          if (!(wq->flags & WQ_RESCUER))
                  return false;
  
          /* mayday mayday mayday */
          cpu = cwq->pool->gcwq->cpu;
          /* WORK_CPU_UNBOUND can't be set in cpumask, use cpu 0 instead */
          if (cpu == WORK_CPU_UNBOUND)
                  cpu = 0;
          if (!mayday_test_and_set_cpu(cpu, wq->mayday_mask))
                  wake_up_process(wq->rescuer->task);
          return true;
  }
  
  static void gcwq_mayday_timeout(unsigned long __pool)
  {
          struct worker_pool *pool = (void *)__pool;
          struct global_cwq *gcwq = pool->gcwq;
          struct work_struct *work;
  
          spin_lock_irq(&gcwq->lock);
  
          if (need_to_create_worker(pool)) {
                  /*
                   * We've been trying to create a new worker but
                   * haven't been successful.  We might be hitting an
                   * allocation deadlock.  Send distress signals to
                   * rescuers.
                   */
                  list_for_each_entry(work, &pool->worklist, entry)
                          send_mayday(work);
          }
  
          spin_unlock_irq(&gcwq->lock);
  
          mod_timer(&pool->mayday_timer, jiffies + MAYDAY_INTERVAL);
  }
  
  /**
   * maybe_create_worker - create a new worker if necessary
   * @pool: pool to create a new worker for
   *
   * Create a new worker for @pool if necessary.  @pool is guaranteed to
   * have at least one idle worker on return from this function.  If
   * creating a new worker takes longer than MAYDAY_INTERVAL, mayday is
   * sent to all rescuers with works scheduled on @pool to resolve
   * possible allocation deadlock.
   *
   * On return, need_to_create_worker() is guaranteed to be false and
   * may_start_working() true.
   *
   * LOCKING:
   * spin_lock_irq(gcwq->lock) which may be released and regrabbed
   * multiple times.  Does GFP_KERNEL allocations.  Called only from
   * manager.
   *
   * RETURNS:
   * false if no action was taken and gcwq->lock stayed locked, true
   * otherwise.
   */
  static bool maybe_create_worker(struct worker_pool *pool)
  __releases(&gcwq->lock)
  __acquires(&gcwq->lock)
  {
          struct global_cwq *gcwq = pool->gcwq;
  
          if (!need_to_create_worker(pool))
                  return false;
  restart:
          spin_unlock_irq(&gcwq->lock);
  
          /* if we don't make progress in MAYDAY_INITIAL_TIMEOUT, call for help */
          mod_timer(&pool->mayday_timer, jiffies + MAYDAY_INITIAL_TIMEOUT);
  
          while (true) {
                  struct worker *worker;
  
                  worker = create_worker(pool);
                  if (worker) {
                          del_timer_sync(&pool->mayday_timer);
                          spin_lock_irq(&gcwq->lock);
                          start_worker(worker);
                          BUG_ON(need_to_create_worker(pool));
                          return true;
                  }
  
                  if (!need_to_create_worker(pool))
                          break;
  
                  __set_current_state(TASK_INTERRUPTIBLE);
                  schedule_timeout(CREATE_COOLDOWN);
  
                  if (!need_to_create_worker(pool))
                          break;
          }
  
          del_timer_sync(&pool->mayday_timer);
          spin_lock_irq(&gcwq->lock);
          if (need_to_create_worker(pool))
                  goto restart;
          return true;
  }
  
  /**
   * maybe_destroy_worker - destroy workers which have been idle for a while
   * @pool: pool to destroy workers for
   *
   * Destroy @pool workers which have been idle for longer than
   * IDLE_WORKER_TIMEOUT.
   *
   * LOCKING:
   * spin_lock_irq(gcwq->lock) which may be released and regrabbed
   * multiple times.  Called only from manager.
   *
   * RETURNS:
   * false if no action was taken and gcwq->lock stayed locked, true
   * otherwise.
   */
  static bool maybe_destroy_workers(struct worker_pool *pool)
  {
          bool ret = false;
  
          while (too_many_workers(pool)) {
                  struct worker *worker;
                  unsigned long expires;
  
                  worker = list_entry(pool->idle_list.prev, struct worker, entry);
                  expires = worker->last_active + IDLE_WORKER_TIMEOUT;
  
                  if (time_before(jiffies, expires)) {
                          mod_timer(&pool->idle_timer, expires);
                          break;
                  }
  
                  destroy_worker(worker);
                  ret = true;
          }
  
          return ret;
  }
  
  /**
   * manage_workers - manage worker pool
   * @worker: self
   *
   * Assume the manager role and manage gcwq worker pool @worker belongs
   * to.  At any given time, there can be only zero or one manager per
   * gcwq.  The exclusion is handled automatically by this function.
   *
   * The caller can safely start processing works on false return.  On
   * true return, it's guaranteed that need_to_create_worker() is false
   * and may_start_working() is true.
   *
   * CONTEXT:
   * spin_lock_irq(gcwq->lock) which may be released and regrabbed
   * multiple times.  Does GFP_KERNEL allocations.
   *
   * RETURNS:
   * false if no action was taken and gcwq->lock stayed locked, true if
   * some action was taken.
   */
  static bool manage_workers(struct worker *worker)
  {
          struct worker_pool *pool = worker->pool;
          bool ret = false;
  
          if (pool->flags & POOL_MANAGING_WORKERS)
                  return ret;
  
          pool->flags |= POOL_MANAGING_WORKERS;
  
          /*
           * To simplify both worker management and CPU hotplug, hold off
           * management while hotplug is in progress.  CPU hotplug path can't
           * grab %POOL_MANAGING_WORKERS to achieve this because that can
           * lead to idle worker depletion (all become busy thinking someone
           * else is managing) which in turn can result in deadlock under
           * extreme circumstances.  Use @pool->assoc_mutex to synchronize
           * manager against CPU hotplug.
           *
           * assoc_mutex would always be free unless CPU hotplug is in
           * progress.  trylock first without dropping @gcwq->lock.
           */
          if (unlikely(!mutex_trylock(&pool->assoc_mutex))) {
                  spin_unlock_irq(&pool->gcwq->lock);
                  mutex_lock(&pool->assoc_mutex);
                  /*
                   * CPU hotplug could have happened while we were waiting
                   * for assoc_mutex.  Hotplug itself can't handle us
                   * because manager isn't either on idle or busy list, and
                   * @gcwq's state and ours could have deviated.
                   *
                   * As hotplug is now excluded via assoc_mutex, we can
                   * simply try to bind.  It will succeed or fail depending
                   * on @gcwq's current state.  Try it and adjust
                   * %WORKER_UNBOUND accordingly.
                   */
                  if (worker_maybe_bind_and_lock(worker))
                          worker->flags &= ~WORKER_UNBOUND;
                  else
                          worker->flags |= WORKER_UNBOUND;
  
                  ret = true;
          }
  
          pool->flags &= ~POOL_MANAGE_WORKERS;
  
          /*
           * Destroy and then create so that may_start_working() is true
           * on return.
           */
          ret |= maybe_destroy_workers(pool);
          ret |= maybe_create_worker(pool);
  
          pool->flags &= ~POOL_MANAGING_WORKERS;
          mutex_unlock(&pool->assoc_mutex);
          return ret;
  }
  
  /**
   * process_one_work - process single work
   * @worker: self
   * @work: work to process
   *
   * Process @work.  This function contains all the logics necessary to
   * process a single work including synchronization against and
   * interaction with other workers on the same cpu, queueing and
   * flushing.  As long as context requirement is met, any worker can
   * call this function to process a work.
   *
   * CONTEXT:
   * spin_lock_irq(gcwq->lock) which is released and regrabbed.
   */
  static void process_one_work(struct worker *worker, struct work_struct *work)
  __releases(&gcwq->lock)
  __acquires(&gcwq->lock)
  {
          struct cpu_workqueue_struct *cwq = get_work_cwq(work);
          struct worker_pool *pool = worker->pool;
          struct global_cwq *gcwq = pool->gcwq;
          struct hlist_head *bwh = busy_worker_head(gcwq, work);
          bool cpu_intensive = cwq->wq->flags & WQ_CPU_INTENSIVE;
          work_func_t f = work->func;
          int work_color;
          struct worker *collision;
  #ifdef CONFIG_LOCKDEP
          /*
           * It is permissible to free the struct work_struct from
           * inside the function that is called from it, this we need to
           * take into account for lockdep too.  To avoid bogus "held
           * lock freed" warnings as well as problems when looking into
           * work->lockdep_map, make a copy and use that here.
           */
          struct lockdep_map lockdep_map;
  
          lockdep_copy_map(&lockdep_map, &work->lockdep_map);
  #endif
          /*
           * Ensure we're on the correct CPU.  DISASSOCIATED test is
           * necessary to avoid spurious warnings from rescuers servicing the
           * unbound or a disassociated gcwq.
           */
          WARN_ON_ONCE(!(worker->flags & WORKER_UNBOUND) &&
                       !(gcwq->flags & GCWQ_DISASSOCIATED) &&
                       raw_smp_processor_id() != gcwq->cpu);
  
          /*
           * A single work shouldn't be executed concurrently by
           * multiple workers on a single cpu.  Check whether anyone is
           * already processing the work.  If so, defer the work to the
           * currently executing one.
           */
          collision = __find_worker_executing_work(gcwq, bwh, work);
          if (unlikely(collision)) {
                  move_linked_works(work, &collision->scheduled, NULL);
                  return;
          }
  
          /* claim and dequeue */
          debug_work_deactivate(work);
          hlist_add_head(&worker->hentry, bwh);
          worker->current_work = work;
          worker->current_cwq = cwq;
          work_color = get_work_color(work);
  
          list_del_init(&work->entry);
  
          /*
           * CPU intensive works don't participate in concurrency
           * management.  They're the scheduler's responsibility.
           */
          if (unlikely(cpu_intensive))
                  worker_set_flags(worker, WORKER_CPU_INTENSIVE, true);
  
          /*
           * Unbound gcwq isn't concurrency managed and work items should be
           * executed ASAP.  Wake up another worker if necessary.
           */
          if ((worker->flags & WORKER_UNBOUND) && need_more_worker(pool))
                  wake_up_worker(pool);
  
          /*
           * Record the last CPU and clear PENDING which should be the last
           * update to @work.  Also, do this inside @gcwq->lock so that
           * PENDING and queued state changes happen together while IRQ is
           * disabled.
           */
          set_work_cpu_and_clear_pending(work, gcwq->cpu);
  
          spin_unlock_irq(&gcwq->lock);
  
          lock_map_acquire_read(&cwq->wq->lockdep_map);
          lock_map_acquire(&lockdep_map);
          trace_workqueue_execute_start(work);
          f(work);
          /*
           * While we must be careful to not use "work" after this, the trace
           * point will only record its address.
           */
          trace_workqueue_execute_end(work);
          lock_map_release(&lockdep_map);
          lock_map_release(&cwq->wq->lockdep_map);
  
          if (unlikely(in_atomic() || lockdep_depth(current) > 0)) {
                  pr_err("BUG: workqueue leaked lock or atomic: %s/0x%08x/%d\n"
                         "     last function: %pf\n",
                         current->comm, preempt_count(), task_pid_nr(current), f);
                  debug_show_held_locks(current);
                  dump_stack();
          }
  
          spin_lock_irq(&gcwq->lock);
  
          /* clear cpu intensive status */
          if (unlikely(cpu_intensive))
                  worker_clr_flags(worker, WORKER_CPU_INTENSIVE);
  
          /* we're done with it, release */
          hlist_del_init(&worker->hentry);
          worker->current_work = NULL;
          worker->current_cwq = NULL;
          cwq_dec_nr_in_flight(cwq, work_color);
  }
  
  /**
   * process_scheduled_works - process scheduled works
   * @worker: self
   *
   * Process all scheduled works.  Please note that the scheduled list
   * may change while processing a work, so this function repeatedly
   * fetches a work from the top and executes it.
   *
   * CONTEXT:
   * spin_lock_irq(gcwq->lock) which may be released and regrabbed
   * multiple times.
   */
  static void process_scheduled_works(struct worker *worker)
  {
          while (!list_empty(&worker->scheduled)) {
                  struct work_struct *work = list_first_entry(&worker->scheduled,
                                                  struct work_struct, entry);
                  process_one_work(worker, work);
          }
  }
  
  /**
   * worker_thread - the worker thread function
   * @__worker: self
   *
   * The gcwq worker thread function.  There's a single dynamic pool of
   * these per each cpu.  These workers process all works regardless of
   * their specific target workqueue.  The only exception is works which
   * belong to workqueues with a rescuer which will be explained in
   * rescuer_thread().
   */
  static int worker_thread(void *__worker)
  {
          struct worker *worker = __worker;
          struct worker_pool *pool = worker->pool;
          struct global_cwq *gcwq = pool->gcwq;
  
          /* tell the scheduler that this is a workqueue worker */
          worker->task->flags |= PF_WQ_WORKER;
  woke_up:
          spin_lock_irq(&gcwq->lock);
  
          /* we are off idle list if destruction or rebind is requested */
          if (unlikely(list_empty(&worker->entry))) {
                  spin_unlock_irq(&gcwq->lock);
  
                  /* if DIE is set, destruction is requested */
                  if (worker->flags & WORKER_DIE) {
                          worker->task->flags &= ~PF_WQ_WORKER;
                          return 0;
                  }
  
                  /* otherwise, rebind */
                  idle_worker_rebind(worker);
                  goto woke_up;
          }
  
          worker_leave_idle(worker);
  recheck:
          /* no more worker necessary? */
          if (!need_more_worker(pool))
                  goto sleep;
  
          /* do we need to manage? */
          if (unlikely(!may_start_working(pool)) && manage_workers(worker))
                  goto recheck;
  
          /*
           * ->scheduled list can only be filled while a worker is
           * preparing to process a work or actually processing it.
           * Make sure nobody diddled with it while I was sleeping.
           */
          BUG_ON(!list_empty(&worker->scheduled));
  
          /*
           * When control reaches this point, we're guaranteed to have
           * at least one idle worker or that someone else has already
           * assumed the manager role.
           */
          worker_clr_flags(worker, WORKER_PREP);
  
          do {
                  struct work_struct *work =
                          list_first_entry(&pool->worklist,
                                           struct work_struct, entry);
  
                  if (likely(!(*work_data_bits(work) & WORK_STRUCT_LINKED))) {
                          /* optimization path, not strictly necessary */
                          process_one_work(worker, work);
                          if (unlikely(!list_empty(&worker->scheduled)))
                                  process_scheduled_works(worker);
                  } else {
                          move_linked_works(work, &worker->scheduled, NULL);
                          process_scheduled_works(worker);
                  }
          } while (keep_working(pool));
  
          worker_set_flags(worker, WORKER_PREP, false);
  sleep:
          if (unlikely(need_to_manage_workers(pool)) && manage_workers(worker))
                  goto recheck;
  
          /*
           * gcwq->lock is held and there's no work to process and no
           * need to manage, sleep.  Workers are woken up only while
           * holding gcwq->lock or from local cpu, so setting the
           * current state before releasing gcwq->lock is enough to
           * prevent losing any event.
           */
          worker_enter_idle(worker);
          __set_current_state(TASK_INTERRUPTIBLE);
          spin_unlock_irq(&gcwq->lock);
          schedule();
          goto woke_up;
  }
  
  /**
   * rescuer_thread - the rescuer thread function
   * @__wq: the associated workqueue
   *
   * Workqueue rescuer thread function.  There's one rescuer for each
   * workqueue which has WQ_RESCUER set.
   *
   * Regular work processing on a gcwq may block trying to create a new
   * worker which uses GFP_KERNEL allocation which has slight chance of
   * developing into deadlock if some works currently on the same queue
   * need to be processed to satisfy the GFP_KERNEL allocation.  This is
   * the problem rescuer solves.
   *
   * When such condition is possible, the gcwq summons rescuers of all
   * workqueues which have works queued on the gcwq and let them process
   * those works so that forward progress can be guaranteed.
   *
   * This should happen rarely.
   */
  static int rescuer_thread(void *__wq)
  {
          struct workqueue_struct *wq = __wq;
          struct worker *rescuer = wq->rescuer;
          struct list_head *scheduled = &rescuer->scheduled;
          bool is_unbound = wq->flags & WQ_UNBOUND;
          unsigned int cpu;
  
          set_user_nice(current, RESCUER_NICE_LEVEL);
  repeat:
          set_current_state(TASK_INTERRUPTIBLE);
  
          if (kthread_should_stop()) {
                  __set_current_state(TASK_RUNNING);
                  return 0;
          }
  
          /*
           * See whether any cpu is asking for help.  Unbounded
           * workqueues use cpu 0 in mayday_mask for CPU_UNBOUND.
           */
          for_each_mayday_cpu(cpu, wq->mayday_mask) {
                  unsigned int tcpu = is_unbound ? WORK_CPU_UNBOUND : cpu;
                  struct cpu_workqueue_struct *cwq = get_cwq(tcpu, wq);
                  struct worker_pool *pool = cwq->pool;
                  struct global_cwq *gcwq = pool->gcwq;
                  struct work_struct *work, *n;
  
                  __set_current_state(TASK_RUNNING);
                  mayday_clear_cpu(cpu, wq->mayday_mask);
  
                  /* migrate to the target cpu if possible */
                  rescuer->pool = pool;
                  worker_maybe_bind_and_lock(rescuer);
  
                  /*
                   * Slurp in all works issued via this workqueue and
                   * process'em.
                   */
                  BUG_ON(!list_empty(&rescuer->scheduled));
                  list_for_each_entry_safe(work, n, &pool->worklist, entry)
                          if (get_work_cwq(work) == cwq)
                                  move_linked_works(work, scheduled, &n);
  
                  process_scheduled_works(rescuer);
  
                  /*
                   * Leave this gcwq.  If keep_working() is %true, notify a
                   * regular worker; otherwise, we end up with 0 concurrency
                   * and stalling the execution.
                   */
                  if (keep_working(pool))
                          wake_up_worker(pool);
  
                  spin_unlock_irq(&gcwq->lock);
          }
  
          schedule();
          goto repeat;
  }
  
  struct wq_barrier {
          struct work_struct      work;
          struct completion       done;
  };
  
  static void wq_barrier_func(struct work_struct *work)
  {
          struct wq_barrier *barr = container_of(work, struct wq_barrier, work);
          complete(&barr->done);
  }
  
  /**
   * insert_wq_barrier - insert a barrier work
   * @cwq: cwq to insert barrier into
   * @barr: wq_barrier to insert
   * @target: target work to attach @barr to
   * @worker: worker currently executing @target, NULL if @target is not executing
   *
   * @barr is linked to @target such that @barr is completed only after
   * @target finishes execution.  Please note that the ordering
   * guarantee is observed only with respect to @target and on the local
   * cpu.
   *
   * Currently, a queued barrier can't be canceled.  This is because
   * try_to_grab_pending() can't determine whether the work to be
   * grabbed is at the head of the queue and thus can't clear LINKED
   * flag of the previous work while there must be a valid next work
   * after a work with LINKED flag set.
   *
   * Note that when @worker is non-NULL, @target may be modified
   * underneath us, so we can't reliably determine cwq from @target.
   *
   * CONTEXT:
   * spin_lock_irq(gcwq->lock).
   */
  static void insert_wq_barrier(struct cpu_workqueue_struct *cwq,
                                struct wq_barrier *barr,
                                struct work_struct *target, struct worker *worker)
  {
          struct list_head *head;
          unsigned int linked = 0;
  
          /*
           * debugobject calls are safe here even with gcwq->lock locked
           * as we know for sure that this will not trigger any of the
           * checks and call back into the fixup functions where we
           * might deadlock.
           */
          INIT_WORK_ONSTACK(&barr->work, wq_barrier_func);
          __set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&barr->work));
          init_completion(&barr->done);
  
          /*
           * If @target is currently being executed, schedule the
           * barrier to the worker; otherwise, put it after @target.
           */
          if (worker)
                  head = worker->scheduled.next;
          else {
                  unsigned long *bits = work_data_bits(target);
  
                  head = target->entry.next;
                  /* there can already be other linked works, inherit and set */
                  linked = *bits & WORK_STRUCT_LINKED;
                  __set_bit(WORK_STRUCT_LINKED_BIT, bits);
          }
  
          debug_work_activate(&barr->work);
          insert_work(cwq, &barr->work, head,
                      work_color_to_flags(WORK_NO_COLOR) | linked);
  }
  
  /**
   * flush_workqueue_prep_cwqs - prepare cwqs for workqueue flushing
   * @wq: workqueue being flushed
   * @flush_color: new flush color, < 0 for no-op
   * @work_color: new work color, < 0 for no-op
   *
   * Prepare cwqs for workqueue flushing.
   *
   * If @flush_color is non-negative, flush_color on all cwqs should be
   * -1.  If no cwq has in-flight commands at the specified color, all
   * cwq->flush_color's stay at -1 and %false is returned.  If any cwq
   * has in flight commands, its cwq->flush_color is set to
   * @flush_color, @wq->nr_cwqs_to_flush is updated accordingly, cwq
   * wakeup logic is armed and %true is returned.
   *
   * The caller should have initialized @wq->first_flusher prior to
   * calling this function with non-negative @flush_color.  If
   * @flush_color is negative, no flush color update is done and %false
   * is returned.
   *
   * If @work_color is non-negative, all cwqs should have the same
   * work_color which is previous to @work_color and all will be
   * advanced to @work_color.
   *
   * CONTEXT:
   * mutex_lock(wq->flush_mutex).
   *
   * RETURNS:
   * %true if @flush_color >= 0 and there's something to flush.  %false
   * otherwise.
   */
  static bool flush_workqueue_prep_cwqs(struct workqueue_struct *wq,
                                        int flush_color, int work_color)
  {
          bool wait = false;
          unsigned int cpu;
  
          if (flush_color >= 0) {
                  BUG_ON(atomic_read(&wq->nr_cwqs_to_flush));
                  atomic_set(&wq->nr_cwqs_to_flush, 1);
          }
  
          for_each_cwq_cpu(cpu, wq) {
                  struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
                  struct global_cwq *gcwq = cwq->pool->gcwq;
  
                  spin_lock_irq(&gcwq->lock);
  
                  if (flush_color >= 0) {
                          BUG_ON(cwq->flush_color != -1);
  
                          if (cwq->nr_in_flight[flush_color]) {
                                  cwq->flush_color = flush_color;
                                  atomic_inc(&wq->nr_cwqs_to_flush);
                                  wait = true;
                          }
                  }
  
                  if (work_color >= 0) {
                          BUG_ON(work_color != work_next_color(cwq->work_color));
                          cwq->work_color = work_color;
                  }
  
                  spin_unlock_irq(&gcwq->lock);
          }
  
          if (flush_color >= 0 && atomic_dec_and_test(&wq->nr_cwqs_to_flush))
                  complete(&wq->first_flusher->done);
  
          return wait;
  }
  
  /**
   * flush_workqueue - ensure that any scheduled work has run to completion.
   * @wq: workqueue to flush
   *
   * Forces execution of the workqueue and blocks until its completion.
   * This is typically used in driver shutdown handlers.
   *
   * We sleep until all works which were queued on entry have been handled,
   * but we are not livelocked by new incoming ones.
   */
  void flush_workqueue(struct workqueue_struct *wq)
  {
          struct wq_flusher this_flusher = {
                  .list = LIST_HEAD_INIT(this_flusher.list),
                  .flush_color = -1,
                  .done = COMPLETION_INITIALIZER_ONSTACK(this_flusher.done),
          };
          int next_color;
  
          lock_map_acquire(&wq->lockdep_map);
          lock_map_release(&wq->lockdep_map);
  
          mutex_lock(&wq->flush_mutex);
  
          /*
           * Start-to-wait phase
           */
          next_color = work_next_color(wq->work_color);
  
          if (next_color != wq->flush_color) {
                  /*
                   * Color space is not full.  The current work_color
                   * becomes our flush_color and work_color is advanced
                   * by one.
                   */
                  BUG_ON(!list_empty(&wq->flusher_overflow));
                  this_flusher.flush_color = wq->work_color;
                  wq->work_color = next_color;
  
                  if (!wq->first_flusher) {
                          /* no flush in progress, become the first flusher */
                          BUG_ON(wq->flush_color != this_flusher.flush_color);
  
                          wq->first_flusher = &this_flusher;
  
                          if (!flush_workqueue_prep_cwqs(wq, wq->flush_color,
                                                         wq->work_color)) {
                                  /* nothing to flush, done */
                                  wq->flush_color = next_color;
                                  wq->first_flusher = NULL;
                                  goto out_unlock;
                          }
                  } else {
                          /* wait in queue */
                          BUG_ON(wq->flush_color == this_flusher.flush_color);
                          list_add_tail(&this_flusher.list, &wq->flusher_queue);
                          flush_workqueue_prep_cwqs(wq, -1, wq->work_color);
                  }
          } else {
                  /*
                   * Oops, color space is full, wait on overflow queue.
                   * The next flush completion will assign us
                   * flush_color and transfer to flusher_queue.
                   */
                  list_add_tail(&this_flusher.list, &wq->flusher_overflow);
          }
  
          mutex_unlock(&wq->flush_mutex);
  
          wait_for_completion(&this_flusher.done);
  
          /*
           * Wake-up-and-cascade phase
           *
           * First flushers are responsible for cascading flushes and
           * handling overflow.  Non-first flushers can simply return.
           */
          if (wq->first_flusher != &this_flusher)
                  return;
  
          mutex_lock(&wq->flush_mutex);
  
          /* we might have raced, check again with mutex held */
          if (wq->first_flusher != &this_flusher)
                  goto out_unlock;
  
          wq->first_flusher = NULL;
  
          BUG_ON(!list_empty(&this_flusher.list));
          BUG_ON(wq->flush_color != this_flusher.flush_color);
  
          while (true) {
                  struct wq_flusher *next, *tmp;
  
                  /* complete all the flushers sharing the current flush color */
                  list_for_each_entry_safe(next, tmp, &wq->flusher_queue, list) {
                          if (next->flush_color != wq->flush_color)
                                  break;
                          list_del_init(&next->list);
                          complete(&next->done);
                  }
  
                  BUG_ON(!list_empty(&wq->flusher_overflow) &&
                         wq->flush_color != work_next_color(wq->work_color));
  
                  /* this flush_color is finished, advance by one */
                  wq->flush_color = work_next_color(wq->flush_color);
  
                  /* one color has been freed, handle overflow queue */
                  if (!list_empty(&wq->flusher_overflow)) {
                          /*
                           * Assign the same color to all overflowed
                           * flushers, advance work_color and append to
                           * flusher_queue.  This is the start-to-wait
                           * phase for these overflowed flushers.
                           */
                          list_for_each_entry(tmp, &wq->flusher_overflow, list)
                                  tmp->flush_color = wq->work_color;
  
                          wq->work_color = work_next_color(wq->work_color);
  
                          list_splice_tail_init(&wq->flusher_overflow,
                                                &wq->flusher_queue);
                          flush_workqueue_prep_cwqs(wq, -1, wq->work_color);
                  }
  
                  if (list_empty(&wq->flusher_queue)) {
                          BUG_ON(wq->flush_color != wq->work_color);
                          break;
                  }
  
                  /*
                   * Need to flush more colors.  Make the next flusher
                   * the new first flusher and arm cwqs.
                   */
                  BUG_ON(wq->flush_color == wq->work_color);
                  BUG_ON(wq->flush_color != next->flush_color);
  
                  list_del_init(&next->list);
                  wq->first_flusher = next;
  
                  if (flush_workqueue_prep_cwqs(wq, wq->flush_color, -1))
                          break;
  
                  /*
                   * Meh... this color is already done, clear first
                   * flusher and repeat cascading.
                   */
                  wq->first_flusher = NULL;
          }
  
  out_unlock:
          mutex_unlock(&wq->flush_mutex);
  }
  EXPORT_SYMBOL_GPL(flush_workqueue);
  
  /**
   * drain_workqueue - drain a workqueue
   * @wq: workqueue to drain
   *
   * Wait until the workqueue becomes empty.  While draining is in progress,
   * only chain queueing is allowed.  IOW, only currently pending or running
   * work items on @wq can queue further work items on it.  @wq is flushed
   * repeatedly until it becomes empty.  The number of flushing is detemined
   * by the depth of chaining and should be relatively short.  Whine if it
   * takes too long.
   */
  void drain_workqueue(struct workqueue_struct *wq)
  {
          unsigned int flush_cnt = 0;
          unsigned int cpu;
  
          /*
           * __queue_work() needs to test whether there are drainers, is much
           * hotter than drain_workqueue() and already looks at @wq->flags.
           * Use WQ_DRAINING so that queue doesn't have to check nr_drainers.
           */
          spin_lock(&workqueue_lock);
          if (!wq->nr_drainers++)
                  wq->flags |= WQ_DRAINING;
          spin_unlock(&workqueue_lock);
  reflush:
          flush_workqueue(wq);
  
          for_each_cwq_cpu(cpu, wq) {
                  struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
                  bool drained;
  
                  spin_lock_irq(&cwq->pool->gcwq->lock);
                  drained = !cwq->nr_active && list_empty(&cwq->delayed_works);
                  spin_unlock_irq(&cwq->pool->gcwq->lock);
  
                  if (drained)
                          continue;
  
                  if (++flush_cnt == 10 ||
                      (flush_cnt % 100 == 0 && flush_cnt <= 1000))
                          pr_warn("workqueue %s: flush on destruction isn't complete after %u tries\n",
                                  wq->name, flush_cnt);
                  goto reflush;
          }
  
          spin_lock(&workqueue_lock);
          if (!--wq->nr_drainers)
                  wq->flags &= ~WQ_DRAINING;
          spin_unlock(&workqueue_lock);
  }
  EXPORT_SYMBOL_GPL(drain_workqueue);
  
  static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr)
  {
          struct worker *worker = NULL;
          struct global_cwq *gcwq;
          struct cpu_workqueue_struct *cwq;
  
          might_sleep();
          gcwq = get_work_gcwq(work);
          if (!gcwq)
                  return false;
  
          spin_lock_irq(&gcwq->lock);
          if (!list_empty(&work->entry)) {
                  /*
                   * See the comment near try_to_grab_pending()->smp_rmb().
                   * If it was re-queued to a different gcwq under us, we
                   * are not going to wait.
                   */
                  smp_rmb();
                  cwq = get_work_cwq(work);
                  if (unlikely(!cwq || gcwq != cwq->pool->gcwq))
                          goto already_gone;
          } else {
                  worker = find_worker_executing_work(gcwq, work);
                  if (!worker)
                          goto already_gone;
                  cwq = worker->current_cwq;
          }
  
          insert_wq_barrier(cwq, barr, work, worker);
          spin_unlock_irq(&gcwq->lock);
  
          /*
           * If @max_active is 1 or rescuer is in use, flushing another work
           * item on the same workqueue may lead to deadlock.  Make sure the
           * flusher is not running on the same workqueue by verifying write
           * access.
           */
          if (cwq->wq->saved_max_active == 1 || cwq->wq->flags & WQ_RESCUER)
                  lock_map_acquire(&cwq->wq->lockdep_map);
          else
                  lock_map_acquire_read(&cwq->wq->lockdep_map);
          lock_map_release(&cwq->wq->lockdep_map);
  
          return true;
  already_gone:
          spin_unlock_irq(&gcwq->lock);
          return false;
  }
  
  /**
   * flush_work - wait for a work to finish executing the last queueing instance
   * @work: the work to flush
   *
   * Wait until @work has finished execution.  @work is guaranteed to be idle
   * on return if it hasn't been requeued since flush started.
   *
   * RETURNS:
   * %true if flush_work() waited for the work to finish execution,
   * %false if it was already idle.
   */
  bool flush_work(struct work_struct *work)
  {
          struct wq_barrier barr;
  
          lock_map_acquire(&work->lockdep_map);
          lock_map_release(&work->lockdep_map);
  
          if (start_flush_work(work, &barr)) {
                  wait_for_completion(&barr.done);
                  destroy_work_on_stack(&barr.work);
                  return true;
          } else {
                  return false;
          }
  }
  EXPORT_SYMBOL_GPL(flush_work);
  
  static bool __cancel_work_timer(struct work_struct *work, bool is_dwork)
  {
          unsigned long flags;
          int ret;
  
          do {
                  ret = try_to_grab_pending(work, is_dwork, &flags);
                  /*
                   * If someone else is canceling, wait for the same event it
                   * would be waiting for before retrying.
                   */
                  if (unlikely(ret == -ENOENT))
                          flush_work(work);
          } while (unlikely(ret < 0));
  
          /* tell other tasks trying to grab @work to back off */
          mark_work_canceling(work);
          local_irq_restore(flags);
  
          flush_work(work);
          clear_work_data(work);
          return ret;
  }
  
  /**
   * cancel_work_sync - cancel a work and wait for it to finish
   * @work: the work to cancel
   *
   * Cancel @work and wait for its execution to finish.  This function
   * can be used even if the work re-queues itself or migrates to
   * another workqueue.  On return from this function, @work is
   * guaranteed to be not pending or executing on any CPU.
   *
   * cancel_work_sync(&delayed_work->work) must not be used for
   * delayed_work's.  Use cancel_delayed_work_sync() instead.
   *
   * The caller must ensure that the workqueue on which @work was last
   * queued can't be destroyed before this function returns.
   *
   * RETURNS:
   * %true if @work was pending, %false otherwise.
   */
  bool cancel_work_sync(struct work_struct *work)
  {
          return __cancel_work_timer(work, false);
  }
  EXPORT_SYMBOL_GPL(cancel_work_sync);
  
  /**
   * flush_delayed_work - wait for a dwork to finish executing the last queueing
   * @dwork: the delayed work to flush
   *
   * Delayed timer is cancelled and the pending work is queued for
   * immediate execution.  Like flush_work(), this function only
   * considers the last queueing instance of @dwork.
   *
   * RETURNS:
   * %true if flush_work() waited for the work to finish execution,
   * %false if it was already idle.
   */
  bool flush_delayed_work(struct delayed_work *dwork)
  {
          local_irq_disable();
          if (del_timer_sync(&dwork->timer))
                  __queue_work(dwork->cpu,
                               get_work_cwq(&dwork->work)->wq, &dwork->work);
          local_irq_enable();
          return flush_work(&dwork->work);
  }
  EXPORT_SYMBOL(flush_delayed_work);
  
  /**
   * cancel_delayed_work - cancel a delayed work
   * @dwork: delayed_work to cancel
   *
   * Kill off a pending delayed_work.  Returns %true if @dwork was pending
   * and canceled; %false if wasn't pending.  Note that the work callback
   * function may still be running on return, unless it returns %true and the
   * work doesn't re-arm itself.  Explicitly flush or use
   * cancel_delayed_work_sync() to wait on it.
   *
   * This function is safe to call from any context including IRQ handler.
   */
  bool cancel_delayed_work(struct delayed_work *dwork)
  {
          unsigned long flags;
          int ret;
  
          do {
                  ret = try_to_grab_pending(&dwork->work, true, &flags);
          } while (unlikely(ret == -EAGAIN));
  
          if (unlikely(ret < 0))
                  return false;
  
          set_work_cpu_and_clear_pending(&dwork->work, work_cpu(&dwork->work));
          local_irq_restore(flags);
          return ret;
  }
  EXPORT_SYMBOL(cancel_delayed_work);
  
  /**
   * cancel_delayed_work_sync - cancel a delayed work and wait for it to finish
   * @dwork: the delayed work cancel
   *
   * This is cancel_work_sync() for delayed works.
   *
   * RETURNS:
   * %true if @dwork was pending, %false otherwise.
   */
  bool cancel_delayed_work_sync(struct delayed_work *dwork)
  {
          return __cancel_work_timer(&dwork->work, true);
  }
  EXPORT_SYMBOL(cancel_delayed_work_sync);
  
  /**
   * schedule_work_on - put work task on a specific cpu
   * @cpu: cpu to put the work task on
   * @work: job to be done
   *
   * This puts a job on a specific cpu
   */
  bool schedule_work_on(int cpu, struct work_struct *work)
  {
          return queue_work_on(cpu, system_wq, work);
  }
  EXPORT_SYMBOL(schedule_work_on);
  
  /**
   * schedule_work - put work task in global workqueue
   * @work: job to be done
   *
   * Returns %false if @work was already on the kernel-global workqueue and
   * %true otherwise.
   *
   * This puts a job in the kernel-global workqueue if it was not already
   * queued and leaves it in the same position on the kernel-global
   * workqueue otherwise.
   */
  bool schedule_work(struct work_struct *work)
  {
          return queue_work(system_wq, work);
  }
  EXPORT_SYMBOL(schedule_work);
  
  /**
   * schedule_delayed_work_on - queue work in global workqueue on CPU after delay
   * @cpu: cpu to use
   * @dwork: job to be done
   * @delay: number of jiffies to wait
   *
   * After waiting for a given time this puts a job in the kernel-global
   * workqueue on the specified CPU.
   */
  bool schedule_delayed_work_on(int cpu, struct delayed_work *dwork,
                                unsigned long delay)
  {
          return queue_delayed_work_on(cpu, system_wq, dwork, delay);
  }
  EXPORT_SYMBOL(schedule_delayed_work_on);
  
  /**
   * schedule_delayed_work - put work task in global workqueue after delay
   * @dwork: job to be done
   * @delay: number of jiffies to wait or 0 for immediate execution
   *
   * After waiting for a given time this puts a job in the kernel-global
   * workqueue.
   */
  bool schedule_delayed_work(struct delayed_work *dwork, unsigned long delay)
  {
          return queue_delayed_work(system_wq, dwork, delay);
  }
  EXPORT_SYMBOL(schedule_delayed_work);
  
  /**
   * schedule_on_each_cpu - execute a function synchronously on each online CPU
   * @func: the function to call
   *
   * schedule_on_each_cpu() executes @func on each online CPU using the
   * system workqueue and blocks until all CPUs have completed.
   * schedule_on_each_cpu() is very slow.
   *
   * RETURNS:
   * 0 on success, -errno on failure.
   */
  int schedule_on_each_cpu(work_func_t func)
  {
          int cpu;
          struct work_struct __percpu *works;
  
          works = alloc_percpu(struct work_struct);
          if (!works)
                  return -ENOMEM;
  
          get_online_cpus();
  
          for_each_online_cpu(cpu) {
                  struct work_struct *work = per_cpu_ptr(works, cpu);
  
                  INIT_WORK(work, func);
                  schedule_work_on(cpu, work);
          }
  
          for_each_online_cpu(cpu)
                  flush_work(per_cpu_ptr(works, cpu));
  
          put_online_cpus();
          free_percpu(works);
          return 0;
  }
  
  /**
   * flush_scheduled_work - ensure that any scheduled work has run to completion.
   *
   * Forces execution of the kernel-global workqueue and blocks until its
   * completion.
   *
   * Think twice before calling this function!  It's very easy to get into
   * trouble if you don't take great care.  Either of the following situations
   * will lead to deadlock:
   *
   *      One of the work items currently on the workqueue needs to acquire
   *      a lock held by your code or its caller.
   *
   *      Your code is running in the context of a work routine.
   *
   * They will be detected by lockdep when they occur, but the first might not
   * occur very often.  It depends on what work items are on the workqueue and
   * what locks they need, which you have no control over.
   *
   * In most situations flushing the entire workqueue is overkill; you merely
   * need to know that a particular work item isn't queued and isn't running.
   * In such cases you should use cancel_delayed_work_sync() or
   * cancel_work_sync() instead.
   */
  void flush_scheduled_work(void)
  {
          flush_workqueue(system_wq);
  }
  EXPORT_SYMBOL(flush_scheduled_work);
  
  /**
   * execute_in_process_context - reliably execute the routine with user context
   * @fn:         the function to execute
   * @ew:         guaranteed storage for the execute work structure (must
   *              be available when the work executes)
   *
   * Executes the function immediately if process context is available,
   * otherwise schedules the function for delayed execution.
   *
   * Returns:     0 - function was executed
   *              1 - function was scheduled for execution
   */
  int execute_in_process_context(work_func_t fn, struct execute_work *ew)
  {
          if (!in_interrupt()) {
                  fn(&ew->work);
                  return 0;
          }
  
          INIT_WORK(&ew->work, fn);
          schedule_work(&ew->work);
  
          return 1;
  }
  EXPORT_SYMBOL_GPL(execute_in_process_context);
  
  int keventd_up(void)
  {
          return system_wq != NULL;
  }
  
  static int alloc_cwqs(struct workqueue_struct *wq)
  {
          /*
           * cwqs are forced aligned according to WORK_STRUCT_FLAG_BITS.
           * Make sure that the alignment isn't lower than that of
           * unsigned long long.
           */
          const size_t size = sizeof(struct cpu_workqueue_struct);
          const size_t align = max_t(size_t, 1 << WORK_STRUCT_FLAG_BITS,
                                     __alignof__(unsigned long long));
  
          if (!(wq->flags & WQ_UNBOUND))
                  wq->cpu_wq.pcpu = __alloc_percpu(size, align);
          else {
                  void *ptr;
  
                  /*
                   * Allocate enough room to align cwq and put an extra
                   * pointer at the end pointing back to the originally
                   * allocated pointer which will be used for free.
                   */
                  ptr = kzalloc(size + align + sizeof(void *), GFP_KERNEL);
                  if (ptr) {
                          wq->cpu_wq.single = PTR_ALIGN(ptr, align);
                          *(void **)(wq->cpu_wq.single + 1) = ptr;
                  }
          }
  
          /* just in case, make sure it's actually aligned */
          BUG_ON(!IS_ALIGNED(wq->cpu_wq.v, align));
          return wq->cpu_wq.v ? 0 : -ENOMEM;
  }
  
  static void free_cwqs(struct workqueue_struct *wq)
  {
          if (!(wq->flags & WQ_UNBOUND))
                  free_percpu(wq->cpu_wq.pcpu);
          else if (wq->cpu_wq.single) {
                  /* the pointer to free is stored right after the cwq */
                  kfree(*(void **)(wq->cpu_wq.single + 1));
          }
  }
  
  static int wq_clamp_max_active(int max_active, unsigned int flags,
                                 const char *name)
  {
          int lim = flags & WQ_UNBOUND ? WQ_UNBOUND_MAX_ACTIVE : WQ_MAX_ACTIVE;
  
          if (max_active < 1 || max_active > lim)
                  pr_warn("workqueue: max_active %d requested for %s is out of range, clamping between %d and %d\n",
                          max_active, name, 1, lim);
  
          return clamp_val(max_active, 1, lim);
  }
  
  struct workqueue_struct *__alloc_workqueue_key(const char *fmt,
                                                 unsigned int flags,
                                                 int max_active,
                                                 struct lock_class_key *key,
                                                 const char *lock_name, ...)
  {
          va_list args, args1;
          struct workqueue_struct *wq;
          unsigned int cpu;
          size_t namelen;
  
          /* determine namelen, allocate wq and format name */
          va_start(args, lock_name);
          va_copy(args1, args);
          namelen = vsnprintf(NULL, 0, fmt, args) + 1;
  
          wq = kzalloc(sizeof(*wq) + namelen, GFP_KERNEL);
          if (!wq)
                  goto err;
  
          vsnprintf(wq->name, namelen, fmt, args1);
          va_end(args);
          va_end(args1);
  
          /*
           * Workqueues which may be used during memory reclaim should
           * have a rescuer to guarantee forward progress.
           */
          if (flags & WQ_MEM_RECLAIM)
                  flags |= WQ_RESCUER;
  
          max_active = max_active ?: WQ_DFL_ACTIVE;
          max_active = wq_clamp_max_active(max_active, flags, wq->name);
  
          /* init wq */
          wq->flags = flags;
          wq->saved_max_active = max_active;
          mutex_init(&wq->flush_mutex);
          atomic_set(&wq->nr_cwqs_to_flush, 0);
          INIT_LIST_HEAD(&wq->flusher_queue);
          INIT_LIST_HEAD(&wq->flusher_overflow);
  
          lockdep_init_map(&wq->lockdep_map, lock_name, key, 0);
          INIT_LIST_HEAD(&wq->list);
  
          if (alloc_cwqs(wq) < 0)
                  goto err;
  
          for_each_cwq_cpu(cpu, wq) {
                  struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
                  struct global_cwq *gcwq = get_gcwq(cpu);
                  int pool_idx = (bool)(flags & WQ_HIGHPRI);
  
                  BUG_ON((unsigned long)cwq & WORK_STRUCT_FLAG_MASK);
                  cwq->pool = &gcwq->pools[pool_idx];
                  cwq->wq = wq;
                  cwq->flush_color = -1;
                  cwq->max_active = max_active;
                  INIT_LIST_HEAD(&cwq->delayed_works);
          }
  
          if (flags & WQ_RESCUER) {
                  struct worker *rescuer;
  
                  if (!alloc_mayday_mask(&wq->mayday_mask, GFP_KERNEL))
                          goto err;
  
                  wq->rescuer = rescuer = alloc_worker();
                  if (!rescuer)
                          goto err;
  
                  rescuer->task = kthread_create(rescuer_thread, wq, "%s",
                                                 wq->name);
                  if (IS_ERR(rescuer->task))
                          goto err;
  
                  rescuer->task->flags |= PF_THREAD_BOUND;
                  wake_up_process(rescuer->task);
          }
  
          /*
           * workqueue_lock protects global freeze state and workqueues
           * list.  Grab it, set max_active accordingly and add the new
           * workqueue to workqueues list.
           */
          spin_lock(&workqueue_lock);
  
          if (workqueue_freezing && wq->flags & WQ_FREEZABLE)
                  for_each_cwq_cpu(cpu, wq)
                          get_cwq(cpu, wq)->max_active = 0;
  
          list_add(&wq->list, &workqueues);
  
          spin_unlock(&workqueue_lock);
  
          return wq;
  err:
          if (wq) {
                  free_cwqs(wq);
                  free_mayday_mask(wq->mayday_mask);
                  kfree(wq->rescuer);
                  kfree(wq);
          }
          return NULL;
  }
  EXPORT_SYMBOL_GPL(__alloc_workqueue_key);
  
  /**
   * destroy_workqueue - safely terminate a workqueue
   * @wq: target workqueue
   *
   * Safely destroy a workqueue. All work currently pending will be done first.
   */
  void destroy_workqueue(struct workqueue_struct *wq)
  {
          unsigned int cpu;
  
          /* drain it before proceeding with destruction */
          drain_workqueue(wq);
  
          /*
           * wq list is used to freeze wq, remove from list after
           * flushing is complete in case freeze races us.
           */
          spin_lock(&workqueue_lock);
          list_del(&wq->list);
          spin_unlock(&workqueue_lock);
  
          /* sanity check */
          for_each_cwq_cpu(cpu, wq) {
                  struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
                  int i;
  
                  for (i = 0; i < WORK_NR_COLORS; i++)
                          BUG_ON(cwq->nr_in_flight[i]);
                  BUG_ON(cwq->nr_active);
                  BUG_ON(!list_empty(&cwq->delayed_works));
          }
  
          if (wq->flags & WQ_RESCUER) {
                  kthread_stop(wq->rescuer->task);
                  free_mayday_mask(wq->mayday_mask);
                  kfree(wq->rescuer);
          }
  
          free_cwqs(wq);
          kfree(wq);
  }
  EXPORT_SYMBOL_GPL(destroy_workqueue);
  
  /**
   * cwq_set_max_active - adjust max_active of a cwq
   * @cwq: target cpu_workqueue_struct
   * @max_active: new max_active value.
   *
   * Set @cwq->max_active to @max_active and activate delayed works if
   * increased.
   *
   * CONTEXT:
   * spin_lock_irq(gcwq->lock).
   */
  static void cwq_set_max_active(struct cpu_workqueue_struct *cwq, int max_active)
  {
          cwq->max_active = max_active;
  
          while (!list_empty(&cwq->delayed_works) &&
                 cwq->nr_active < cwq->max_active)
                  cwq_activate_first_delayed(cwq);
  }
  
  /**
   * workqueue_set_max_active - adjust max_active of a workqueue
   * @wq: target workqueue
   * @max_active: new max_active value.
   *
   * Set max_active of @wq to @max_active.
   *
   * CONTEXT:
   * Don't call from IRQ context.
   */
  void workqueue_set_max_active(struct workqueue_struct *wq, int max_active)
  {
          unsigned int cpu;
  
          max_active = wq_clamp_max_active(max_active, wq->flags, wq->name);
  
          spin_lock(&workqueue_lock);
  
          wq->saved_max_active = max_active;
  
          for_each_cwq_cpu(cpu, wq) {
                  struct global_cwq *gcwq = get_gcwq(cpu);
  
                  spin_lock_irq(&gcwq->lock);
  
                  if (!(wq->flags & WQ_FREEZABLE) ||
                      !(gcwq->flags & GCWQ_FREEZING))
                          cwq_set_max_active(get_cwq(gcwq->cpu, wq), max_active);
  
                  spin_unlock_irq(&gcwq->lock);
          }
  
          spin_unlock(&workqueue_lock);
  }
  EXPORT_SYMBOL_GPL(workqueue_set_max_active);
  
  /**
   * workqueue_congested - test whether a workqueue is congested
   * @cpu: CPU in question
   * @wq: target workqueue
   *
   * Test whether @wq's cpu workqueue for @cpu is congested.  There is
   * no synchronization around this function and the test result is
   * unreliable and only useful as advisory hints or for debugging.
   *
   * RETURNS:
   * %true if congested, %false otherwise.
   */
  bool workqueue_congested(unsigned int cpu, struct workqueue_struct *wq)
  {
          struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
  
          return !list_empty(&cwq->delayed_works);
  }
  EXPORT_SYMBOL_GPL(workqueue_congested);
  
  /**
   * work_cpu - return the last known associated cpu for @work
   * @work: the work of interest
   *
   * RETURNS:
   * CPU number if @work was ever queued.  WORK_CPU_NONE otherwise.
   */
  unsigned int work_cpu(struct work_struct *work)
  {
          struct global_cwq *gcwq = get_work_gcwq(work);
  
          return gcwq ? gcwq->cpu : WORK_CPU_NONE;
  }
  EXPORT_SYMBOL_GPL(work_cpu);
  
  /**
   * work_busy - test whether a work is currently pending or running
   * @work: the work to be tested
   *
   * Test whether @work is currently pending or running.  There is no
   * synchronization around this function and the test result is
   * unreliable and only useful as advisory hints or for debugging.
   * Especially for reentrant wqs, the pending state might hide the
   * running state.
   *
   * RETURNS:
   * OR'd bitmask of WORK_BUSY_* bits.
   */
  unsigned int work_busy(struct work_struct *work)
  {
          struct global_cwq *gcwq = get_work_gcwq(work);
          unsigned long flags;
          unsigned int ret = 0;
  
          if (!gcwq)
                  return 0;
  
          spin_lock_irqsave(&gcwq->lock, flags);
  
          if (work_pending(work))
                  ret |= WORK_BUSY_PENDING;
          if (find_worker_executing_work(gcwq, work))
                  ret |= WORK_BUSY_RUNNING;
  
          spin_unlock_irqrestore(&gcwq->lock, flags);
  
          return ret;
  }
  EXPORT_SYMBOL_GPL(work_busy);
  
  /*
   * CPU hotplug.
   *
   * There are two challenges in supporting CPU hotplug.  Firstly, there
   * are a lot of assumptions on strong associations among work, cwq and
   * gcwq which make migrating pending and scheduled works very
   * difficult to implement without impacting hot paths.  Secondly,
   * gcwqs serve mix of short, long and very long running works making
   * blocked draining impractical.
   *
   * This is solved by allowing a gcwq to be disassociated from the CPU
   * running as an unbound one and allowing it to be reattached later if the
   * cpu comes back online.
   */
  
  /* claim manager positions of all pools */
  static void gcwq_claim_assoc_and_lock(struct global_cwq *gcwq)
  {
          struct worker_pool *pool;
  
          for_each_worker_pool(pool, gcwq)
                  mutex_lock_nested(&pool->assoc_mutex, pool - gcwq->pools);
          spin_lock_irq(&gcwq->lock);
  }
  
  /* release manager positions */
  static void gcwq_release_assoc_and_unlock(struct global_cwq *gcwq)
  {
          struct worker_pool *pool;
  
          spin_unlock_irq(&gcwq->lock);
          for_each_worker_pool(pool, gcwq)
                  mutex_unlock(&pool->assoc_mutex);
  }
  
  static void gcwq_unbind_fn(struct work_struct *work)
  {
          struct global_cwq *gcwq = get_gcwq(smp_processor_id());
          struct worker_pool *pool;
          struct worker *worker;
          struct hlist_node *pos;
          int i;
  
          BUG_ON(gcwq->cpu != smp_processor_id());
  
          gcwq_claim_assoc_and_lock(gcwq);
  
          /*
           * We've claimed all manager positions.  Make all workers unbound
           * and set DISASSOCIATED.  Before this, all workers except for the
           * ones which are still executing works from before the last CPU
           * down must be on the cpu.  After this, they may become diasporas.
           */
          for_each_worker_pool(pool, gcwq)
                  list_for_each_entry(worker, &pool->idle_list, entry)
                          worker->flags |= WORKER_UNBOUND;
  
          for_each_busy_worker(worker, i, pos, gcwq)
                  worker->flags |= WORKER_UNBOUND;
  
          gcwq->flags |= GCWQ_DISASSOCIATED;
  
          gcwq_release_assoc_and_unlock(gcwq);
  
          /*
           * Call schedule() so that we cross rq->lock and thus can guarantee
           * sched callbacks see the %WORKER_UNBOUND flag.  This is necessary
           * as scheduler callbacks may be invoked from other cpus.
           */
          schedule();
  
          /*
           * Sched callbacks are disabled now.  Zap nr_running.  After this,
           * nr_running stays zero and need_more_worker() and keep_working()
           * are always true as long as the worklist is not empty.  @gcwq now
           * behaves as unbound (in terms of concurrency management) gcwq
           * which is served by workers tied to the CPU.
           *
           * On return from this function, the current worker would trigger
           * unbound chain execution of pending work items if other workers
           * didn't already.
           */
          for_each_worker_pool(pool, gcwq)
                  atomic_set(get_pool_nr_running(pool), 0);
  }
  
  /*
   * Workqueues should be brought up before normal priority CPU notifiers.
   * This will be registered high priority CPU notifier.
   */
  static int __cpuinit workqueue_cpu_up_callback(struct notifier_block *nfb,
                                                 unsigned long action,
                                                 void *hcpu)
  {
          unsigned int cpu = (unsigned long)hcpu;
          struct global_cwq *gcwq = get_gcwq(cpu);
          struct worker_pool *pool;
  
          switch (action & ~CPU_TASKS_FROZEN) {
          case CPU_UP_PREPARE:
                  for_each_worker_pool(pool, gcwq) {
                          struct worker *worker;
  
                          if (pool->nr_workers)
                                  continue;
  
                          worker = create_worker(pool);
                          if (!worker)
                                  return NOTIFY_BAD;
  
                          spin_lock_irq(&gcwq->lock);
                          start_worker(worker);
                          spin_unlock_irq(&gcwq->lock);
                  }
                  break;
  
          case CPU_DOWN_FAILED:
          case CPU_ONLINE:
                  gcwq_claim_assoc_and_lock(gcwq);
                  gcwq->flags &= ~GCWQ_DISASSOCIATED;
                  rebind_workers(gcwq);
                  gcwq_release_assoc_and_unlock(gcwq);
                  break;
          }
          return NOTIFY_OK;
  }
  
  /*
   * Workqueues should be brought down after normal priority CPU notifiers.
   * This will be registered as low priority CPU notifier.
   */
  static int __cpuinit workqueue_cpu_down_callback(struct notifier_block *nfb,
                                                   unsigned long action,
                                                   void *hcpu)
  {
          unsigned int cpu = (unsigned long)hcpu;
          struct work_struct unbind_work;
  
          switch (action & ~CPU_TASKS_FROZEN) {
          case CPU_DOWN_PREPARE:
                  /* unbinding should happen on the local CPU */
                  INIT_WORK_ONSTACK(&unbind_work, gcwq_unbind_fn);
                  queue_work_on(cpu, system_highpri_wq, &unbind_work);
                  flush_work(&unbind_work);
                  break;
          }
          return NOTIFY_OK;
  }
  
  #ifdef CONFIG_SMP
  
  struct work_for_cpu {
          struct work_struct work;
          long (*fn)(void *);
          void *arg;
          long ret;
  };
  
  static void work_for_cpu_fn(struct work_struct *work)
  {
          struct work_for_cpu *wfc = container_of(work, struct work_for_cpu, work);
  
          wfc->ret = wfc->fn(wfc->arg);
  }
  
  /**
   * work_on_cpu - run a function in user context on a particular cpu
   * @cpu: the cpu to run on
   * @fn: the function to run
   * @arg: the function arg
   *
   * This will return the value @fn returns.
   * It is up to the caller to ensure that the cpu doesn't go offline.
   * The caller must not hold any locks which would prevent @fn from completing.
   */
  long work_on_cpu(unsigned int cpu, long (*fn)(void *), void *arg)
  {
          struct work_for_cpu wfc = { .fn = fn, .arg = arg };
  
          INIT_WORK_ONSTACK(&wfc.work, work_for_cpu_fn);
          schedule_work_on(cpu, &wfc.work);
          flush_work(&wfc.work);
          return wfc.ret;
  }
  EXPORT_SYMBOL_GPL(work_on_cpu);
  #endif /* CONFIG_SMP */
  
  #ifdef CONFIG_FREEZER
  
  /**
   * freeze_workqueues_begin - begin freezing workqueues
   *
   * Start freezing workqueues.  After this function returns, all freezable
   * workqueues will queue new works to their frozen_works list instead of
   * gcwq->worklist.
   *
   * CONTEXT:
   * Grabs and releases workqueue_lock and gcwq->lock's.
   */
  void freeze_workqueues_begin(void)
  {
          unsigned int cpu;
  
          spin_lock(&workqueue_lock);
  
          BUG_ON(workqueue_freezing);
          workqueue_freezing = true;
  
          for_each_gcwq_cpu(cpu) {
                  struct global_cwq *gcwq = get_gcwq(cpu);
                  struct workqueue_struct *wq;
  
                  spin_lock_irq(&gcwq->lock);
  
                  BUG_ON(gcwq->flags & GCWQ_FREEZING);
                  gcwq->flags |= GCWQ_FREEZING;
  
                  list_for_each_entry(wq, &workqueues, list) {
                          struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
  
                          if (cwq && wq->flags & WQ_FREEZABLE)
                                  cwq->max_active = 0;
                  }
  
                  spin_unlock_irq(&gcwq->lock);
          }
  
          spin_unlock(&workqueue_lock);
  }
  
  /**
   * freeze_workqueues_busy - are freezable workqueues still busy?
   *
   * Check whether freezing is complete.  This function must be called
   * between freeze_workqueues_begin() and thaw_workqueues().
   *
   * CONTEXT:
   * Grabs and releases workqueue_lock.
   *
   * RETURNS:
   * %true if some freezable workqueues are still busy.  %false if freezing
   * is complete.
   */
  bool freeze_workqueues_busy(void)
  {
          unsigned int cpu;
          bool busy = false;
  
          spin_lock(&workqueue_lock);
  
          BUG_ON(!workqueue_freezing);
  
          for_each_gcwq_cpu(cpu) {
                  struct workqueue_struct *wq;
                  /*
                   * nr_active is monotonically decreasing.  It's safe
                   * to peek without lock.
                   */
                  list_for_each_entry(wq, &workqueues, list) {
                          struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
  
                          if (!cwq || !(wq->flags & WQ_FREEZABLE))
                                  continue;
  
                          BUG_ON(cwq->nr_active < 0);
                          if (cwq->nr_active) {
                                  busy = true;
                                  goto out_unlock;
                          }
                  }
          }
  out_unlock:
          spin_unlock(&workqueue_lock);
          return busy;
  }
  
  /**
   * thaw_workqueues - thaw workqueues
   *
   * Thaw workqueues.  Normal queueing is restored and all collected
   * frozen works are transferred to their respective gcwq worklists.
   *
   * CONTEXT:
   * Grabs and releases workqueue_lock and gcwq->lock's.
   */
  void thaw_workqueues(void)
  {
          unsigned int cpu;
  
          spin_lock(&workqueue_lock);
  
          if (!workqueue_freezing)
                  goto out_unlock;
  
          for_each_gcwq_cpu(cpu) {
                  struct global_cwq *gcwq = get_gcwq(cpu);
                  struct worker_pool *pool;
                  struct workqueue_struct *wq;
  
                  spin_lock_irq(&gcwq->lock);
  
                  BUG_ON(!(gcwq->flags & GCWQ_FREEZING));
                  gcwq->flags &= ~GCWQ_FREEZING;
  
                  list_for_each_entry(wq, &workqueues, list) {
                          struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
  
                          if (!cwq || !(wq->flags & WQ_FREEZABLE))
                                  continue;
  
                          /* restore max_active and repopulate worklist */
                          cwq_set_max_active(cwq, wq->saved_max_active);
                  }
  
                  for_each_worker_pool(pool, gcwq)
                          wake_up_worker(pool);
  
                  spin_unlock_irq(&gcwq->lock);
          }
  
          workqueue_freezing = false;
  out_unlock:
          spin_unlock(&workqueue_lock);
  }
  #endif /* CONFIG_FREEZER */
  
  static int __init init_workqueues(void)
  {
          unsigned int cpu;
          int i;
  
          /* make sure we have enough bits for OFFQ CPU number */
          BUILD_BUG_ON((1LU << (BITS_PER_LONG - WORK_OFFQ_CPU_SHIFT)) <
                       WORK_CPU_LAST);
  
          cpu_notifier(workqueue_cpu_up_callback, CPU_PRI_WORKQUEUE_UP);
          hotcpu_notifier(workqueue_cpu_down_callback, CPU_PRI_WORKQUEUE_DOWN);
  
          /* initialize gcwqs */
          for_each_gcwq_cpu(cpu) {
                  struct global_cwq *gcwq = get_gcwq(cpu);
                  struct worker_pool *pool;
  
                  spin_lock_init(&gcwq->lock);
                  gcwq->cpu = cpu;
                  gcwq->flags |= GCWQ_DISASSOCIATED;
  
                  for (i = 0; i < BUSY_WORKER_HASH_SIZE; i++)
                          INIT_HLIST_HEAD(&gcwq->busy_hash[i]);
  
                  for_each_worker_pool(pool, gcwq) {
                          pool->gcwq = gcwq;
                          INIT_LIST_HEAD(&pool->worklist);
                          INIT_LIST_HEAD(&pool->idle_list);
  
                          init_timer_deferrable(&pool->idle_timer);
                          pool->idle_timer.function = idle_worker_timeout;
                          pool->idle_timer.data = (unsigned long)pool;
  
                          setup_timer(&pool->mayday_timer, gcwq_mayday_timeout,
                                      (unsigned long)pool);
  
                          mutex_init(&pool->assoc_mutex);
                          ida_init(&pool->worker_ida);
                  }
          }
  
          /* create the initial worker */
          for_each_online_gcwq_cpu(cpu) {
                  struct global_cwq *gcwq = get_gcwq(cpu);
                  struct worker_pool *pool;
  
                  if (cpu != WORK_CPU_UNBOUND)
                          gcwq->flags &= ~GCWQ_DISASSOCIATED;
  
                  for_each_worker_pool(pool, gcwq) {
                          struct worker *worker;
  
                          worker = create_worker(pool);
                          BUG_ON(!worker);
                          spin_lock_irq(&gcwq->lock);
                          start_worker(worker);
                          spin_unlock_irq(&gcwq->lock);
                  }
          }
  
          system_wq = alloc_workqueue("events", 0, 0);
          system_highpri_wq = alloc_workqueue("events_highpri", WQ_HIGHPRI, 0);
          system_long_wq = alloc_workqueue("events_long", 0, 0);
          system_unbound_wq = alloc_workqueue("events_unbound", WQ_UNBOUND,
                                              WQ_UNBOUND_MAX_ACTIVE);
          system_freezable_wq = alloc_workqueue("events_freezable",
                                                WQ_FREEZABLE, 0);
          BUG_ON(!system_wq || !system_highpri_wq || !system_long_wq ||
                 !system_unbound_wq || !system_freezable_wq);
          return 0;
  }
  early_initcall(init_workqueues);