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

     /*
      * Read-Copy Update mechanism for mutual exclusion
      *
      * This program is free software; you can redistribute it and/or modify
      * it under the terms of the GNU General Public License as published by
      * the Free Software Foundation; either version 2 of the License, or
      * (at your option) any later version.
      *
      * This program is distributed in the hope that it will be useful,
     * but WITHOUT ANY WARRANTY; without even the implied warranty of
     * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     * GNU General Public License for more details.
     *
     * You should have received a copy of the GNU General Public License
     * along with this program; if not, write to the Free Software
     * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
     *
     * Copyright IBM Corporation, 2008
     *
     * Authors: Dipankar Sarma <dipankar@in.ibm.com>
     *          Manfred Spraul <manfred@colorfullife.com>
     *          Paul E. McKenney <paulmck@linux.vnet.ibm.com> Hierarchical version
     *
     * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
     * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
     *
     * For detailed explanation of Read-Copy Update mechanism see -
     *      Documentation/RCU
     */
    #include <linux/types.h>
    #include <linux/kernel.h>
    #include <linux/init.h>
    #include <linux/spinlock.h>
    #include <linux/smp.h>
    #include <linux/rcupdate.h>
    #include <linux/interrupt.h>
    #include <linux/sched.h>
    #include <linux/nmi.h>
    #include <linux/atomic.h>
    #include <linux/bitops.h>
    #include <linux/export.h>
    #include <linux/completion.h>
    #include <linux/moduleparam.h>
    #include <linux/percpu.h>
    #include <linux/notifier.h>
    #include <linux/cpu.h>
    #include <linux/mutex.h>
    #include <linux/time.h>
    #include <linux/kernel_stat.h>
    #include <linux/wait.h>
    #include <linux/kthread.h>
    #include <linux/prefetch.h>
    #include <linux/delay.h>
    #include <linux/stop_machine.h>
    #include <linux/random.h>
    
    #include "rcutree.h"
    #include <trace/events/rcu.h>
    
    #include "rcu.h"
    
    /* Data structures. */
    
    static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
    static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
    
    #define RCU_STATE_INITIALIZER(sname, cr) { \
            .level = { &sname##_state.node[0] }, \
            .call = cr, \
            .fqs_state = RCU_GP_IDLE, \
            .gpnum = 0UL - 300UL, \
            .completed = 0UL - 300UL, \
            .orphan_lock = __RAW_SPIN_LOCK_UNLOCKED(&sname##_state.orphan_lock), \
            .orphan_nxttail = &sname##_state.orphan_nxtlist, \
            .orphan_donetail = &sname##_state.orphan_donelist, \
            .barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
            .onoff_mutex = __MUTEX_INITIALIZER(sname##_state.onoff_mutex), \
            .name = #sname, \
    }
    
    struct rcu_state rcu_sched_state =
            RCU_STATE_INITIALIZER(rcu_sched, call_rcu_sched);
    DEFINE_PER_CPU(struct rcu_data, rcu_sched_data);
    
    struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh, call_rcu_bh);
    DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);
    
    static struct rcu_state *rcu_state;
    LIST_HEAD(rcu_struct_flavors);
    
    /* Increase (but not decrease) the CONFIG_RCU_FANOUT_LEAF at boot time. */
    static int rcu_fanout_leaf = CONFIG_RCU_FANOUT_LEAF;
    module_param(rcu_fanout_leaf, int, 0444);
    int rcu_num_lvls __read_mostly = RCU_NUM_LVLS;
    static int num_rcu_lvl[] = {  /* Number of rcu_nodes at specified level. */
            NUM_RCU_LVL_0,
            NUM_RCU_LVL_1,
            NUM_RCU_LVL_2,
            NUM_RCU_LVL_3,
           NUM_RCU_LVL_4,
   };
   int rcu_num_nodes __read_mostly = NUM_RCU_NODES; /* Total # rcu_nodes in use. */
   
   /*
    * The rcu_scheduler_active variable transitions from zero to one just
    * before the first task is spawned.  So when this variable is zero, RCU
    * can assume that there is but one task, allowing RCU to (for example)
    * optimized synchronize_sched() to a simple barrier().  When this variable
    * is one, RCU must actually do all the hard work required to detect real
    * grace periods.  This variable is also used to suppress boot-time false
    * positives from lockdep-RCU error checking.
    */
   int rcu_scheduler_active __read_mostly;
   EXPORT_SYMBOL_GPL(rcu_scheduler_active);
   
   /*
    * The rcu_scheduler_fully_active variable transitions from zero to one
    * during the early_initcall() processing, which is after the scheduler
    * is capable of creating new tasks.  So RCU processing (for example,
    * creating tasks for RCU priority boosting) must be delayed until after
    * rcu_scheduler_fully_active transitions from zero to one.  We also
    * currently delay invocation of any RCU callbacks until after this point.
    *
    * It might later prove better for people registering RCU callbacks during
    * early boot to take responsibility for these callbacks, but one step at
    * a time.
    */
   static int rcu_scheduler_fully_active __read_mostly;
   
   #ifdef CONFIG_RCU_BOOST
   
   /*
    * Control variables for per-CPU and per-rcu_node kthreads.  These
    * handle all flavors of RCU.
    */
   static DEFINE_PER_CPU(struct task_struct *, rcu_cpu_kthread_task);
   DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
   DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_loops);
   DEFINE_PER_CPU(char, rcu_cpu_has_work);
   
   #endif /* #ifdef CONFIG_RCU_BOOST */
   
   static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu);
   static void invoke_rcu_core(void);
   static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);
   
   /*
    * Track the rcutorture test sequence number and the update version
    * number within a given test.  The rcutorture_testseq is incremented
    * on every rcutorture module load and unload, so has an odd value
    * when a test is running.  The rcutorture_vernum is set to zero
    * when rcutorture starts and is incremented on each rcutorture update.
    * These variables enable correlating rcutorture output with the
    * RCU tracing information.
    */
   unsigned long rcutorture_testseq;
   unsigned long rcutorture_vernum;
   
   /*
    * Return true if an RCU grace period is in progress.  The ACCESS_ONCE()s
    * permit this function to be invoked without holding the root rcu_node
    * structure's ->lock, but of course results can be subject to change.
    */
   static int rcu_gp_in_progress(struct rcu_state *rsp)
   {
           return ACCESS_ONCE(rsp->completed) != ACCESS_ONCE(rsp->gpnum);
   }
   
   /*
    * Note a quiescent state.  Because we do not need to know
    * how many quiescent states passed, just if there was at least
    * one since the start of the grace period, this just sets a flag.
    * The caller must have disabled preemption.
    */
   void rcu_sched_qs(int cpu)
   {
           struct rcu_data *rdp = &per_cpu(rcu_sched_data, cpu);
   
           if (rdp->passed_quiesce == 0)
                   trace_rcu_grace_period("rcu_sched", rdp->gpnum, "cpuqs");
           rdp->passed_quiesce = 1;
   }
   
   void rcu_bh_qs(int cpu)
   {
           struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu);
   
           if (rdp->passed_quiesce == 0)
                   trace_rcu_grace_period("rcu_bh", rdp->gpnum, "cpuqs");
           rdp->passed_quiesce = 1;
   }
   
   /*
    * Note a context switch.  This is a quiescent state for RCU-sched,
    * and requires special handling for preemptible RCU.
    * The caller must have disabled preemption.
    */
   void rcu_note_context_switch(int cpu)
   {
           trace_rcu_utilization("Start context switch");
           rcu_sched_qs(cpu);
           rcu_preempt_note_context_switch(cpu);
           trace_rcu_utilization("End context switch");
   }
   EXPORT_SYMBOL_GPL(rcu_note_context_switch);
   
   DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
           .dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
           .dynticks = ATOMIC_INIT(1),
   };
   
   static long blimit = 10;        /* Maximum callbacks per rcu_do_batch. */
   static long qhimark = 10000;    /* If this many pending, ignore blimit. */
   static long qlowmark = 100;     /* Once only this many pending, use blimit. */
   
   module_param(blimit, long, 0444);
   module_param(qhimark, long, 0444);
   module_param(qlowmark, long, 0444);
   
   int rcu_cpu_stall_suppress __read_mostly; /* 1 = suppress stall warnings. */
   int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;
   
   module_param(rcu_cpu_stall_suppress, int, 0644);
   module_param(rcu_cpu_stall_timeout, int, 0644);
   
   static ulong jiffies_till_first_fqs = RCU_JIFFIES_TILL_FORCE_QS;
   static ulong jiffies_till_next_fqs = RCU_JIFFIES_TILL_FORCE_QS;
   
   module_param(jiffies_till_first_fqs, ulong, 0644);
   module_param(jiffies_till_next_fqs, ulong, 0644);
   
   static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *));
   static void force_quiescent_state(struct rcu_state *rsp);
   static int rcu_pending(int cpu);
   
   /*
    * Return the number of RCU-sched batches processed thus far for debug & stats.
    */
   long rcu_batches_completed_sched(void)
   {
           return rcu_sched_state.completed;
   }
   EXPORT_SYMBOL_GPL(rcu_batches_completed_sched);
   
   /*
    * Return the number of RCU BH batches processed thus far for debug & stats.
    */
   long rcu_batches_completed_bh(void)
   {
           return rcu_bh_state.completed;
   }
   EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
   
   /*
    * Force a quiescent state for RCU BH.
    */
   void rcu_bh_force_quiescent_state(void)
   {
           force_quiescent_state(&rcu_bh_state);
   }
   EXPORT_SYMBOL_GPL(rcu_bh_force_quiescent_state);
   
   /*
    * Record the number of times rcutorture tests have been initiated and
    * terminated.  This information allows the debugfs tracing stats to be
    * correlated to the rcutorture messages, even when the rcutorture module
    * is being repeatedly loaded and unloaded.  In other words, we cannot
    * store this state in rcutorture itself.
    */
   void rcutorture_record_test_transition(void)
   {
           rcutorture_testseq++;
           rcutorture_vernum = 0;
   }
   EXPORT_SYMBOL_GPL(rcutorture_record_test_transition);
   
   /*
    * Record the number of writer passes through the current rcutorture test.
    * This is also used to correlate debugfs tracing stats with the rcutorture
    * messages.
    */
   void rcutorture_record_progress(unsigned long vernum)
   {
           rcutorture_vernum++;
   }
   EXPORT_SYMBOL_GPL(rcutorture_record_progress);
   
   /*
    * Force a quiescent state for RCU-sched.
    */
   void rcu_sched_force_quiescent_state(void)
   {
           force_quiescent_state(&rcu_sched_state);
   }
   EXPORT_SYMBOL_GPL(rcu_sched_force_quiescent_state);
   
   /*
    * Does the CPU have callbacks ready to be invoked?
    */
   static int
   cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp)
   {
           return &rdp->nxtlist != rdp->nxttail[RCU_DONE_TAIL] &&
                  rdp->nxttail[RCU_DONE_TAIL] != NULL;
   }
   
   /*
    * Does the current CPU require a yet-as-unscheduled grace period?
    */
   static int
   cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
   {
           struct rcu_head **ntp;
   
           ntp = rdp->nxttail[RCU_DONE_TAIL +
                              (ACCESS_ONCE(rsp->completed) != rdp->completed)];
           return rdp->nxttail[RCU_DONE_TAIL] && ntp && *ntp &&
                  !rcu_gp_in_progress(rsp);
   }
   
   /*
    * Return the root node of the specified rcu_state structure.
    */
   static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
   {
           return &rsp->node[0];
   }
   
   /*
    * rcu_eqs_enter_common - current CPU is moving towards extended quiescent state
    *
    * If the new value of the ->dynticks_nesting counter now is zero,
    * we really have entered idle, and must do the appropriate accounting.
    * The caller must have disabled interrupts.
    */
   static void rcu_eqs_enter_common(struct rcu_dynticks *rdtp, long long oldval,
                                   bool user)
   {
           trace_rcu_dyntick("Start", oldval, 0);
           if (!user && !is_idle_task(current)) {
                   struct task_struct *idle = idle_task(smp_processor_id());
   
                   trace_rcu_dyntick("Error on entry: not idle task", oldval, 0);
                   ftrace_dump(DUMP_ORIG);
                   WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
                             current->pid, current->comm,
                             idle->pid, idle->comm); /* must be idle task! */
           }
           rcu_prepare_for_idle(smp_processor_id());
           /* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
           smp_mb__before_atomic_inc();  /* See above. */
           atomic_inc(&rdtp->dynticks);
           smp_mb__after_atomic_inc();  /* Force ordering with next sojourn. */
           WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);
   
           /*
            * It is illegal to enter an extended quiescent state while
            * in an RCU read-side critical section.
            */
           rcu_lockdep_assert(!lock_is_held(&rcu_lock_map),
                              "Illegal idle entry in RCU read-side critical section.");
           rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map),
                              "Illegal idle entry in RCU-bh read-side critical section.");
           rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map),
                              "Illegal idle entry in RCU-sched read-side critical section.");
   }
   
   /*
    * Enter an RCU extended quiescent state, which can be either the
    * idle loop or adaptive-tickless usermode execution.
    */
   static void rcu_eqs_enter(bool user)
   {
           long long oldval;
           struct rcu_dynticks *rdtp;
   
           rdtp = &__get_cpu_var(rcu_dynticks);
           oldval = rdtp->dynticks_nesting;
           WARN_ON_ONCE((oldval & DYNTICK_TASK_NEST_MASK) == 0);
           if ((oldval & DYNTICK_TASK_NEST_MASK) == DYNTICK_TASK_NEST_VALUE)
                   rdtp->dynticks_nesting = 0;
           else
                   rdtp->dynticks_nesting -= DYNTICK_TASK_NEST_VALUE;
           rcu_eqs_enter_common(rdtp, oldval, user);
   }
   
   /**
    * rcu_idle_enter - inform RCU that current CPU is entering idle
    *
    * Enter idle mode, in other words, -leave- the mode in which RCU
    * read-side critical sections can occur.  (Though RCU read-side
    * critical sections can occur in irq handlers in idle, a possibility
    * handled by irq_enter() and irq_exit().)
    *
    * We crowbar the ->dynticks_nesting field to zero to allow for
    * the possibility of usermode upcalls having messed up our count
    * of interrupt nesting level during the prior busy period.
    */
   void rcu_idle_enter(void)
   {
           unsigned long flags;
   
           local_irq_save(flags);
           rcu_eqs_enter(false);
           local_irq_restore(flags);
   }
   EXPORT_SYMBOL_GPL(rcu_idle_enter);
   
   #ifdef CONFIG_RCU_USER_QS
   /**
    * rcu_user_enter - inform RCU that we are resuming userspace.
    *
    * Enter RCU idle mode right before resuming userspace.  No use of RCU
    * is permitted between this call and rcu_user_exit(). This way the
    * CPU doesn't need to maintain the tick for RCU maintenance purposes
    * when the CPU runs in userspace.
    */
   void rcu_user_enter(void)
   {
           rcu_eqs_enter(1);
   }
   
   /**
    * rcu_user_enter_after_irq - inform RCU that we are going to resume userspace
    * after the current irq returns.
    *
    * This is similar to rcu_user_enter() but in the context of a non-nesting
    * irq. After this call, RCU enters into idle mode when the interrupt
    * returns.
    */
   void rcu_user_enter_after_irq(void)
   {
           unsigned long flags;
           struct rcu_dynticks *rdtp;
   
           local_irq_save(flags);
           rdtp = &__get_cpu_var(rcu_dynticks);
           /* Ensure this irq is interrupting a non-idle RCU state.  */
           WARN_ON_ONCE(!(rdtp->dynticks_nesting & DYNTICK_TASK_MASK));
           rdtp->dynticks_nesting = 1;
           local_irq_restore(flags);
   }
   #endif /* CONFIG_RCU_USER_QS */
   
   /**
    * rcu_irq_exit - inform RCU that current CPU is exiting irq towards idle
    *
    * Exit from an interrupt handler, which might possibly result in entering
    * idle mode, in other words, leaving the mode in which read-side critical
    * sections can occur.
    *
    * This code assumes that the idle loop never does anything that might
    * result in unbalanced calls to irq_enter() and irq_exit().  If your
    * architecture violates this assumption, RCU will give you what you
    * deserve, good and hard.  But very infrequently and irreproducibly.
    *
    * Use things like work queues to work around this limitation.
    *
    * You have been warned.
    */
   void rcu_irq_exit(void)
   {
           unsigned long flags;
           long long oldval;
           struct rcu_dynticks *rdtp;
   
           local_irq_save(flags);
           rdtp = &__get_cpu_var(rcu_dynticks);
           oldval = rdtp->dynticks_nesting;
           rdtp->dynticks_nesting--;
           WARN_ON_ONCE(rdtp->dynticks_nesting < 0);
           if (rdtp->dynticks_nesting)
                   trace_rcu_dyntick("--=", oldval, rdtp->dynticks_nesting);
           else
                   rcu_eqs_enter_common(rdtp, oldval, true);
           local_irq_restore(flags);
   }
   
   /*
    * rcu_eqs_exit_common - current CPU moving away from extended quiescent state
    *
    * If the new value of the ->dynticks_nesting counter was previously zero,
    * we really have exited idle, and must do the appropriate accounting.
    * The caller must have disabled interrupts.
    */
   static void rcu_eqs_exit_common(struct rcu_dynticks *rdtp, long long oldval,
                                  int user)
   {
           smp_mb__before_atomic_inc();  /* Force ordering w/previous sojourn. */
           atomic_inc(&rdtp->dynticks);
           /* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
           smp_mb__after_atomic_inc();  /* See above. */
           WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
           rcu_cleanup_after_idle(smp_processor_id());
           trace_rcu_dyntick("End", oldval, rdtp->dynticks_nesting);
           if (!user && !is_idle_task(current)) {
                   struct task_struct *idle = idle_task(smp_processor_id());
   
                   trace_rcu_dyntick("Error on exit: not idle task",
                                     oldval, rdtp->dynticks_nesting);
                   ftrace_dump(DUMP_ORIG);
                   WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
                             current->pid, current->comm,
                             idle->pid, idle->comm); /* must be idle task! */
           }
   }
   
   /*
    * Exit an RCU extended quiescent state, which can be either the
    * idle loop or adaptive-tickless usermode execution.
    */
   static void rcu_eqs_exit(bool user)
   {
           struct rcu_dynticks *rdtp;
           long long oldval;
   
           rdtp = &__get_cpu_var(rcu_dynticks);
           oldval = rdtp->dynticks_nesting;
           WARN_ON_ONCE(oldval < 0);
           if (oldval & DYNTICK_TASK_NEST_MASK)
                   rdtp->dynticks_nesting += DYNTICK_TASK_NEST_VALUE;
           else
                   rdtp->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
           rcu_eqs_exit_common(rdtp, oldval, user);
   }
   
   /**
    * rcu_idle_exit - inform RCU that current CPU is leaving idle
    *
    * Exit idle mode, in other words, -enter- the mode in which RCU
    * read-side critical sections can occur.
    *
    * We crowbar the ->dynticks_nesting field to DYNTICK_TASK_NEST to
    * allow for the possibility of usermode upcalls messing up our count
    * of interrupt nesting level during the busy period that is just
    * now starting.
    */
   void rcu_idle_exit(void)
   {
           unsigned long flags;
   
           local_irq_save(flags);
           rcu_eqs_exit(false);
           local_irq_restore(flags);
   }
   EXPORT_SYMBOL_GPL(rcu_idle_exit);
   
   #ifdef CONFIG_RCU_USER_QS
   /**
    * rcu_user_exit - inform RCU that we are exiting userspace.
    *
    * Exit RCU idle mode while entering the kernel because it can
    * run a RCU read side critical section anytime.
    */
   void rcu_user_exit(void)
   {
           rcu_eqs_exit(1);
   }
   
   /**
    * rcu_user_exit_after_irq - inform RCU that we won't resume to userspace
    * idle mode after the current non-nesting irq returns.
    *
    * This is similar to rcu_user_exit() but in the context of an irq.
    * This is called when the irq has interrupted a userspace RCU idle mode
    * context. When the current non-nesting interrupt returns after this call,
    * the CPU won't restore the RCU idle mode.
    */
   void rcu_user_exit_after_irq(void)
   {
           unsigned long flags;
           struct rcu_dynticks *rdtp;
   
           local_irq_save(flags);
           rdtp = &__get_cpu_var(rcu_dynticks);
           /* Ensure we are interrupting an RCU idle mode. */
           WARN_ON_ONCE(rdtp->dynticks_nesting & DYNTICK_TASK_NEST_MASK);
           rdtp->dynticks_nesting += DYNTICK_TASK_EXIT_IDLE;
           local_irq_restore(flags);
   }
   #endif /* CONFIG_RCU_USER_QS */
   
   /**
    * rcu_irq_enter - inform RCU that current CPU is entering irq away from idle
    *
    * Enter an interrupt handler, which might possibly result in exiting
    * idle mode, in other words, entering the mode in which read-side critical
    * sections can occur.
    *
    * Note that the Linux kernel is fully capable of entering an interrupt
    * handler that it never exits, for example when doing upcalls to
    * user mode!  This code assumes that the idle loop never does upcalls to
    * user mode.  If your architecture does do upcalls from the idle loop (or
    * does anything else that results in unbalanced calls to the irq_enter()
    * and irq_exit() functions), RCU will give you what you deserve, good
    * and hard.  But very infrequently and irreproducibly.
    *
    * Use things like work queues to work around this limitation.
    *
    * You have been warned.
    */
   void rcu_irq_enter(void)
   {
           unsigned long flags;
           struct rcu_dynticks *rdtp;
           long long oldval;
   
           local_irq_save(flags);
           rdtp = &__get_cpu_var(rcu_dynticks);
           oldval = rdtp->dynticks_nesting;
           rdtp->dynticks_nesting++;
           WARN_ON_ONCE(rdtp->dynticks_nesting == 0);
           if (oldval)
                   trace_rcu_dyntick("++=", oldval, rdtp->dynticks_nesting);
           else
                   rcu_eqs_exit_common(rdtp, oldval, true);
           local_irq_restore(flags);
   }
   
   /**
    * rcu_nmi_enter - inform RCU of entry to NMI context
    *
    * If the CPU was idle with dynamic ticks active, and there is no
    * irq handler running, this updates rdtp->dynticks_nmi to let the
    * RCU grace-period handling know that the CPU is active.
    */
   void rcu_nmi_enter(void)
   {
           struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
   
           if (rdtp->dynticks_nmi_nesting == 0 &&
               (atomic_read(&rdtp->dynticks) & 0x1))
                   return;
           rdtp->dynticks_nmi_nesting++;
           smp_mb__before_atomic_inc();  /* Force delay from prior write. */
           atomic_inc(&rdtp->dynticks);
           /* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
           smp_mb__after_atomic_inc();  /* See above. */
           WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
   }
   
   /**
    * rcu_nmi_exit - inform RCU of exit from NMI context
    *
    * If the CPU was idle with dynamic ticks active, and there is no
    * irq handler running, this updates rdtp->dynticks_nmi to let the
    * RCU grace-period handling know that the CPU is no longer active.
    */
   void rcu_nmi_exit(void)
   {
           struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
   
           if (rdtp->dynticks_nmi_nesting == 0 ||
               --rdtp->dynticks_nmi_nesting != 0)
                   return;
           /* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
           smp_mb__before_atomic_inc();  /* See above. */
           atomic_inc(&rdtp->dynticks);
           smp_mb__after_atomic_inc();  /* Force delay to next write. */
           WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);
   }
   
   /**
    * rcu_is_cpu_idle - see if RCU thinks that the current CPU is idle
    *
    * If the current CPU is in its idle loop and is neither in an interrupt
    * or NMI handler, return true.
    */
   int rcu_is_cpu_idle(void)
   {
           int ret;
   
           preempt_disable();
           ret = (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
           preempt_enable();
           return ret;
   }
   EXPORT_SYMBOL(rcu_is_cpu_idle);
   
   #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU)
   
   /*
    * Is the current CPU online?  Disable preemption to avoid false positives
    * that could otherwise happen due to the current CPU number being sampled,
    * this task being preempted, its old CPU being taken offline, resuming
    * on some other CPU, then determining that its old CPU is now offline.
    * It is OK to use RCU on an offline processor during initial boot, hence
    * the check for rcu_scheduler_fully_active.  Note also that it is OK
    * for a CPU coming online to use RCU for one jiffy prior to marking itself
    * online in the cpu_online_mask.  Similarly, it is OK for a CPU going
    * offline to continue to use RCU for one jiffy after marking itself
    * offline in the cpu_online_mask.  This leniency is necessary given the
    * non-atomic nature of the online and offline processing, for example,
    * the fact that a CPU enters the scheduler after completing the CPU_DYING
    * notifiers.
    *
    * This is also why RCU internally marks CPUs online during the
    * CPU_UP_PREPARE phase and offline during the CPU_DEAD phase.
    *
    * Disable checking if in an NMI handler because we cannot safely report
    * errors from NMI handlers anyway.
    */
   bool rcu_lockdep_current_cpu_online(void)
   {
           struct rcu_data *rdp;
           struct rcu_node *rnp;
           bool ret;
   
           if (in_nmi())
                   return 1;
           preempt_disable();
           rdp = &__get_cpu_var(rcu_sched_data);
           rnp = rdp->mynode;
           ret = (rdp->grpmask & rnp->qsmaskinit) ||
                 !rcu_scheduler_fully_active;
           preempt_enable();
           return ret;
   }
   EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);
   
   #endif /* #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU) */
   
   /**
    * rcu_is_cpu_rrupt_from_idle - see if idle or immediately interrupted from idle
    *
    * If the current CPU is idle or running at a first-level (not nested)
    * interrupt from idle, return true.  The caller must have at least
    * disabled preemption.
    */
   int rcu_is_cpu_rrupt_from_idle(void)
   {
           return __get_cpu_var(rcu_dynticks).dynticks_nesting <= 1;
   }
   
   /*
    * Snapshot the specified CPU's dynticks counter so that we can later
    * credit them with an implicit quiescent state.  Return 1 if this CPU
    * is in dynticks idle mode, which is an extended quiescent state.
    */
   static int dyntick_save_progress_counter(struct rcu_data *rdp)
   {
           rdp->dynticks_snap = atomic_add_return(0, &rdp->dynticks->dynticks);
           return (rdp->dynticks_snap & 0x1) == 0;
   }
   
   /*
    * Return true if the specified CPU has passed through a quiescent
    * state by virtue of being in or having passed through an dynticks
    * idle state since the last call to dyntick_save_progress_counter()
    * for this same CPU, or by virtue of having been offline.
    */
   static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
   {
           unsigned int curr;
           unsigned int snap;
   
           curr = (unsigned int)atomic_add_return(0, &rdp->dynticks->dynticks);
           snap = (unsigned int)rdp->dynticks_snap;
   
           /*
            * If the CPU passed through or entered a dynticks idle phase with
            * no active irq/NMI handlers, then we can safely pretend that the CPU
            * already acknowledged the request to pass through a quiescent
            * state.  Either way, that CPU cannot possibly be in an RCU
            * read-side critical section that started before the beginning
            * of the current RCU grace period.
            */
           if ((curr & 0x1) == 0 || UINT_CMP_GE(curr, snap + 2)) {
                   trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, "dti");
                   rdp->dynticks_fqs++;
                   return 1;
           }
   
           /*
            * Check for the CPU being offline, but only if the grace period
            * is old enough.  We don't need to worry about the CPU changing
            * state: If we see it offline even once, it has been through a
            * quiescent state.
            *
            * The reason for insisting that the grace period be at least
            * one jiffy old is that CPUs that are not quite online and that
            * have just gone offline can still execute RCU read-side critical
            * sections.
            */
           if (ULONG_CMP_GE(rdp->rsp->gp_start + 2, jiffies))
                   return 0;  /* Grace period is not old enough. */
           barrier();
           if (cpu_is_offline(rdp->cpu)) {
                   trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, "ofl");
                   rdp->offline_fqs++;
                   return 1;
           }
           return 0;
   }
   
   static int jiffies_till_stall_check(void)
   {
           int till_stall_check = ACCESS_ONCE(rcu_cpu_stall_timeout);
   
           /*
            * Limit check must be consistent with the Kconfig limits
            * for CONFIG_RCU_CPU_STALL_TIMEOUT.
            */
           if (till_stall_check < 3) {
                   ACCESS_ONCE(rcu_cpu_stall_timeout) = 3;
                   till_stall_check = 3;
           } else if (till_stall_check > 300) {
                   ACCESS_ONCE(rcu_cpu_stall_timeout) = 300;
                   till_stall_check = 300;
           }
           return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
   }
   
   static void record_gp_stall_check_time(struct rcu_state *rsp)
   {
           rsp->gp_start = jiffies;
           rsp->jiffies_stall = jiffies + jiffies_till_stall_check();
   }
   
   /*
    * Dump stacks of all tasks running on stalled CPUs.  This is a fallback
    * for architectures that do not implement trigger_all_cpu_backtrace().
    * The NMI-triggered stack traces are more accurate because they are
    * printed by the target CPU.
    */
   static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
   {
           int cpu;
           unsigned long flags;
           struct rcu_node *rnp;
   
           rcu_for_each_leaf_node(rsp, rnp) {
                   raw_spin_lock_irqsave(&rnp->lock, flags);
                   if (rnp->qsmask != 0) {
                           for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
                                   if (rnp->qsmask & (1UL << cpu))
                                           dump_cpu_task(rnp->grplo + cpu);
                   }
                   raw_spin_unlock_irqrestore(&rnp->lock, flags);
           }
   }
   
   static void print_other_cpu_stall(struct rcu_state *rsp)
   {
           int cpu;
           long delta;
           unsigned long flags;
           int ndetected = 0;
           struct rcu_node *rnp = rcu_get_root(rsp);
           long totqlen = 0;
   
           /* Only let one CPU complain about others per time interval. */
   
           raw_spin_lock_irqsave(&rnp->lock, flags);
           delta = jiffies - rsp->jiffies_stall;
           if (delta < RCU_STALL_RAT_DELAY || !rcu_gp_in_progress(rsp)) {
                   raw_spin_unlock_irqrestore(&rnp->lock, flags);
                   return;
           }
           rsp->jiffies_stall = jiffies + 3 * jiffies_till_stall_check() + 3;
           raw_spin_unlock_irqrestore(&rnp->lock, flags);
   
           /*
            * OK, time to rat on our buddy...
            * See Documentation/RCU/stallwarn.txt for info on how to debug
            * RCU CPU stall warnings.
            */
           printk(KERN_ERR "INFO: %s detected stalls on CPUs/tasks:",
                  rsp->name);
           print_cpu_stall_info_begin();
           rcu_for_each_leaf_node(rsp, rnp) {
                   raw_spin_lock_irqsave(&rnp->lock, flags);
                   ndetected += rcu_print_task_stall(rnp);
                   if (rnp->qsmask != 0) {
                           for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
                                   if (rnp->qsmask & (1UL << cpu)) {
                                           print_cpu_stall_info(rsp,
                                                                rnp->grplo + cpu);
                                           ndetected++;
                                   }
                   }
                   raw_spin_unlock_irqrestore(&rnp->lock, flags);
           }
   
           /*
            * Now rat on any tasks that got kicked up to the root rcu_node
            * due to CPU offlining.
            */
           rnp = rcu_get_root(rsp);
           raw_spin_lock_irqsave(&rnp->lock, flags);
           ndetected += rcu_print_task_stall(rnp);
           raw_spin_unlock_irqrestore(&rnp->lock, flags);
   
           print_cpu_stall_info_end();
           for_each_possible_cpu(cpu)
                   totqlen += per_cpu_ptr(rsp->rda, cpu)->qlen;
           pr_cont("(detected by %d, t=%ld jiffies, g=%lu, c=%lu, q=%lu)\n",
                  smp_processor_id(), (long)(jiffies - rsp->gp_start),
                  rsp->gpnum, rsp->completed, totqlen);
           if (ndetected == 0)
                   printk(KERN_ERR "INFO: Stall ended before state dump start\n");
           else if (!trigger_all_cpu_backtrace())
                   rcu_dump_cpu_stacks(rsp);
   
           /* Complain about tasks blocking the grace period. */
   
           rcu_print_detail_task_stall(rsp);
   
           force_quiescent_state(rsp);  /* Kick them all. */
   }
   
   static void print_cpu_stall(struct rcu_state *rsp)
   {
           int cpu;
           unsigned long flags;
           struct rcu_node *rnp = rcu_get_root(rsp);
           long totqlen = 0;
   
           /*
            * OK, time to rat on ourselves...
            * See Documentation/RCU/stallwarn.txt for info on how to debug
            * RCU CPU stall warnings.
            */
           printk(KERN_ERR "INFO: %s self-detected stall on CPU", rsp->name);
           print_cpu_stall_info_begin();
           print_cpu_stall_info(rsp, smp_processor_id());
           print_cpu_stall_info_end();
           for_each_possible_cpu(cpu)
                   totqlen += per_cpu_ptr(rsp->rda, cpu)->qlen;
           pr_cont(" (t=%lu jiffies g=%lu c=%lu q=%lu)\n",
                   jiffies - rsp->gp_start, rsp->gpnum, rsp->completed, totqlen);
           if (!trigger_all_cpu_backtrace())
                   dump_stack();
   
           raw_spin_lock_irqsave(&rnp->lock, flags);
           if (ULONG_CMP_GE(jiffies, rsp->jiffies_stall))
                   rsp->jiffies_stall = jiffies +
                                        3 * jiffies_till_stall_check() + 3;
           raw_spin_unlock_irqrestore(&rnp->lock, flags);
   
           set_need_resched();  /* kick ourselves to get things going. */
   }
   
   static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
   {
           unsigned long j;
           unsigned long js;
           struct rcu_node *rnp;
   
           if (rcu_cpu_stall_suppress)
                   return;
           j = ACCESS_ONCE(jiffies);
           js = ACCESS_ONCE(rsp->jiffies_stall);
           rnp = rdp->mynode;
           if (rcu_gp_in_progress(rsp) &&
               (ACCESS_ONCE(rnp->qsmask) & rdp->grpmask) && ULONG_CMP_GE(j, js)) {
   
                   /* We haven't checked in, so go dump stack. */
                   print_cpu_stall(rsp);
   
           } else if (rcu_gp_in_progress(rsp) &&
                      ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY)) {
   
                   /* They had a few time units to dump stack, so complain. */
                   print_other_cpu_stall(rsp);
           }
   }
   
   static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
   {
           rcu_cpu_stall_suppress = 1;
           return NOTIFY_DONE;
   }
   
   /**
    * rcu_cpu_stall_reset - prevent further stall warnings in current grace period
    *
    * Set the stall-warning timeout way off into the future, thus preventing
    * any RCU CPU stall-warning messages from appearing in the current set of
    * RCU grace periods.
    *
    * The caller must disable hard irqs.
    */
   void rcu_cpu_stall_reset(void)
   {
           struct rcu_state *rsp;
   
           for_each_rcu_flavor(rsp)
                   rsp->jiffies_stall = jiffies + ULONG_MAX / 2;
   }
   
   static struct notifier_block rcu_panic_block = {
           .notifier_call = rcu_panic,
   };
   
   static void __init check_cpu_stall_init(void)
   {
           atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
   }
  
  /*
   * Update CPU-local rcu_data state to record the newly noticed grace period.
   * This is used both when we started the grace period and when we notice
   * that someone else started the grace period.  The caller must hold the
   * ->lock of the leaf rcu_node structure corresponding to the current CPU,
   *  and must have irqs disabled.
   */
  static void __note_new_gpnum(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
  {
          if (rdp->gpnum != rnp->gpnum) {
                  /*
                   * If the current grace period is waiting for this CPU,
                   * set up to detect a quiescent state, otherwise don't
                   * go looking for one.
                   */
                  rdp->gpnum = rnp->gpnum;
                  trace_rcu_grace_period(rsp->name, rdp->gpnum, "cpustart");
                  rdp->passed_quiesce = 0;
                  rdp->qs_pending = !!(rnp->qsmask & rdp->grpmask);
                  zero_cpu_stall_ticks(rdp);
          }
  }
  
  static void note_new_gpnum(struct rcu_state *rsp, struct rcu_data *rdp)
  {
          unsigned long flags;
          struct rcu_node *rnp;
  
          local_irq_save(flags);
          rnp = rdp->mynode;
          if (rdp->gpnum == ACCESS_ONCE(rnp->gpnum) || /* outside lock. */
              !raw_spin_trylock(&rnp->lock)) { /* irqs already off, so later. */
                  local_irq_restore(flags);
                  return;
          }
          __note_new_gpnum(rsp, rnp, rdp);
          raw_spin_unlock_irqrestore(&rnp->lock, flags);
  }
  
  /*
   * Did someone else start a new RCU grace period start since we last
   * checked?  Update local state appropriately if so.  Must be called
   * on the CPU corresponding to rdp.
   */
  static int
  check_for_new_grace_period(struct rcu_state *rsp, struct rcu_data *rdp)
  {
          unsigned long flags;
          int ret = 0;
  
          local_irq_save(flags);
          if (rdp->gpnum != rsp->gpnum) {
                  note_new_gpnum(rsp, rdp);
                  ret = 1;
          }
          local_irq_restore(flags);
          return ret;
  }
  
  /*
   * Initialize the specified rcu_data structure's callback list to empty.
   */
  static void init_callback_list(struct rcu_data *rdp)
  {
          int i;
  
          rdp->nxtlist = NULL;
          for (i = 0; i < RCU_NEXT_SIZE; i++)
                  rdp->nxttail[i] = &rdp->nxtlist;
          init_nocb_callback_list(rdp);
  }
  
  /*
   * Advance this CPU's callbacks, but only if the current grace period
   * has ended.  This may be called only from the CPU to whom the rdp
   * belongs.  In addition, the corresponding leaf rcu_node structure's
   * ->lock must be held by the caller, with irqs disabled.
   */
  static void
  __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
  {
          /* Did another grace period end? */
          if (rdp->completed != rnp->completed) {
  
                  /* Advance callbacks.  No harm if list empty. */
                  rdp->nxttail[RCU_DONE_TAIL] = rdp->nxttail[RCU_WAIT_TAIL];
                  rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_READY_TAIL];
                  rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
  
                  /* Remember that we saw this grace-period completion. */
                  rdp->completed = rnp->completed;
                  trace_rcu_grace_period(rsp->name, rdp->gpnum, "cpuend");
  
                  /*
                   * If we were in an extended quiescent state, we may have
                   * missed some grace periods that others CPUs handled on
                   * our behalf. Catch up with this state to avoid noting
                   * spurious new grace periods.  If another grace period
                   * has started, then rnp->gpnum will have advanced, so
                   * we will detect this later on.  Of course, any quiescent
                   * states we found for the old GP are now invalid.
                   */
                  if (ULONG_CMP_LT(rdp->gpnum, rdp->completed)) {
                          rdp->gpnum = rdp->completed;
                          rdp->passed_quiesce = 0;
                  }
  
                  /*
                   * If RCU does not need a quiescent state from this CPU,
                   * then make sure that this CPU doesn't go looking for one.
                   */
                  if ((rnp->qsmask & rdp->grpmask) == 0)
                          rdp->qs_pending = 0;
          }
  }
  
  /*
   * Advance this CPU's callbacks, but only if the current grace period
   * has ended.  This may be called only from the CPU to whom the rdp
   * belongs.
   */
  static void
  rcu_process_gp_end(struct rcu_state *rsp, struct rcu_data *rdp)
  {
          unsigned long flags;
          struct rcu_node *rnp;
  
          local_irq_save(flags);
          rnp = rdp->mynode;
          if (rdp->completed == ACCESS_ONCE(rnp->completed) || /* outside lock. */
              !raw_spin_trylock(&rnp->lock)) { /* irqs already off, so later. */
                  local_irq_restore(flags);
                  return;
          }
          __rcu_process_gp_end(rsp, rnp, rdp);
          raw_spin_unlock_irqrestore(&rnp->lock, flags);
  }
  
  /*
   * Do per-CPU grace-period initialization for running CPU.  The caller
   * must hold the lock of the leaf rcu_node structure corresponding to
   * this CPU.
   */
  static void
  rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
  {
          /* Prior grace period ended, so advance callbacks for current CPU. */
          __rcu_process_gp_end(rsp, rnp, rdp);
  
          /* Set state so that this CPU will detect the next quiescent state. */
          __note_new_gpnum(rsp, rnp, rdp);
  }
  
  /*
   * Initialize a new grace period.
   */
  static int rcu_gp_init(struct rcu_state *rsp)
  {
          struct rcu_data *rdp;
          struct rcu_node *rnp = rcu_get_root(rsp);
  
          raw_spin_lock_irq(&rnp->lock);
          rsp->gp_flags = 0; /* Clear all flags: New grace period. */
  
          if (rcu_gp_in_progress(rsp)) {
                  /* Grace period already in progress, don't start another.  */
                  raw_spin_unlock_irq(&rnp->lock);
                  return 0;
          }
  
          /* Advance to a new grace period and initialize state. */
          rsp->gpnum++;
          trace_rcu_grace_period(rsp->name, rsp->gpnum, "start");
          record_gp_stall_check_time(rsp);
          raw_spin_unlock_irq(&rnp->lock);
  
          /* Exclude any concurrent CPU-hotplug operations. */
          mutex_lock(&rsp->onoff_mutex);
  
          /*
           * Set the quiescent-state-needed bits in all the rcu_node
           * structures for all currently online CPUs in breadth-first order,
           * starting from the root rcu_node structure, relying on the layout
           * of the tree within the rsp->node[] array.  Note that other CPUs
           * will access only the leaves of the hierarchy, thus seeing that no
           * grace period is in progress, at least until the corresponding
           * leaf node has been initialized.  In addition, we have excluded
           * CPU-hotplug operations.
           *
           * The grace period cannot complete until the initialization
           * process finishes, because this kthread handles both.
           */
          rcu_for_each_node_breadth_first(rsp, rnp) {
                  raw_spin_lock_irq(&rnp->lock);
                  rdp = this_cpu_ptr(rsp->rda);
                  rcu_preempt_check_blocked_tasks(rnp);
                  rnp->qsmask = rnp->qsmaskinit;
                  rnp->gpnum = rsp->gpnum;
                  WARN_ON_ONCE(rnp->completed != rsp->completed);
                  rnp->completed = rsp->completed;
                  if (rnp == rdp->mynode)
                          rcu_start_gp_per_cpu(rsp, rnp, rdp);
                  rcu_preempt_boost_start_gp(rnp);
                  trace_rcu_grace_period_init(rsp->name, rnp->gpnum,
                                              rnp->level, rnp->grplo,
                                              rnp->grphi, rnp->qsmask);
                  raw_spin_unlock_irq(&rnp->lock);
  #ifdef CONFIG_PROVE_RCU_DELAY
                  if ((random32() % (rcu_num_nodes * 8)) == 0)
                          schedule_timeout_uninterruptible(2);
  #endif /* #ifdef CONFIG_PROVE_RCU_DELAY */
                  cond_resched();
          }
  
          mutex_unlock(&rsp->onoff_mutex);
          return 1;
  }
  
  /*
   * Do one round of quiescent-state forcing.
   */
  int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
  {
          int fqs_state = fqs_state_in;
          struct rcu_node *rnp = rcu_get_root(rsp);
  
          rsp->n_force_qs++;
          if (fqs_state == RCU_SAVE_DYNTICK) {
                  /* Collect dyntick-idle snapshots. */
                  force_qs_rnp(rsp, dyntick_save_progress_counter);
                  fqs_state = RCU_FORCE_QS;
          } else {
                  /* Handle dyntick-idle and offline CPUs. */
                  force_qs_rnp(rsp, rcu_implicit_dynticks_qs);
          }
          /* Clear flag to prevent immediate re-entry. */
          if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
                  raw_spin_lock_irq(&rnp->lock);
                  rsp->gp_flags &= ~RCU_GP_FLAG_FQS;
                  raw_spin_unlock_irq(&rnp->lock);
          }
          return fqs_state;
  }
  
  /*
   * Clean up after the old grace period.
   */
  static void rcu_gp_cleanup(struct rcu_state *rsp)
  {
          unsigned long gp_duration;
          struct rcu_data *rdp;
          struct rcu_node *rnp = rcu_get_root(rsp);
  
          raw_spin_lock_irq(&rnp->lock);
          gp_duration = jiffies - rsp->gp_start;
          if (gp_duration > rsp->gp_max)
                  rsp->gp_max = gp_duration;
  
          /*
           * We know the grace period is complete, but to everyone else
           * it appears to still be ongoing.  But it is also the case
           * that to everyone else it looks like there is nothing that
           * they can do to advance the grace period.  It is therefore
           * safe for us to drop the lock in order to mark the grace
           * period as completed in all of the rcu_node structures.
           */
          raw_spin_unlock_irq(&rnp->lock);
  
          /*
           * Propagate new ->completed value to rcu_node structures so
           * that other CPUs don't have to wait until the start of the next
           * grace period to process their callbacks.  This also avoids
           * some nasty RCU grace-period initialization races by forcing
           * the end of the current grace period to be completely recorded in
           * all of the rcu_node structures before the beginning of the next
           * grace period is recorded in any of the rcu_node structures.
           */
          rcu_for_each_node_breadth_first(rsp, rnp) {
                  raw_spin_lock_irq(&rnp->lock);
                  rnp->completed = rsp->gpnum;
                  raw_spin_unlock_irq(&rnp->lock);
                  cond_resched();
          }
          rnp = rcu_get_root(rsp);
          raw_spin_lock_irq(&rnp->lock);
  
          rsp->completed = rsp->gpnum; /* Declare grace period done. */
          trace_rcu_grace_period(rsp->name, rsp->completed, "end");
          rsp->fqs_state = RCU_GP_IDLE;
          rdp = this_cpu_ptr(rsp->rda);
          if (cpu_needs_another_gp(rsp, rdp))
                  rsp->gp_flags = 1;
          raw_spin_unlock_irq(&rnp->lock);
  }
  
  /*
   * Body of kthread that handles grace periods.
   */
  static int __noreturn rcu_gp_kthread(void *arg)
  {
          int fqs_state;
          unsigned long j;
          int ret;
          struct rcu_state *rsp = arg;
          struct rcu_node *rnp = rcu_get_root(rsp);
  
          for (;;) {
  
                  /* Handle grace-period start. */
                  for (;;) {
                          wait_event_interruptible(rsp->gp_wq,
                                                   rsp->gp_flags &
                                                   RCU_GP_FLAG_INIT);
                          if ((rsp->gp_flags & RCU_GP_FLAG_INIT) &&
                              rcu_gp_init(rsp))
                                  break;
                          cond_resched();
                          flush_signals(current);
                  }
  
                  /* Handle quiescent-state forcing. */
                  fqs_state = RCU_SAVE_DYNTICK;
                  j = jiffies_till_first_fqs;
                  if (j > HZ) {
                          j = HZ;
                          jiffies_till_first_fqs = HZ;
                  }
                  for (;;) {
                          rsp->jiffies_force_qs = jiffies + j;
                          ret = wait_event_interruptible_timeout(rsp->gp_wq,
                                          (rsp->gp_flags & RCU_GP_FLAG_FQS) ||
                                          (!ACCESS_ONCE(rnp->qsmask) &&
                                           !rcu_preempt_blocked_readers_cgp(rnp)),
                                          j);
                          /* If grace period done, leave loop. */
                          if (!ACCESS_ONCE(rnp->qsmask) &&
                              !rcu_preempt_blocked_readers_cgp(rnp))
                                  break;
                          /* If time for quiescent-state forcing, do it. */
                          if (ret == 0 || (rsp->gp_flags & RCU_GP_FLAG_FQS)) {
                                  fqs_state = rcu_gp_fqs(rsp, fqs_state);
                                  cond_resched();
                          } else {
                                  /* Deal with stray signal. */
                                  cond_resched();
                                  flush_signals(current);
                          }
                          j = jiffies_till_next_fqs;
                          if (j > HZ) {
                                  j = HZ;
                                  jiffies_till_next_fqs = HZ;
                          } else if (j < 1) {
                                  j = 1;
                                  jiffies_till_next_fqs = 1;
                          }
                  }
  
                  /* Handle grace-period end. */
                  rcu_gp_cleanup(rsp);
          }
  }
  
  /*
   * Start a new RCU grace period if warranted, re-initializing the hierarchy
   * in preparation for detecting the next grace period.  The caller must hold
   * the root node's ->lock, which is released before return.  Hard irqs must
   * be disabled.
   *
   * Note that it is legal for a dying CPU (which is marked as offline) to
   * invoke this function.  This can happen when the dying CPU reports its
   * quiescent state.
   */
  static void
  rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
          __releases(rcu_get_root(rsp)->lock)
  {
          struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
          struct rcu_node *rnp = rcu_get_root(rsp);
  
          if (!rsp->gp_kthread ||
              !cpu_needs_another_gp(rsp, rdp)) {
                  /*
                   * Either we have not yet spawned the grace-period
                   * task, this CPU does not need another grace period,
                   * or a grace period is already in progress.
                   * Either way, don't start a new grace period.
                   */
                  raw_spin_unlock_irqrestore(&rnp->lock, flags);
                  return;
          }
  
          /*
           * Because there is no grace period in progress right now,
           * any callbacks we have up to this point will be satisfied
           * by the next grace period.  So promote all callbacks to be
           * handled after the end of the next grace period.  If the
           * CPU is not yet aware of the end of the previous grace period,
           * we need to allow for the callback advancement that will
           * occur when it does become aware.  Deadlock prevents us from
           * making it aware at this point: We cannot acquire a leaf
           * rcu_node ->lock while holding the root rcu_node ->lock.
           */
          rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
          if (rdp->completed == rsp->completed)
                  rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
  
          rsp->gp_flags = RCU_GP_FLAG_INIT;
          raw_spin_unlock(&rnp->lock); /* Interrupts remain disabled. */
  
          /* Ensure that CPU is aware of completion of last grace period. */
          rcu_process_gp_end(rsp, rdp);
          local_irq_restore(flags);
  
          /* Wake up rcu_gp_kthread() to start the grace period. */
          wake_up(&rsp->gp_wq);
  }
  
  /*
   * Report a full set of quiescent states to the specified rcu_state
   * data structure.  This involves cleaning up after the prior grace
   * period and letting rcu_start_gp() start up the next grace period
   * if one is needed.  Note that the caller must hold rnp->lock, as
   * required by rcu_start_gp(), which will release it.
   */
  static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
          __releases(rcu_get_root(rsp)->lock)
  {
          WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
          raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
          wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
  }
  
  /*
   * Similar to rcu_report_qs_rdp(), for which it is a helper function.
   * Allows quiescent states for a group of CPUs to be reported at one go
   * to the specified rcu_node structure, though all the CPUs in the group
   * must be represented by the same rcu_node structure (which need not be
   * a leaf rcu_node structure, though it often will be).  That structure's
   * lock must be held upon entry, and it is released before return.
   */
  static void
  rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
                    struct rcu_node *rnp, unsigned long flags)
          __releases(rnp->lock)
  {
          struct rcu_node *rnp_c;
  
          /* Walk up the rcu_node hierarchy. */
          for (;;) {
                  if (!(rnp->qsmask & mask)) {
  
                          /* Our bit has already been cleared, so done. */
                          raw_spin_unlock_irqrestore(&rnp->lock, flags);
                          return;
                  }
                  rnp->qsmask &= ~mask;
                  trace_rcu_quiescent_state_report(rsp->name, rnp->gpnum,
                                                   mask, rnp->qsmask, rnp->level,
                                                   rnp->grplo, rnp->grphi,
                                                   !!rnp->gp_tasks);
                  if (rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {
  
                          /* Other bits still set at this level, so done. */
                          raw_spin_unlock_irqrestore(&rnp->lock, flags);
                          return;
                  }
                  mask = rnp->grpmask;
                  if (rnp->parent == NULL) {
  
                          /* No more levels.  Exit loop holding root lock. */
  
                          break;
                  }
                  raw_spin_unlock_irqrestore(&rnp->lock, flags);
                  rnp_c = rnp;
                  rnp = rnp->parent;
                  raw_spin_lock_irqsave(&rnp->lock, flags);
                  WARN_ON_ONCE(rnp_c->qsmask);
          }
  
          /*
           * Get here if we are the last CPU to pass through a quiescent
           * state for this grace period.  Invoke rcu_report_qs_rsp()
           * to clean up and start the next grace period if one is needed.
           */
          rcu_report_qs_rsp(rsp, flags); /* releases rnp->lock. */
  }
  
  /*
   * Record a quiescent state for the specified CPU to that CPU's rcu_data
   * structure.  This must be either called from the specified CPU, or
   * called when the specified CPU is known to be offline (and when it is
   * also known that no other CPU is concurrently trying to help the offline
   * CPU).  The lastcomp argument is used to make sure we are still in the
   * grace period of interest.  We don't want to end the current grace period
   * based on quiescent states detected in an earlier grace period!
   */
  static void
  rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp)
  {
          unsigned long flags;
          unsigned long mask;
          struct rcu_node *rnp;
  
          rnp = rdp->mynode;
          raw_spin_lock_irqsave(&rnp->lock, flags);
          if (rdp->passed_quiesce == 0 || rdp->gpnum != rnp->gpnum ||
              rnp->completed == rnp->gpnum) {
  
                  /*
                   * The grace period in which this quiescent state was
                   * recorded has ended, so don't report it upwards.
                   * We will instead need a new quiescent state that lies
                   * within the current grace period.
                   */
                  rdp->passed_quiesce = 0;        /* need qs for new gp. */
                  raw_spin_unlock_irqrestore(&rnp->lock, flags);
                  return;
          }
          mask = rdp->grpmask;
          if ((rnp->qsmask & mask) == 0) {
                  raw_spin_unlock_irqrestore(&rnp->lock, flags);
          } else {
                  rdp->qs_pending = 0;
  
                  /*
                   * This GP can't end until cpu checks in, so all of our
                   * callbacks can be processed during the next GP.
                   */
                  rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
  
                  rcu_report_qs_rnp(mask, rsp, rnp, flags); /* rlses rnp->lock */
          }
  }
  
  /*
   * Check to see if there is a new grace period of which this CPU
   * is not yet aware, and if so, set up local rcu_data state for it.
   * Otherwise, see if this CPU has just passed through its first
   * quiescent state for this grace period, and record that fact if so.
   */
  static void
  rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
  {
          /* If there is now a new grace period, record and return. */
          if (check_for_new_grace_period(rsp, rdp))
                  return;
  
          /*
           * Does this CPU still need to do its part for current grace period?
           * If no, return and let the other CPUs do their part as well.
           */
          if (!rdp->qs_pending)
                  return;
  
          /*
           * Was there a quiescent state since the beginning of the grace
           * period? If no, then exit and wait for the next call.
           */
          if (!rdp->passed_quiesce)
                  return;
  
          /*
           * Tell RCU we are done (but rcu_report_qs_rdp() will be the
           * judge of that).
           */
          rcu_report_qs_rdp(rdp->cpu, rsp, rdp);
  }
  
  #ifdef CONFIG_HOTPLUG_CPU
  
  /*
   * Send the specified CPU's RCU callbacks to the orphanage.  The
   * specified CPU must be offline, and the caller must hold the
   * ->orphan_lock.
   */
  static void
  rcu_send_cbs_to_orphanage(int cpu, struct rcu_state *rsp,
                            struct rcu_node *rnp, struct rcu_data *rdp)
  {
          /* No-CBs CPUs do not have orphanable callbacks. */
          if (is_nocb_cpu(rdp->cpu))
                  return;
  
          /*
           * Orphan the callbacks.  First adjust the counts.  This is safe
           * because _rcu_barrier() excludes CPU-hotplug operations, so it
           * cannot be running now.  Thus no memory barrier is required.
           */
          if (rdp->nxtlist != NULL) {
                  rsp->qlen_lazy += rdp->qlen_lazy;
                  rsp->qlen += rdp->qlen;
                  rdp->n_cbs_orphaned += rdp->qlen;
                  rdp->qlen_lazy = 0;
                  ACCESS_ONCE(rdp->qlen) = 0;
          }
  
          /*
           * Next, move those callbacks still needing a grace period to
           * the orphanage, where some other CPU will pick them up.
           * Some of the callbacks might have gone partway through a grace
           * period, but that is too bad.  They get to start over because we
           * cannot assume that grace periods are synchronized across CPUs.
           * We don't bother updating the ->nxttail[] array yet, instead
           * we just reset the whole thing later on.
           */
          if (*rdp->nxttail[RCU_DONE_TAIL] != NULL) {
                  *rsp->orphan_nxttail = *rdp->nxttail[RCU_DONE_TAIL];
                  rsp->orphan_nxttail = rdp->nxttail[RCU_NEXT_TAIL];
                  *rdp->nxttail[RCU_DONE_TAIL] = NULL;
          }
  
          /*
           * Then move the ready-to-invoke callbacks to the orphanage,
           * where some other CPU will pick them up.  These will not be
           * required to pass though another grace period: They are done.
           */
          if (rdp->nxtlist != NULL) {
                  *rsp->orphan_donetail = rdp->nxtlist;
                  rsp->orphan_donetail = rdp->nxttail[RCU_DONE_TAIL];
          }
  
          /* Finally, initialize the rcu_data structure's list to empty.  */
          init_callback_list(rdp);
  }
  
  /*
   * Adopt the RCU callbacks from the specified rcu_state structure's
   * orphanage.  The caller must hold the ->orphan_lock.
   */
  static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
  {
          int i;
          struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
  
          /* No-CBs CPUs are handled specially. */
          if (rcu_nocb_adopt_orphan_cbs(rsp, rdp))
                  return;
  
          /* Do the accounting first. */
          rdp->qlen_lazy += rsp->qlen_lazy;
          rdp->qlen += rsp->qlen;
          rdp->n_cbs_adopted += rsp->qlen;
          if (rsp->qlen_lazy != rsp->qlen)
                  rcu_idle_count_callbacks_posted();
          rsp->qlen_lazy = 0;
          rsp->qlen = 0;
  
          /*
           * We do not need a memory barrier here because the only way we
           * can get here if there is an rcu_barrier() in flight is if
           * we are the task doing the rcu_barrier().
           */
  
          /* First adopt the ready-to-invoke callbacks. */
          if (rsp->orphan_donelist != NULL) {
                  *rsp->orphan_donetail = *rdp->nxttail[RCU_DONE_TAIL];
                  *rdp->nxttail[RCU_DONE_TAIL] = rsp->orphan_donelist;
                  for (i = RCU_NEXT_SIZE - 1; i >= RCU_DONE_TAIL; i--)
                          if (rdp->nxttail[i] == rdp->nxttail[RCU_DONE_TAIL])
                                  rdp->nxttail[i] = rsp->orphan_donetail;
                  rsp->orphan_donelist = NULL;
                  rsp->orphan_donetail = &rsp->orphan_donelist;
          }
  
          /* And then adopt the callbacks that still need a grace period. */
          if (rsp->orphan_nxtlist != NULL) {
                  *rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxtlist;
                  rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxttail;
                  rsp->orphan_nxtlist = NULL;
                  rsp->orphan_nxttail = &rsp->orphan_nxtlist;
          }
  }
  
  /*
   * Trace the fact that this CPU is going offline.
   */
  static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
  {
          RCU_TRACE(unsigned long mask);
          RCU_TRACE(struct rcu_data *rdp = this_cpu_ptr(rsp->rda));
          RCU_TRACE(struct rcu_node *rnp = rdp->mynode);
  
          RCU_TRACE(mask = rdp->grpmask);
          trace_rcu_grace_period(rsp->name,
                                 rnp->gpnum + 1 - !!(rnp->qsmask & mask),
                                 "cpuofl");
  }
  
  /*
   * The CPU has been completely removed, and some other CPU is reporting
   * this fact from process context.  Do the remainder of the cleanup,
   * including orphaning the outgoing CPU's RCU callbacks, and also
   * adopting them.  There can only be one CPU hotplug operation at a time,
   * so no other CPU can be attempting to update rcu_cpu_kthread_task.
   */
  static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
  {
          unsigned long flags;
          unsigned long mask;
          int need_report = 0;
          struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
          struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
  
          /* Adjust any no-longer-needed kthreads. */
          rcu_boost_kthread_setaffinity(rnp, -1);
  
          /* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */
  
          /* Exclude any attempts to start a new grace period. */
          mutex_lock(&rsp->onoff_mutex);
          raw_spin_lock_irqsave(&rsp->orphan_lock, flags);
  
          /* Orphan the dead CPU's callbacks, and adopt them if appropriate. */
          rcu_send_cbs_to_orphanage(cpu, rsp, rnp, rdp);
          rcu_adopt_orphan_cbs(rsp);
  
          /* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
          mask = rdp->grpmask;    /* rnp->grplo is constant. */
          do {
                  raw_spin_lock(&rnp->lock);      /* irqs already disabled. */
                  rnp->qsmaskinit &= ~mask;
                  if (rnp->qsmaskinit != 0) {
                          if (rnp != rdp->mynode)
                                  raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
                          break;
                  }
                  if (rnp == rdp->mynode)
                          need_report = rcu_preempt_offline_tasks(rsp, rnp, rdp);
                  else
                          raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
                  mask = rnp->grpmask;
                  rnp = rnp->parent;
          } while (rnp != NULL);
  
          /*
           * We still hold the leaf rcu_node structure lock here, and
           * irqs are still disabled.  The reason for this subterfuge is
           * because invoking rcu_report_unblock_qs_rnp() with ->orphan_lock
           * held leads to deadlock.
           */
          raw_spin_unlock(&rsp->orphan_lock); /* irqs remain disabled. */
          rnp = rdp->mynode;
          if (need_report & RCU_OFL_TASKS_NORM_GP)
                  rcu_report_unblock_qs_rnp(rnp, flags);
          else
                  raw_spin_unlock_irqrestore(&rnp->lock, flags);
          if (need_report & RCU_OFL_TASKS_EXP_GP)
                  rcu_report_exp_rnp(rsp, rnp, true);
          WARN_ONCE(rdp->qlen != 0 || rdp->nxtlist != NULL,
                    "rcu_cleanup_dead_cpu: Callbacks on offline CPU %d: qlen=%lu, nxtlist=%p\n",
                    cpu, rdp->qlen, rdp->nxtlist);
          init_callback_list(rdp);
          /* Disallow further callbacks on this CPU. */
          rdp->nxttail[RCU_NEXT_TAIL] = NULL;
          mutex_unlock(&rsp->onoff_mutex);
  }
  
  #else /* #ifdef CONFIG_HOTPLUG_CPU */
  
  static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
  {
  }
  
  static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
  {
  }
  
  #endif /* #else #ifdef CONFIG_HOTPLUG_CPU */
  
  /*
   * Invoke any RCU callbacks that have made it to the end of their grace
   * period.  Thottle as specified by rdp->blimit.
   */
  static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
  {
          unsigned long flags;
          struct rcu_head *next, *list, **tail;
          long bl, count, count_lazy;
          int i;
  
          /* If no callbacks are ready, just return.*/
          if (!cpu_has_callbacks_ready_to_invoke(rdp)) {
                  trace_rcu_batch_start(rsp->name, rdp->qlen_lazy, rdp->qlen, 0);
                  trace_rcu_batch_end(rsp->name, 0, !!ACCESS_ONCE(rdp->nxtlist),
                                      need_resched(), is_idle_task(current),
                                      rcu_is_callbacks_kthread());
                  return;
          }
  
          /*
           * Extract the list of ready callbacks, disabling to prevent
           * races with call_rcu() from interrupt handlers.
           */
          local_irq_save(flags);
          WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
          bl = rdp->blimit;
          trace_rcu_batch_start(rsp->name, rdp->qlen_lazy, rdp->qlen, bl);
          list = rdp->nxtlist;
          rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
          *rdp->nxttail[RCU_DONE_TAIL] = NULL;
          tail = rdp->nxttail[RCU_DONE_TAIL];
          for (i = RCU_NEXT_SIZE - 1; i >= 0; i--)
                  if (rdp->nxttail[i] == rdp->nxttail[RCU_DONE_TAIL])
                          rdp->nxttail[i] = &rdp->nxtlist;
          local_irq_restore(flags);
  
          /* Invoke callbacks. */
          count = count_lazy = 0;
          while (list) {
                  next = list->next;
                  prefetch(next);
                  debug_rcu_head_unqueue(list);
                  if (__rcu_reclaim(rsp->name, list))
                          count_lazy++;
                  list = next;
                  /* Stop only if limit reached and CPU has something to do. */
                  if (++count >= bl &&
                      (need_resched() ||
                       (!is_idle_task(current) && !rcu_is_callbacks_kthread())))
                          break;
          }
  
          local_irq_save(flags);
          trace_rcu_batch_end(rsp->name, count, !!list, need_resched(),
                              is_idle_task(current),
                              rcu_is_callbacks_kthread());
  
          /* Update count, and requeue any remaining callbacks. */
          if (list != NULL) {
                  *tail = rdp->nxtlist;
                  rdp->nxtlist = list;
                  for (i = 0; i < RCU_NEXT_SIZE; i++)
                          if (&rdp->nxtlist == rdp->nxttail[i])
                                  rdp->nxttail[i] = tail;
                          else
                                  break;
          }
          smp_mb(); /* List handling before counting for rcu_barrier(). */
          rdp->qlen_lazy -= count_lazy;
          ACCESS_ONCE(rdp->qlen) -= count;
          rdp->n_cbs_invoked += count;
  
          /* Reinstate batch limit if we have worked down the excess. */
          if (rdp->blimit == LONG_MAX && rdp->qlen <= qlowmark)
                  rdp->blimit = blimit;
  
          /* Reset ->qlen_last_fqs_check trigger if enough CBs have drained. */
          if (rdp->qlen == 0 && rdp->qlen_last_fqs_check != 0) {
                  rdp->qlen_last_fqs_check = 0;
                  rdp->n_force_qs_snap = rsp->n_force_qs;
          } else if (rdp->qlen < rdp->qlen_last_fqs_check - qhimark)
                  rdp->qlen_last_fqs_check = rdp->qlen;
          WARN_ON_ONCE((rdp->nxtlist == NULL) != (rdp->qlen == 0));
  
          local_irq_restore(flags);
  
          /* Re-invoke RCU core processing if there are callbacks remaining. */
          if (cpu_has_callbacks_ready_to_invoke(rdp))
                  invoke_rcu_core();
  }
  
  /*
   * Check to see if this CPU is in a non-context-switch quiescent state
   * (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
   * Also schedule RCU core processing.
   *
   * This function must be called from hardirq context.  It is normally
   * invoked from the scheduling-clock interrupt.  If rcu_pending returns
   * false, there is no point in invoking rcu_check_callbacks().
   */
  void rcu_check_callbacks(int cpu, int user)
  {
          trace_rcu_utilization("Start scheduler-tick");
          increment_cpu_stall_ticks();
          if (user || rcu_is_cpu_rrupt_from_idle()) {
  
                  /*
                   * Get here if this CPU took its interrupt from user
                   * mode or from the idle loop, and if this is not a
                   * nested interrupt.  In this case, the CPU is in
                   * a quiescent state, so note it.
                   *
                   * No memory barrier is required here because both
                   * rcu_sched_qs() and rcu_bh_qs() reference only CPU-local
                   * variables that other CPUs neither access nor modify,
                   * at least not while the corresponding CPU is online.
                   */
  
                  rcu_sched_qs(cpu);
                  rcu_bh_qs(cpu);
  
          } else if (!in_softirq()) {
  
                  /*
                   * Get here if this CPU did not take its interrupt from
                   * softirq, in other words, if it is not interrupting
                   * a rcu_bh read-side critical section.  This is an _bh
                   * critical section, so note it.
                   */
  
                  rcu_bh_qs(cpu);
          }
          rcu_preempt_check_callbacks(cpu);
          if (rcu_pending(cpu))
                  invoke_rcu_core();
          trace_rcu_utilization("End scheduler-tick");
  }
  
  /*
   * Scan the leaf rcu_node structures, processing dyntick state for any that
   * have not yet encountered a quiescent state, using the function specified.
   * Also initiate boosting for any threads blocked on the root rcu_node.
   *
   * The caller must have suppressed start of new grace periods.
   */
  static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
  {
          unsigned long bit;
          int cpu;
          unsigned long flags;
          unsigned long mask;
          struct rcu_node *rnp;
  
          rcu_for_each_leaf_node(rsp, rnp) {
                  cond_resched();
                  mask = 0;
                  raw_spin_lock_irqsave(&rnp->lock, flags);
                  if (!rcu_gp_in_progress(rsp)) {
                          raw_spin_unlock_irqrestore(&rnp->lock, flags);
                          return;
                  }
                  if (rnp->qsmask == 0) {
                          rcu_initiate_boost(rnp, flags); /* releases rnp->lock */
                          continue;
                  }
                  cpu = rnp->grplo;
                  bit = 1;
                  for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {
                          if ((rnp->qsmask & bit) != 0 &&
                              f(per_cpu_ptr(rsp->rda, cpu)))
                                  mask |= bit;
                  }
                  if (mask != 0) {
  
                          /* rcu_report_qs_rnp() releases rnp->lock. */
                          rcu_report_qs_rnp(mask, rsp, rnp, flags);
                          continue;
                  }
                  raw_spin_unlock_irqrestore(&rnp->lock, flags);
          }
          rnp = rcu_get_root(rsp);
          if (rnp->qsmask == 0) {
                  raw_spin_lock_irqsave(&rnp->lock, flags);
                  rcu_initiate_boost(rnp, flags); /* releases rnp->lock. */
          }
  }
  
  /*
   * Force quiescent states on reluctant CPUs, and also detect which
   * CPUs are in dyntick-idle mode.
   */
  static void force_quiescent_state(struct rcu_state *rsp)
  {
          unsigned long flags;
          bool ret;
          struct rcu_node *rnp;
          struct rcu_node *rnp_old = NULL;
  
          /* Funnel through hierarchy to reduce memory contention. */
          rnp = per_cpu_ptr(rsp->rda, raw_smp_processor_id())->mynode;
          for (; rnp != NULL; rnp = rnp->parent) {
                  ret = (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) ||
                        !raw_spin_trylock(&rnp->fqslock);
                  if (rnp_old != NULL)
                          raw_spin_unlock(&rnp_old->fqslock);
                  if (ret) {
                          rsp->n_force_qs_lh++;
                          return;
                  }
                  rnp_old = rnp;
          }
          /* rnp_old == rcu_get_root(rsp), rnp == NULL. */
  
          /* Reached the root of the rcu_node tree, acquire lock. */
          raw_spin_lock_irqsave(&rnp_old->lock, flags);
          raw_spin_unlock(&rnp_old->fqslock);
          if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
                  rsp->n_force_qs_lh++;
                  raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
                  return;  /* Someone beat us to it. */
          }
          rsp->gp_flags |= RCU_GP_FLAG_FQS;
          raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
          wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
  }
  
  /*
   * This does the RCU core processing work for the specified rcu_state
   * and rcu_data structures.  This may be called only from the CPU to
   * whom the rdp belongs.
   */
  static void
  __rcu_process_callbacks(struct rcu_state *rsp)
  {
          unsigned long flags;
          struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
  
          WARN_ON_ONCE(rdp->beenonline == 0);
  
          /*
           * Advance callbacks in response to end of earlier grace
           * period that some other CPU ended.
           */
          rcu_process_gp_end(rsp, rdp);
  
          /* Update RCU state based on any recent quiescent states. */
          rcu_check_quiescent_state(rsp, rdp);
  
          /* Does this CPU require a not-yet-started grace period? */
          if (cpu_needs_another_gp(rsp, rdp)) {
                  raw_spin_lock_irqsave(&rcu_get_root(rsp)->lock, flags);
                  rcu_start_gp(rsp, flags);  /* releases above lock */
          }
  
          /* If there are callbacks ready, invoke them. */
          if (cpu_has_callbacks_ready_to_invoke(rdp))
                  invoke_rcu_callbacks(rsp, rdp);
  }
  
  /*
   * Do RCU core processing for the current CPU.
   */
  static void rcu_process_callbacks(struct softirq_action *unused)
  {
          struct rcu_state *rsp;
  
          if (cpu_is_offline(smp_processor_id()))
                  return;
          trace_rcu_utilization("Start RCU core");
          for_each_rcu_flavor(rsp)
                  __rcu_process_callbacks(rsp);
          trace_rcu_utilization("End RCU core");
  }
  
  /*
   * Schedule RCU callback invocation.  If the specified type of RCU
   * does not support RCU priority boosting, just do a direct call,
   * otherwise wake up the per-CPU kernel kthread.  Note that because we
   * are running on the current CPU with interrupts disabled, the
   * rcu_cpu_kthread_task cannot disappear out from under us.
   */
  static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp)
  {
          if (unlikely(!ACCESS_ONCE(rcu_scheduler_fully_active)))
                  return;
          if (likely(!rsp->boost)) {
                  rcu_do_batch(rsp, rdp);
                  return;
          }
          invoke_rcu_callbacks_kthread();
  }
  
  static void invoke_rcu_core(void)
  {
          raise_softirq(RCU_SOFTIRQ);
  }
  
  /*
   * Handle any core-RCU processing required by a call_rcu() invocation.
   */
  static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp,
                              struct rcu_head *head, unsigned long flags)
  {
          /*
           * If called from an extended quiescent state, invoke the RCU
           * core in order to force a re-evaluation of RCU's idleness.
           */
          if (rcu_is_cpu_idle() && cpu_online(smp_processor_id()))
                  invoke_rcu_core();
  
          /* If interrupts were disabled or CPU offline, don't invoke RCU core. */
          if (irqs_disabled_flags(flags) || cpu_is_offline(smp_processor_id()))
                  return;
  
          /*
           * Force the grace period if too many callbacks or too long waiting.
           * Enforce hysteresis, and don't invoke force_quiescent_state()
           * if some other CPU has recently done so.  Also, don't bother
           * invoking force_quiescent_state() if the newly enqueued callback
           * is the only one waiting for a grace period to complete.
           */
          if (unlikely(rdp->qlen > rdp->qlen_last_fqs_check + qhimark)) {
  
                  /* Are we ignoring a completed grace period? */
                  rcu_process_gp_end(rsp, rdp);
                  check_for_new_grace_period(rsp, rdp);
  
                  /* Start a new grace period if one not already started. */
                  if (!rcu_gp_in_progress(rsp)) {
                          unsigned long nestflag;
                          struct rcu_node *rnp_root = rcu_get_root(rsp);
  
                          raw_spin_lock_irqsave(&rnp_root->lock, nestflag);
                          rcu_start_gp(rsp, nestflag);  /* rlses rnp_root->lock */
                  } else {
                          /* Give the grace period a kick. */
                          rdp->blimit = LONG_MAX;
                          if (rsp->n_force_qs == rdp->n_force_qs_snap &&
                              *rdp->nxttail[RCU_DONE_TAIL] != head)
                                  force_quiescent_state(rsp);
                          rdp->n_force_qs_snap = rsp->n_force_qs;
                          rdp->qlen_last_fqs_check = rdp->qlen;
                  }
          }
  }
  
  /*
   * Helper function for call_rcu() and friends.  The cpu argument will
   * normally be -1, indicating "currently running CPU".  It may specify
   * a CPU only if that CPU is a no-CBs CPU.  Currently, only _rcu_barrier()
   * is expected to specify a CPU.
   */
  static void
  __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
             struct rcu_state *rsp, int cpu, bool lazy)
  {
          unsigned long flags;
          struct rcu_data *rdp;
  
          WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */
          debug_rcu_head_queue(head);
          head->func = func;
          head->next = NULL;
  
          /*
           * Opportunistically note grace-period endings and beginnings.
           * Note that we might see a beginning right after we see an
           * end, but never vice versa, since this CPU has to pass through
           * a quiescent state betweentimes.
           */
          local_irq_save(flags);
          rdp = this_cpu_ptr(rsp->rda);
  
          /* Add the callback to our list. */
          if (unlikely(rdp->nxttail[RCU_NEXT_TAIL] == NULL) || cpu != -1) {
                  int offline;
  
                  if (cpu != -1)
                          rdp = per_cpu_ptr(rsp->rda, cpu);
                  offline = !__call_rcu_nocb(rdp, head, lazy);
                  WARN_ON_ONCE(offline);
                  /* _call_rcu() is illegal on offline CPU; leak the callback. */
                  local_irq_restore(flags);
                  return;
          }
          ACCESS_ONCE(rdp->qlen)++;
          if (lazy)
                  rdp->qlen_lazy++;
          else
                  rcu_idle_count_callbacks_posted();
          smp_mb();  /* Count before adding callback for rcu_barrier(). */
          *rdp->nxttail[RCU_NEXT_TAIL] = head;
          rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
  
          if (__is_kfree_rcu_offset((unsigned long)func))
                  trace_rcu_kfree_callback(rsp->name, head, (unsigned long)func,
                                           rdp->qlen_lazy, rdp->qlen);
          else
                  trace_rcu_callback(rsp->name, head, rdp->qlen_lazy, rdp->qlen);
  
          /* Go handle any RCU core processing required. */
          __call_rcu_core(rsp, rdp, head, flags);
          local_irq_restore(flags);
  }
  
  /*
   * Queue an RCU-sched callback for invocation after a grace period.
   */
  void call_rcu_sched(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
  {
          __call_rcu(head, func, &rcu_sched_state, -1, 0);
  }
  EXPORT_SYMBOL_GPL(call_rcu_sched);
  
  /*
   * Queue an RCU callback for invocation after a quicker grace period.
   */
  void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
  {
          __call_rcu(head, func, &rcu_bh_state, -1, 0);
  }
  EXPORT_SYMBOL_GPL(call_rcu_bh);
  
  /*
   * Because a context switch is a grace period for RCU-sched and RCU-bh,
   * any blocking grace-period wait automatically implies a grace period
   * if there is only one CPU online at any point time during execution
   * of either synchronize_sched() or synchronize_rcu_bh().  It is OK to
   * occasionally incorrectly indicate that there are multiple CPUs online
   * when there was in fact only one the whole time, as this just adds
   * some overhead: RCU still operates correctly.
   */
  static inline int rcu_blocking_is_gp(void)
  {
          int ret;
  
          might_sleep();  /* Check for RCU read-side critical section. */
          preempt_disable();
          ret = num_online_cpus() <= 1;
          preempt_enable();
          return ret;
  }
  
  /**
   * synchronize_sched - wait until an rcu-sched grace period has elapsed.
   *
   * Control will return to the caller some time after a full rcu-sched
   * grace period has elapsed, in other words after all currently executing
   * rcu-sched read-side critical sections have completed.   These read-side
   * critical sections are delimited by rcu_read_lock_sched() and
   * rcu_read_unlock_sched(), and may be nested.  Note that preempt_disable(),
   * local_irq_disable(), and so on may be used in place of
   * rcu_read_lock_sched().
   *
   * This means that all preempt_disable code sequences, including NMI and
   * non-threaded hardware-interrupt handlers, in progress on entry will
   * have completed before this primitive returns.  However, this does not
   * guarantee that softirq handlers will have completed, since in some
   * kernels, these handlers can run in process context, and can block.
   *
   * Note that this guarantee implies further memory-ordering guarantees.
   * On systems with more than one CPU, when synchronize_sched() returns,
   * each CPU is guaranteed to have executed a full memory barrier since the
   * end of its last RCU-sched read-side critical section whose beginning
   * preceded the call to synchronize_sched().  In addition, each CPU having
   * an RCU read-side critical section that extends beyond the return from
   * synchronize_sched() is guaranteed to have executed a full memory barrier
   * after the beginning of synchronize_sched() and before the beginning of
   * that RCU read-side critical section.  Note that these guarantees include
   * CPUs that are offline, idle, or executing in user mode, as well as CPUs
   * that are executing in the kernel.
   *
   * Furthermore, if CPU A invoked synchronize_sched(), which returned
   * to its caller on CPU B, then both CPU A and CPU B are guaranteed
   * to have executed a full memory barrier during the execution of
   * synchronize_sched() -- even if CPU A and CPU B are the same CPU (but
   * again only if the system has more than one CPU).
   *
   * This primitive provides the guarantees made by the (now removed)
   * synchronize_kernel() API.  In contrast, synchronize_rcu() only
   * guarantees that rcu_read_lock() sections will have completed.
   * In "classic RCU", these two guarantees happen to be one and
   * the same, but can differ in realtime RCU implementations.
   */
  void synchronize_sched(void)
  {
          rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map) &&
                             !lock_is_held(&rcu_lock_map) &&
                             !lock_is_held(&rcu_sched_lock_map),
                             "Illegal synchronize_sched() in RCU-sched read-side critical section");
          if (rcu_blocking_is_gp())
                  return;
          if (rcu_expedited)
                  synchronize_sched_expedited();
          else
                  wait_rcu_gp(call_rcu_sched);
  }
  EXPORT_SYMBOL_GPL(synchronize_sched);
  
  /**
   * synchronize_rcu_bh - wait until an rcu_bh grace period has elapsed.
   *
   * Control will return to the caller some time after a full rcu_bh grace
   * period has elapsed, in other words after all currently executing rcu_bh
   * read-side critical sections have completed.  RCU read-side critical
   * sections are delimited by rcu_read_lock_bh() and rcu_read_unlock_bh(),
   * and may be nested.
   *
   * See the description of synchronize_sched() for more detailed information
   * on memory ordering guarantees.
   */
  void synchronize_rcu_bh(void)
  {
          rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map) &&
                             !lock_is_held(&rcu_lock_map) &&
                             !lock_is_held(&rcu_sched_lock_map),
                             "Illegal synchronize_rcu_bh() in RCU-bh read-side critical section");
          if (rcu_blocking_is_gp())
                  return;
          if (rcu_expedited)
                  synchronize_rcu_bh_expedited();
          else
                  wait_rcu_gp(call_rcu_bh);
  }
  EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
  
  static int synchronize_sched_expedited_cpu_stop(void *data)
  {
          /*
           * There must be a full memory barrier on each affected CPU
           * between the time that try_stop_cpus() is called and the
           * time that it returns.
           *
           * In the current initial implementation of cpu_stop, the
           * above condition is already met when the control reaches
           * this point and the following smp_mb() is not strictly
           * necessary.  Do smp_mb() anyway for documentation and
           * robustness against future implementation changes.
           */
          smp_mb(); /* See above comment block. */
          return 0;
  }
  
  /**
   * synchronize_sched_expedited - Brute-force RCU-sched grace period
   *
   * Wait for an RCU-sched grace period to elapse, but use a "big hammer"
   * approach to force the grace period to end quickly.  This consumes
   * significant time on all CPUs and is unfriendly to real-time workloads,
   * so is thus not recommended for any sort of common-case code.  In fact,
   * if you are using synchronize_sched_expedited() in a loop, please
   * restructure your code to batch your updates, and then use a single
   * synchronize_sched() instead.
   *
   * Note that it is illegal to call this function while holding any lock
   * that is acquired by a CPU-hotplug notifier.  And yes, it is also illegal
   * to call this function from a CPU-hotplug notifier.  Failing to observe
   * these restriction will result in deadlock.
   *
   * This implementation can be thought of as an application of ticket
   * locking to RCU, with sync_sched_expedited_started and
   * sync_sched_expedited_done taking on the roles of the halves
   * of the ticket-lock word.  Each task atomically increments
   * sync_sched_expedited_started upon entry, snapshotting the old value,
   * then attempts to stop all the CPUs.  If this succeeds, then each
   * CPU will have executed a context switch, resulting in an RCU-sched
   * grace period.  We are then done, so we use atomic_cmpxchg() to
   * update sync_sched_expedited_done to match our snapshot -- but
   * only if someone else has not already advanced past our snapshot.
   *
   * On the other hand, if try_stop_cpus() fails, we check the value
   * of sync_sched_expedited_done.  If it has advanced past our
   * initial snapshot, then someone else must have forced a grace period
   * some time after we took our snapshot.  In this case, our work is
   * done for us, and we can simply return.  Otherwise, we try again,
   * but keep our initial snapshot for purposes of checking for someone
   * doing our work for us.
   *
   * If we fail too many times in a row, we fall back to synchronize_sched().
   */
  void synchronize_sched_expedited(void)
  {
          long firstsnap, s, snap;
          int trycount = 0;
          struct rcu_state *rsp = &rcu_sched_state;
  
          /*
           * If we are in danger of counter wrap, just do synchronize_sched().
           * By allowing sync_sched_expedited_started to advance no more than
           * ULONG_MAX/8 ahead of sync_sched_expedited_done, we are ensuring
           * that more than 3.5 billion CPUs would be required to force a
           * counter wrap on a 32-bit system.  Quite a few more CPUs would of
           * course be required on a 64-bit system.
           */
          if (ULONG_CMP_GE((ulong)atomic_long_read(&rsp->expedited_start),
                           (ulong)atomic_long_read(&rsp->expedited_done) +
                           ULONG_MAX / 8)) {
                  synchronize_sched();
                  atomic_long_inc(&rsp->expedited_wrap);
                  return;
          }
  
          /*
           * Take a ticket.  Note that atomic_inc_return() implies a
           * full memory barrier.
           */
          snap = atomic_long_inc_return(&rsp->expedited_start);
          firstsnap = snap;
          get_online_cpus();
          WARN_ON_ONCE(cpu_is_offline(raw_smp_processor_id()));
  
          /*
           * Each pass through the following loop attempts to force a
           * context switch on each CPU.
           */
          while (try_stop_cpus(cpu_online_mask,
                               synchronize_sched_expedited_cpu_stop,
                               NULL) == -EAGAIN) {
                  put_online_cpus();
                  atomic_long_inc(&rsp->expedited_tryfail);
  
                  /* Check to see if someone else did our work for us. */
                  s = atomic_long_read(&rsp->expedited_done);
                  if (ULONG_CMP_GE((ulong)s, (ulong)firstsnap)) {
                          /* ensure test happens before caller kfree */
                          smp_mb__before_atomic_inc(); /* ^^^ */
                          atomic_long_inc(&rsp->expedited_workdone1);
                          return;
                  }
  
                  /* No joy, try again later.  Or just synchronize_sched(). */
                  if (trycount++ < 10) {
                          udelay(trycount * num_online_cpus());
                  } else {
                          wait_rcu_gp(call_rcu_sched);
                          atomic_long_inc(&rsp->expedited_normal);
                          return;
                  }
  
                  /* Recheck to see if someone else did our work for us. */
                  s = atomic_long_read(&rsp->expedited_done);
                  if (ULONG_CMP_GE((ulong)s, (ulong)firstsnap)) {
                          /* ensure test happens before caller kfree */
                          smp_mb__before_atomic_inc(); /* ^^^ */
                          atomic_long_inc(&rsp->expedited_workdone2);
                          return;
                  }
  
                  /*
                   * Refetching sync_sched_expedited_started allows later
                   * callers to piggyback on our grace period.  We retry
                   * after they started, so our grace period works for them,
                   * and they started after our first try, so their grace
                   * period works for us.
                   */
                  get_online_cpus();
                  snap = atomic_long_read(&rsp->expedited_start);
                  smp_mb(); /* ensure read is before try_stop_cpus(). */
          }
          atomic_long_inc(&rsp->expedited_stoppedcpus);
  
          /*
           * Everyone up to our most recent fetch is covered by our grace
           * period.  Update the counter, but only if our work is still
           * relevant -- which it won't be if someone who started later
           * than we did already did their update.
           */
          do {
                  atomic_long_inc(&rsp->expedited_done_tries);
                  s = atomic_long_read(&rsp->expedited_done);
                  if (ULONG_CMP_GE((ulong)s, (ulong)snap)) {
                          /* ensure test happens before caller kfree */
                          smp_mb__before_atomic_inc(); /* ^^^ */
                          atomic_long_inc(&rsp->expedited_done_lost);
                          break;
                  }
          } while (atomic_long_cmpxchg(&rsp->expedited_done, s, snap) != s);
          atomic_long_inc(&rsp->expedited_done_exit);
  
          put_online_cpus();
  }
  EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
  
  /*
   * Check to see if there is any immediate RCU-related work to be done
   * by the current CPU, for the specified type of RCU, returning 1 if so.
   * The checks are in order of increasing expense: checks that can be
   * carried out against CPU-local state are performed first.  However,
   * we must check for CPU stalls first, else we might not get a chance.
   */
  static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
  {
          struct rcu_node *rnp = rdp->mynode;
  
          rdp->n_rcu_pending++;
  
          /* Check for CPU stalls, if enabled. */
          check_cpu_stall(rsp, rdp);
  
          /* Is the RCU core waiting for a quiescent state from this CPU? */
          if (rcu_scheduler_fully_active &&
              rdp->qs_pending && !rdp->passed_quiesce) {
                  rdp->n_rp_qs_pending++;
          } else if (rdp->qs_pending && rdp->passed_quiesce) {
                  rdp->n_rp_report_qs++;
                  return 1;
          }
  
          /* Does this CPU have callbacks ready to invoke? */
          if (cpu_has_callbacks_ready_to_invoke(rdp)) {
                  rdp->n_rp_cb_ready++;
                  return 1;
          }
  
          /* Has RCU gone idle with this CPU needing another grace period? */
          if (cpu_needs_another_gp(rsp, rdp)) {
                  rdp->n_rp_cpu_needs_gp++;
                  return 1;
          }
  
          /* Has another RCU grace period completed?  */
          if (ACCESS_ONCE(rnp->completed) != rdp->completed) { /* outside lock */
                  rdp->n_rp_gp_completed++;
                  return 1;
          }
  
          /* Has a new RCU grace period started? */
          if (ACCESS_ONCE(rnp->gpnum) != rdp->gpnum) { /* outside lock */
                  rdp->n_rp_gp_started++;
                  return 1;
          }
  
          /* nothing to do */
          rdp->n_rp_need_nothing++;
          return 0;
  }
  
  /*
   * Check to see if there is any immediate RCU-related work to be done
   * by the current CPU, returning 1 if so.  This function is part of the
   * RCU implementation; it is -not- an exported member of the RCU API.
   */
  static int rcu_pending(int cpu)
  {
          struct rcu_state *rsp;
  
          for_each_rcu_flavor(rsp)
                  if (__rcu_pending(rsp, per_cpu_ptr(rsp->rda, cpu)))
                          return 1;
          return 0;
  }
  
  /*
   * Check to see if any future RCU-related work will need to be done
   * by the current CPU, even if none need be done immediately, returning
   * 1 if so.
   */
  static int rcu_cpu_has_callbacks(int cpu)
  {
          struct rcu_state *rsp;
  
          /* RCU callbacks either ready or pending? */
          for_each_rcu_flavor(rsp)
                  if (per_cpu_ptr(rsp->rda, cpu)->nxtlist)
                          return 1;
          return 0;
  }
  
  /*
   * Helper function for _rcu_barrier() tracing.  If tracing is disabled,
   * the compiler is expected to optimize this away.
   */
  static void _rcu_barrier_trace(struct rcu_state *rsp, char *s,
                                 int cpu, unsigned long done)
  {
          trace_rcu_barrier(rsp->name, s, cpu,
                            atomic_read(&rsp->barrier_cpu_count), done);
  }
  
  /*
   * RCU callback function for _rcu_barrier().  If we are last, wake
   * up the task executing _rcu_barrier().
   */
  static void rcu_barrier_callback(struct rcu_head *rhp)
  {
          struct rcu_data *rdp = container_of(rhp, struct rcu_data, barrier_head);
          struct rcu_state *rsp = rdp->rsp;
  
          if (atomic_dec_and_test(&rsp->barrier_cpu_count)) {
                  _rcu_barrier_trace(rsp, "LastCB", -1, rsp->n_barrier_done);
                  complete(&rsp->barrier_completion);
          } else {
                  _rcu_barrier_trace(rsp, "CB", -1, rsp->n_barrier_done);
          }
  }
  
  /*
   * Called with preemption disabled, and from cross-cpu IRQ context.
   */
  static void rcu_barrier_func(void *type)
  {
          struct rcu_state *rsp = type;
          struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
  
          _rcu_barrier_trace(rsp, "IRQ", -1, rsp->n_barrier_done);
          atomic_inc(&rsp->barrier_cpu_count);
          rsp->call(&rdp->barrier_head, rcu_barrier_callback);
  }
  
  /*
   * Orchestrate the specified type of RCU barrier, waiting for all
   * RCU callbacks of the specified type to complete.
   */
  static void _rcu_barrier(struct rcu_state *rsp)
  {
          int cpu;
          struct rcu_data *rdp;
          unsigned long snap = ACCESS_ONCE(rsp->n_barrier_done);
          unsigned long snap_done;
  
          _rcu_barrier_trace(rsp, "Begin", -1, snap);
  
          /* Take mutex to serialize concurrent rcu_barrier() requests. */
          mutex_lock(&rsp->barrier_mutex);
  
          /*
           * Ensure that all prior references, including to ->n_barrier_done,
           * are ordered before the _rcu_barrier() machinery.
           */
          smp_mb();  /* See above block comment. */
  
          /*
           * Recheck ->n_barrier_done to see if others did our work for us.
           * This means checking ->n_barrier_done for an even-to-odd-to-even
           * transition.  The "if" expression below therefore rounds the old
           * value up to the next even number and adds two before comparing.
           */
          snap_done = ACCESS_ONCE(rsp->n_barrier_done);
          _rcu_barrier_trace(rsp, "Check", -1, snap_done);
          if (ULONG_CMP_GE(snap_done, ((snap + 1) & ~0x1) + 2)) {
                  _rcu_barrier_trace(rsp, "EarlyExit", -1, snap_done);
                  smp_mb(); /* caller's subsequent code after above check. */
                  mutex_unlock(&rsp->barrier_mutex);
                  return;
          }
  
          /*
           * Increment ->n_barrier_done to avoid duplicate work.  Use
           * ACCESS_ONCE() to prevent the compiler from speculating
           * the increment to precede the early-exit check.
           */
          ACCESS_ONCE(rsp->n_barrier_done)++;
          WARN_ON_ONCE((rsp->n_barrier_done & 0x1) != 1);
          _rcu_barrier_trace(rsp, "Inc1", -1, rsp->n_barrier_done);
          smp_mb(); /* Order ->n_barrier_done increment with below mechanism. */
  
          /*
           * Initialize the count to one rather than to zero in order to
           * avoid a too-soon return to zero in case of a short grace period
           * (or preemption of this task).  Exclude CPU-hotplug operations
           * to ensure that no offline CPU has callbacks queued.
           */
          init_completion(&rsp->barrier_completion);
          atomic_set(&rsp->barrier_cpu_count, 1);
          get_online_cpus();
  
          /*
           * Force each CPU with callbacks to register a new callback.
           * When that callback is invoked, we will know that all of the
           * corresponding CPU's preceding callbacks have been invoked.
           */
          for_each_possible_cpu(cpu) {
                  if (!cpu_online(cpu) && !is_nocb_cpu(cpu))
                          continue;
                  rdp = per_cpu_ptr(rsp->rda, cpu);
                  if (is_nocb_cpu(cpu)) {
                          _rcu_barrier_trace(rsp, "OnlineNoCB", cpu,
                                             rsp->n_barrier_done);
                          atomic_inc(&rsp->barrier_cpu_count);
                          __call_rcu(&rdp->barrier_head, rcu_barrier_callback,
                                     rsp, cpu, 0);
                  } else if (ACCESS_ONCE(rdp->qlen)) {
                          _rcu_barrier_trace(rsp, "OnlineQ", cpu,
                                             rsp->n_barrier_done);
                          smp_call_function_single(cpu, rcu_barrier_func, rsp, 1);
                  } else {
                          _rcu_barrier_trace(rsp, "OnlineNQ", cpu,
                                             rsp->n_barrier_done);
                  }
          }
          put_online_cpus();
  
          /*
           * Now that we have an rcu_barrier_callback() callback on each
           * CPU, and thus each counted, remove the initial count.
           */
          if (atomic_dec_and_test(&rsp->barrier_cpu_count))
                  complete(&rsp->barrier_completion);
  
          /* Increment ->n_barrier_done to prevent duplicate work. */
          smp_mb(); /* Keep increment after above mechanism. */
          ACCESS_ONCE(rsp->n_barrier_done)++;
          WARN_ON_ONCE((rsp->n_barrier_done & 0x1) != 0);
          _rcu_barrier_trace(rsp, "Inc2", -1, rsp->n_barrier_done);
          smp_mb(); /* Keep increment before caller's subsequent code. */
  
          /* Wait for all rcu_barrier_callback() callbacks to be invoked. */
          wait_for_completion(&rsp->barrier_completion);
  
          /* Other rcu_barrier() invocations can now safely proceed. */
          mutex_unlock(&rsp->barrier_mutex);
  }
  
  /**
   * rcu_barrier_bh - Wait until all in-flight call_rcu_bh() callbacks complete.
   */
  void rcu_barrier_bh(void)
  {
          _rcu_barrier(&rcu_bh_state);
  }
  EXPORT_SYMBOL_GPL(rcu_barrier_bh);
  
  /**
   * rcu_barrier_sched - Wait for in-flight call_rcu_sched() callbacks.
   */
  void rcu_barrier_sched(void)
  {
          _rcu_barrier(&rcu_sched_state);
  }
  EXPORT_SYMBOL_GPL(rcu_barrier_sched);
  
  /*
   * Do boot-time initialization of a CPU's per-CPU RCU data.
   */
  static void __init
  rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp)
  {
          unsigned long flags;
          struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
          struct rcu_node *rnp = rcu_get_root(rsp);
  
          /* Set up local state, ensuring consistent view of global state. */
          raw_spin_lock_irqsave(&rnp->lock, flags);
          rdp->grpmask = 1UL << (cpu - rdp->mynode->grplo);
          init_callback_list(rdp);
          rdp->qlen_lazy = 0;
          ACCESS_ONCE(rdp->qlen) = 0;
          rdp->dynticks = &per_cpu(rcu_dynticks, cpu);
          WARN_ON_ONCE(rdp->dynticks->dynticks_nesting != DYNTICK_TASK_EXIT_IDLE);
          WARN_ON_ONCE(atomic_read(&rdp->dynticks->dynticks) != 1);
  #ifdef CONFIG_RCU_USER_QS
          WARN_ON_ONCE(rdp->dynticks->in_user);
  #endif
          rdp->cpu = cpu;
          rdp->rsp = rsp;
          rcu_boot_init_nocb_percpu_data(rdp);
          raw_spin_unlock_irqrestore(&rnp->lock, flags);
  }
  
  /*
   * Initialize a CPU's per-CPU RCU data.  Note that only one online or
   * offline event can be happening at a given time.  Note also that we
   * can accept some slop in the rsp->completed access due to the fact
   * that this CPU cannot possibly have any RCU callbacks in flight yet.
   */
  static void __cpuinit
  rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
  {
          unsigned long flags;
          unsigned long mask;
          struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
          struct rcu_node *rnp = rcu_get_root(rsp);
  
          /* Exclude new grace periods. */
          mutex_lock(&rsp->onoff_mutex);
  
          /* Set up local state, ensuring consistent view of global state. */
          raw_spin_lock_irqsave(&rnp->lock, flags);
          rdp->beenonline = 1;     /* We have now been online. */
          rdp->preemptible = preemptible;
          rdp->qlen_last_fqs_check = 0;
          rdp->n_force_qs_snap = rsp->n_force_qs;
          rdp->blimit = blimit;
          init_callback_list(rdp);  /* Re-enable callbacks on this CPU. */
          rdp->dynticks->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
          atomic_set(&rdp->dynticks->dynticks,
                     (atomic_read(&rdp->dynticks->dynticks) & ~0x1) + 1);
          rcu_prepare_for_idle_init(cpu);
          raw_spin_unlock(&rnp->lock);            /* irqs remain disabled. */
  
          /* Add CPU to rcu_node bitmasks. */
          rnp = rdp->mynode;
          mask = rdp->grpmask;
          do {
                  /* Exclude any attempts to start a new GP on small systems. */
                  raw_spin_lock(&rnp->lock);      /* irqs already disabled. */
                  rnp->qsmaskinit |= mask;
                  mask = rnp->grpmask;
                  if (rnp == rdp->mynode) {
                          /*
                           * If there is a grace period in progress, we will
                           * set up to wait for it next time we run the
                           * RCU core code.
                           */
                          rdp->gpnum = rnp->completed;
                          rdp->completed = rnp->completed;
                          rdp->passed_quiesce = 0;
                          rdp->qs_pending = 0;
                          trace_rcu_grace_period(rsp->name, rdp->gpnum, "cpuonl");
                  }
                  raw_spin_unlock(&rnp->lock); /* irqs already disabled. */
                  rnp = rnp->parent;
          } while (rnp != NULL && !(rnp->qsmaskinit & mask));
          local_irq_restore(flags);
  
          mutex_unlock(&rsp->onoff_mutex);
  }
  
  static void __cpuinit rcu_prepare_cpu(int cpu)
  {
          struct rcu_state *rsp;
  
          for_each_rcu_flavor(rsp)
                  rcu_init_percpu_data(cpu, rsp,
                                       strcmp(rsp->name, "rcu_preempt") == 0);
  }
  
  /*
   * Handle CPU online/offline notification events.
   */
  static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
                                      unsigned long action, void *hcpu)
  {
          long cpu = (long)hcpu;
          struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
          struct rcu_node *rnp = rdp->mynode;
          struct rcu_state *rsp;
          int ret = NOTIFY_OK;
  
          trace_rcu_utilization("Start CPU hotplug");
          switch (action) {
          case CPU_UP_PREPARE:
          case CPU_UP_PREPARE_FROZEN:
                  rcu_prepare_cpu(cpu);
                  rcu_prepare_kthreads(cpu);
                  break;
          case CPU_ONLINE:
          case CPU_DOWN_FAILED:
                  rcu_boost_kthread_setaffinity(rnp, -1);
                  break;
          case CPU_DOWN_PREPARE:
                  if (nocb_cpu_expendable(cpu))
                          rcu_boost_kthread_setaffinity(rnp, cpu);
                  else
                          ret = NOTIFY_BAD;
                  break;
          case CPU_DYING:
          case CPU_DYING_FROZEN:
                  /*
                   * The whole machine is "stopped" except this CPU, so we can
                   * touch any data without introducing corruption. We send the
                   * dying CPU's callbacks to an arbitrarily chosen online CPU.
                   */
                  for_each_rcu_flavor(rsp)
                          rcu_cleanup_dying_cpu(rsp);
                  rcu_cleanup_after_idle(cpu);
                  break;
          case CPU_DEAD:
          case CPU_DEAD_FROZEN:
          case CPU_UP_CANCELED:
          case CPU_UP_CANCELED_FROZEN:
                  for_each_rcu_flavor(rsp)
                          rcu_cleanup_dead_cpu(cpu, rsp);
                  break;
          default:
                  break;
          }
          trace_rcu_utilization("End CPU hotplug");
          return ret;
  }
  
  /*
   * Spawn the kthread that handles this RCU flavor's grace periods.
   */
  static int __init rcu_spawn_gp_kthread(void)
  {
          unsigned long flags;
          struct rcu_node *rnp;
          struct rcu_state *rsp;
          struct task_struct *t;
  
          for_each_rcu_flavor(rsp) {
                  t = kthread_run(rcu_gp_kthread, rsp, rsp->name);
                  BUG_ON(IS_ERR(t));
                  rnp = rcu_get_root(rsp);
                  raw_spin_lock_irqsave(&rnp->lock, flags);
                  rsp->gp_kthread = t;
                  raw_spin_unlock_irqrestore(&rnp->lock, flags);
                  rcu_spawn_nocb_kthreads(rsp);
          }
          return 0;
  }
  early_initcall(rcu_spawn_gp_kthread);
  
  /*
   * This function is invoked towards the end of the scheduler's initialization
   * process.  Before this is called, the idle task might contain
   * RCU read-side critical sections (during which time, this idle
   * task is booting the system).  After this function is called, the
   * idle tasks are prohibited from containing RCU read-side critical
   * sections.  This function also enables RCU lockdep checking.
   */
  void rcu_scheduler_starting(void)
  {
          WARN_ON(num_online_cpus() != 1);
          WARN_ON(nr_context_switches() > 0);
          rcu_scheduler_active = 1;
  }
  
  /*
   * Compute the per-level fanout, either using the exact fanout specified
   * or balancing the tree, depending on CONFIG_RCU_FANOUT_EXACT.
   */
  #ifdef CONFIG_RCU_FANOUT_EXACT
  static void __init rcu_init_levelspread(struct rcu_state *rsp)
  {
          int i;
  
          for (i = rcu_num_lvls - 1; i > 0; i--)
                  rsp->levelspread[i] = CONFIG_RCU_FANOUT;
          rsp->levelspread[0] = rcu_fanout_leaf;
  }
  #else /* #ifdef CONFIG_RCU_FANOUT_EXACT */
  static void __init rcu_init_levelspread(struct rcu_state *rsp)
  {
          int ccur;
          int cprv;
          int i;
  
          cprv = nr_cpu_ids;
          for (i = rcu_num_lvls - 1; i >= 0; i--) {
                  ccur = rsp->levelcnt[i];
                  rsp->levelspread[i] = (cprv + ccur - 1) / ccur;
                  cprv = ccur;
          }
  }
  #endif /* #else #ifdef CONFIG_RCU_FANOUT_EXACT */
  
  /*
   * Helper function for rcu_init() that initializes one rcu_state structure.
   */
  static void __init rcu_init_one(struct rcu_state *rsp,
                  struct rcu_data __percpu *rda)
  {
          static char *buf[] = { "rcu_node_0",
                                 "rcu_node_1",
                                 "rcu_node_2",
                                 "rcu_node_3" };  /* Match MAX_RCU_LVLS */
          static char *fqs[] = { "rcu_node_fqs_0",
                                 "rcu_node_fqs_1",
                                 "rcu_node_fqs_2",
                                 "rcu_node_fqs_3" };  /* Match MAX_RCU_LVLS */
          int cpustride = 1;
          int i;
          int j;
          struct rcu_node *rnp;
  
          BUILD_BUG_ON(MAX_RCU_LVLS > ARRAY_SIZE(buf));  /* Fix buf[] init! */
  
          /* Initialize the level-tracking arrays. */
  
          for (i = 0; i < rcu_num_lvls; i++)
                  rsp->levelcnt[i] = num_rcu_lvl[i];
          for (i = 1; i < rcu_num_lvls; i++)
                  rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
          rcu_init_levelspread(rsp);
  
          /* Initialize the elements themselves, starting from the leaves. */
  
          for (i = rcu_num_lvls - 1; i >= 0; i--) {
                  cpustride *= rsp->levelspread[i];
                  rnp = rsp->level[i];
                  for (j = 0; j < rsp->levelcnt[i]; j++, rnp++) {
                          raw_spin_lock_init(&rnp->lock);
                          lockdep_set_class_and_name(&rnp->lock,
                                                     &rcu_node_class[i], buf[i]);
                          raw_spin_lock_init(&rnp->fqslock);
                          lockdep_set_class_and_name(&rnp->fqslock,
                                                     &rcu_fqs_class[i], fqs[i]);
                          rnp->gpnum = rsp->gpnum;
                          rnp->completed = rsp->completed;
                          rnp->qsmask = 0;
                          rnp->qsmaskinit = 0;
                          rnp->grplo = j * cpustride;
                          rnp->grphi = (j + 1) * cpustride - 1;
                          if (rnp->grphi >= NR_CPUS)
                                  rnp->grphi = NR_CPUS - 1;
                          if (i == 0) {
                                  rnp->grpnum = 0;
                                  rnp->grpmask = 0;
                                  rnp->parent = NULL;
                          } else {
                                  rnp->grpnum = j % rsp->levelspread[i - 1];
                                  rnp->grpmask = 1UL << rnp->grpnum;
                                  rnp->parent = rsp->level[i - 1] +
                                                j / rsp->levelspread[i - 1];
                          }
                          rnp->level = i;
                          INIT_LIST_HEAD(&rnp->blkd_tasks);
                  }
          }
  
          rsp->rda = rda;
          init_waitqueue_head(&rsp->gp_wq);
          rnp = rsp->level[rcu_num_lvls - 1];
          for_each_possible_cpu(i) {
                  while (i > rnp->grphi)
                          rnp++;
                  per_cpu_ptr(rsp->rda, i)->mynode = rnp;
                  rcu_boot_init_percpu_data(i, rsp);
          }
          list_add(&rsp->flavors, &rcu_struct_flavors);
  }
  
  /*
   * Compute the rcu_node tree geometry from kernel parameters.  This cannot
   * replace the definitions in rcutree.h because those are needed to size
   * the ->node array in the rcu_state structure.
   */
  static void __init rcu_init_geometry(void)
  {
          int i;
          int j;
          int n = nr_cpu_ids;
          int rcu_capacity[MAX_RCU_LVLS + 1];
  
          /* If the compile-time values are accurate, just leave. */
          if (rcu_fanout_leaf == CONFIG_RCU_FANOUT_LEAF &&
              nr_cpu_ids == NR_CPUS)
                  return;
  
          /*
           * Compute number of nodes that can be handled an rcu_node tree
           * with the given number of levels.  Setting rcu_capacity[0] makes
           * some of the arithmetic easier.
           */
          rcu_capacity[0] = 1;
          rcu_capacity[1] = rcu_fanout_leaf;
          for (i = 2; i <= MAX_RCU_LVLS; i++)
                  rcu_capacity[i] = rcu_capacity[i - 1] * CONFIG_RCU_FANOUT;
  
          /*
           * The boot-time rcu_fanout_leaf parameter is only permitted
           * to increase the leaf-level fanout, not decrease it.  Of course,
           * the leaf-level fanout cannot exceed the number of bits in
           * the rcu_node masks.  Finally, the tree must be able to accommodate
           * the configured number of CPUs.  Complain and fall back to the
           * compile-time values if these limits are exceeded.
           */
          if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
              rcu_fanout_leaf > sizeof(unsigned long) * 8 ||
              n > rcu_capacity[MAX_RCU_LVLS]) {
                  WARN_ON(1);
                  return;
          }
  
          /* Calculate the number of rcu_nodes at each level of the tree. */
          for (i = 1; i <= MAX_RCU_LVLS; i++)
                  if (n <= rcu_capacity[i]) {
                          for (j = 0; j <= i; j++)
                                  num_rcu_lvl[j] =
                                          DIV_ROUND_UP(n, rcu_capacity[i - j]);
                          rcu_num_lvls = i;
                          for (j = i + 1; j <= MAX_RCU_LVLS; j++)
                                  num_rcu_lvl[j] = 0;
                          break;
                  }
  
          /* Calculate the total number of rcu_node structures. */
          rcu_num_nodes = 0;
          for (i = 0; i <= MAX_RCU_LVLS; i++)
                  rcu_num_nodes += num_rcu_lvl[i];
          rcu_num_nodes -= n;
  }
  
  void __init rcu_init(void)
  {
          int cpu;
  
          rcu_bootup_announce();
          rcu_init_geometry();
          rcu_init_one(&rcu_sched_state, &rcu_sched_data);
          rcu_init_one(&rcu_bh_state, &rcu_bh_data);
          __rcu_init_preempt();
          rcu_init_nocb();
           open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
  
          /*
           * We don't need protection against CPU-hotplug here because
           * this is called early in boot, before either interrupts
           * or the scheduler are operational.
           */
          cpu_notifier(rcu_cpu_notify, 0);
          for_each_online_cpu(cpu)
                  rcu_cpu_notify(NULL, CPU_UP_PREPARE, (void *)(long)cpu);
          check_cpu_stall_init();
  }
  
  #include "rcutree_plugin.h"

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