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

     /*
      * Sleepable 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 (C) IBM Corporation, 2006
     * Copyright (C) Fujitsu, 2012
     *
     * Author: Paul McKenney <paulmck@us.ibm.com>
     *         Lai Jiangshan <laijs@cn.fujitsu.com>
     *
     * For detailed explanation of Read-Copy Update mechanism see -
     *              Documentation/RCU/ *.txt
     *
     */
    
    #include <linux/export.h>
    #include <linux/mutex.h>
    #include <linux/percpu.h>
    #include <linux/preempt.h>
    #include <linux/rcupdate.h>
    #include <linux/sched.h>
    #include <linux/smp.h>
    #include <linux/delay.h>
    #include <linux/srcu.h>
    
    #include <trace/events/rcu.h>
    
    #include "rcu.h"
    
    /*
     * Initialize an rcu_batch structure to empty.
     */
    static inline void rcu_batch_init(struct rcu_batch *b)
    {
            b->head = NULL;
            b->tail = &b->head;
    }
    
    /*
     * Enqueue a callback onto the tail of the specified rcu_batch structure.
     */
    static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head)
    {
            *b->tail = head;
            b->tail = &head->next;
    }
    
    /*
     * Is the specified rcu_batch structure empty?
     */
    static inline bool rcu_batch_empty(struct rcu_batch *b)
    {
            return b->tail == &b->head;
    }
    
    /*
     * Remove the callback at the head of the specified rcu_batch structure
     * and return a pointer to it, or return NULL if the structure is empty.
     */
    static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b)
    {
            struct rcu_head *head;
    
            if (rcu_batch_empty(b))
                    return NULL;
    
            head = b->head;
            b->head = head->next;
            if (b->tail == &head->next)
                    rcu_batch_init(b);
    
            return head;
    }
    
    /*
     * Move all callbacks from the rcu_batch structure specified by "from" to
     * the structure specified by "to".
     */
    static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from)
    {
            if (!rcu_batch_empty(from)) {
                    *to->tail = from->head;
                    to->tail = from->tail;
                    rcu_batch_init(from);
            }
    }
   
   static int init_srcu_struct_fields(struct srcu_struct *sp)
   {
           sp->completed = 0;
           spin_lock_init(&sp->queue_lock);
           sp->running = false;
           rcu_batch_init(&sp->batch_queue);
           rcu_batch_init(&sp->batch_check0);
           rcu_batch_init(&sp->batch_check1);
           rcu_batch_init(&sp->batch_done);
           INIT_DELAYED_WORK(&sp->work, process_srcu);
           sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
           return sp->per_cpu_ref ? 0 : -ENOMEM;
   }
   
   #ifdef CONFIG_DEBUG_LOCK_ALLOC
   
   int __init_srcu_struct(struct srcu_struct *sp, const char *name,
                          struct lock_class_key *key)
   {
           /* Don't re-initialize a lock while it is held. */
           debug_check_no_locks_freed((void *)sp, sizeof(*sp));
           lockdep_init_map(&sp->dep_map, name, key, 0);
           return init_srcu_struct_fields(sp);
   }
   EXPORT_SYMBOL_GPL(__init_srcu_struct);
   
   #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
   
   /**
    * init_srcu_struct - initialize a sleep-RCU structure
    * @sp: structure to initialize.
    *
    * Must invoke this on a given srcu_struct before passing that srcu_struct
    * to any other function.  Each srcu_struct represents a separate domain
    * of SRCU protection.
    */
   int init_srcu_struct(struct srcu_struct *sp)
   {
           return init_srcu_struct_fields(sp);
   }
   EXPORT_SYMBOL_GPL(init_srcu_struct);
   
   #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
   
   /*
    * Returns approximate total of the readers' ->seq[] values for the
    * rank of per-CPU counters specified by idx.
    */
   static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
   {
           int cpu;
           unsigned long sum = 0;
           unsigned long t;
   
           for_each_possible_cpu(cpu) {
                   t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
                   sum += t;
           }
           return sum;
   }
   
   /*
    * Returns approximate number of readers active on the specified rank
    * of the per-CPU ->c[] counters.
    */
   static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
   {
           int cpu;
           unsigned long sum = 0;
           unsigned long t;
   
           for_each_possible_cpu(cpu) {
                   t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
                   sum += t;
           }
           return sum;
   }
   
   /*
    * Return true if the number of pre-existing readers is determined to
    * be stably zero.  An example unstable zero can occur if the call
    * to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
    * but due to task migration, sees the corresponding __srcu_read_unlock()
    * decrement.  This can happen because srcu_readers_active_idx() takes
    * time to sum the array, and might in fact be interrupted or preempted
    * partway through the summation.
    */
   static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
   {
           unsigned long seq;
   
           seq = srcu_readers_seq_idx(sp, idx);
   
           /*
            * The following smp_mb() A pairs with the smp_mb() B located in
            * __srcu_read_lock().  This pairing ensures that if an
            * __srcu_read_lock() increments its counter after the summation
            * in srcu_readers_active_idx(), then the corresponding SRCU read-side
            * critical section will see any changes made prior to the start
            * of the current SRCU grace period.
            *
            * Also, if the above call to srcu_readers_seq_idx() saw the
            * increment of ->seq[], then the call to srcu_readers_active_idx()
            * must see the increment of ->c[].
            */
           smp_mb(); /* A */
   
           /*
            * Note that srcu_readers_active_idx() can incorrectly return
            * zero even though there is a pre-existing reader throughout.
            * To see this, suppose that task A is in a very long SRCU
            * read-side critical section that started on CPU 0, and that
            * no other reader exists, so that the sum of the counters
            * is equal to one.  Then suppose that task B starts executing
            * srcu_readers_active_idx(), summing up to CPU 1, and then that
            * task C starts reading on CPU 0, so that its increment is not
            * summed, but finishes reading on CPU 2, so that its decrement
            * -is- summed.  Then when task B completes its sum, it will
            * incorrectly get zero, despite the fact that task A has been
            * in its SRCU read-side critical section the whole time.
            *
            * We therefore do a validation step should srcu_readers_active_idx()
            * return zero.
            */
           if (srcu_readers_active_idx(sp, idx) != 0)
                   return false;
   
           /*
            * The remainder of this function is the validation step.
            * The following smp_mb() D pairs with the smp_mb() C in
            * __srcu_read_unlock().  If the __srcu_read_unlock() was seen
            * by srcu_readers_active_idx() above, then any destructive
            * operation performed after the grace period will happen after
            * the corresponding SRCU read-side critical section.
            *
            * Note that there can be at most NR_CPUS worth of readers using
            * the old index, which is not enough to overflow even a 32-bit
            * integer.  (Yes, this does mean that systems having more than
            * a billion or so CPUs need to be 64-bit systems.)  Therefore,
            * the sum of the ->seq[] counters cannot possibly overflow.
            * Therefore, the only way that the return values of the two
            * calls to srcu_readers_seq_idx() can be equal is if there were
            * no increments of the corresponding rank of ->seq[] counts
            * in the interim.  But the missed-increment scenario laid out
            * above includes an increment of the ->seq[] counter by
            * the corresponding __srcu_read_lock().  Therefore, if this
            * scenario occurs, the return values from the two calls to
            * srcu_readers_seq_idx() will differ, and thus the validation
            * step below suffices.
            */
           smp_mb(); /* D */
   
           return srcu_readers_seq_idx(sp, idx) == seq;
   }
   
   /**
    * srcu_readers_active - returns approximate number of readers.
    * @sp: which srcu_struct to count active readers (holding srcu_read_lock).
    *
    * Note that this is not an atomic primitive, and can therefore suffer
    * severe errors when invoked on an active srcu_struct.  That said, it
    * can be useful as an error check at cleanup time.
    */
   static int srcu_readers_active(struct srcu_struct *sp)
   {
           int cpu;
           unsigned long sum = 0;
   
           for_each_possible_cpu(cpu) {
                   sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]);
                   sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]);
           }
           return sum;
   }
   
   /**
    * cleanup_srcu_struct - deconstruct a sleep-RCU structure
    * @sp: structure to clean up.
    *
    * Must invoke this after you are finished using a given srcu_struct that
    * was initialized via init_srcu_struct(), else you leak memory.
    */
   void cleanup_srcu_struct(struct srcu_struct *sp)
   {
           int sum;
   
           sum = srcu_readers_active(sp);
           WARN_ON(sum);  /* Leakage unless caller handles error. */
           if (sum != 0)
                   return;
           free_percpu(sp->per_cpu_ref);
           sp->per_cpu_ref = NULL;
   }
   EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
   
   /*
    * Counts the new reader in the appropriate per-CPU element of the
    * srcu_struct.  Must be called from process context.
    * Returns an index that must be passed to the matching srcu_read_unlock().
    */
   int __srcu_read_lock(struct srcu_struct *sp)
   {
           int idx;
   
           preempt_disable();
           idx = rcu_dereference_index_check(sp->completed,
                                             rcu_read_lock_sched_held()) & 0x1;
           ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
           smp_mb(); /* B */  /* Avoid leaking the critical section. */
           ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
           preempt_enable();
           return idx;
   }
   EXPORT_SYMBOL_GPL(__srcu_read_lock);
   
   /*
    * Removes the count for the old reader from the appropriate per-CPU
    * element of the srcu_struct.  Note that this may well be a different
    * CPU than that which was incremented by the corresponding srcu_read_lock().
    * Must be called from process context.
    */
   void __srcu_read_unlock(struct srcu_struct *sp, int idx)
   {
           preempt_disable();
           smp_mb(); /* C */  /* Avoid leaking the critical section. */
           ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) -= 1;
           preempt_enable();
   }
   EXPORT_SYMBOL_GPL(__srcu_read_unlock);
   
   /*
    * We use an adaptive strategy for synchronize_srcu() and especially for
    * synchronize_srcu_expedited().  We spin for a fixed time period
    * (defined below) to allow SRCU readers to exit their read-side critical
    * sections.  If there are still some readers after 10 microseconds,
    * we repeatedly block for 1-millisecond time periods.  This approach
    * has done well in testing, so there is no need for a config parameter.
    */
   #define SRCU_RETRY_CHECK_DELAY          5
   #define SYNCHRONIZE_SRCU_TRYCOUNT       2
   #define SYNCHRONIZE_SRCU_EXP_TRYCOUNT   12
   
   /*
    * @@@ Wait until all pre-existing readers complete.  Such readers
    * will have used the index specified by "idx".
    * the caller should ensures the ->completed is not changed while checking
    * and idx = (->completed & 1) ^ 1
    */
   static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
   {
           for (;;) {
                   if (srcu_readers_active_idx_check(sp, idx))
                           return true;
                   if (--trycount <= 0)
                           return false;
                   udelay(SRCU_RETRY_CHECK_DELAY);
           }
   }
   
   /*
    * Increment the ->completed counter so that future SRCU readers will
    * use the other rank of the ->c[] and ->seq[] arrays.  This allows
    * us to wait for pre-existing readers in a starvation-free manner.
    */
   static void srcu_flip(struct srcu_struct *sp)
   {
           sp->completed++;
   }
   
   /*
    * Enqueue an SRCU callback on the specified srcu_struct structure,
    * initiating grace-period processing if it is not already running.
    */
   void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
                   void (*func)(struct rcu_head *head))
   {
           unsigned long flags;
   
           head->next = NULL;
           head->func = func;
           spin_lock_irqsave(&sp->queue_lock, flags);
           rcu_batch_queue(&sp->batch_queue, head);
           if (!sp->running) {
                   sp->running = true;
                   schedule_delayed_work(&sp->work, 0);
           }
           spin_unlock_irqrestore(&sp->queue_lock, flags);
   }
   EXPORT_SYMBOL_GPL(call_srcu);
   
   struct rcu_synchronize {
           struct rcu_head head;
           struct completion completion;
   };
   
   /*
    * Awaken the corresponding synchronize_srcu() instance now that a
    * grace period has elapsed.
    */
   static void wakeme_after_rcu(struct rcu_head *head)
   {
           struct rcu_synchronize *rcu;
   
           rcu = container_of(head, struct rcu_synchronize, head);
           complete(&rcu->completion);
   }
   
   static void srcu_advance_batches(struct srcu_struct *sp, int trycount);
   static void srcu_reschedule(struct srcu_struct *sp);
   
   /*
    * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
    */
   static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
   {
           struct rcu_synchronize rcu;
           struct rcu_head *head = &rcu.head;
           bool done = false;
   
           rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
                              !lock_is_held(&rcu_bh_lock_map) &&
                              !lock_is_held(&rcu_lock_map) &&
                              !lock_is_held(&rcu_sched_lock_map),
                              "Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section");
   
           init_completion(&rcu.completion);
   
           head->next = NULL;
           head->func = wakeme_after_rcu;
           spin_lock_irq(&sp->queue_lock);
           if (!sp->running) {
                   /* steal the processing owner */
                   sp->running = true;
                   rcu_batch_queue(&sp->batch_check0, head);
                   spin_unlock_irq(&sp->queue_lock);
   
                   srcu_advance_batches(sp, trycount);
                   if (!rcu_batch_empty(&sp->batch_done)) {
                           BUG_ON(sp->batch_done.head != head);
                           rcu_batch_dequeue(&sp->batch_done);
                           done = true;
                   }
                   /* give the processing owner to work_struct */
                   srcu_reschedule(sp);
           } else {
                   rcu_batch_queue(&sp->batch_queue, head);
                   spin_unlock_irq(&sp->queue_lock);
           }
   
           if (!done)
                   wait_for_completion(&rcu.completion);
   }
   
   /**
    * synchronize_srcu - wait for prior SRCU read-side critical-section completion
    * @sp: srcu_struct with which to synchronize.
    *
    * Flip the completed counter, and wait for the old count to drain to zero.
    * As with classic RCU, the updater must use some separate means of
    * synchronizing concurrent updates.  Can block; must be called from
    * process context.
    *
    * Note that it is illegal to call synchronize_srcu() from the corresponding
    * SRCU read-side critical section; doing so will result in deadlock.
    * However, it is perfectly legal to call synchronize_srcu() on one
    * srcu_struct from some other srcu_struct's read-side critical section.
    */
   void synchronize_srcu(struct srcu_struct *sp)
   {
           __synchronize_srcu(sp, rcu_expedited
                              ? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
                              : SYNCHRONIZE_SRCU_TRYCOUNT);
   }
   EXPORT_SYMBOL_GPL(synchronize_srcu);
   
   /**
    * synchronize_srcu_expedited - Brute-force SRCU grace period
    * @sp: srcu_struct with which to synchronize.
    *
    * Wait for an SRCU grace period to elapse, but be more aggressive about
    * spinning rather than blocking when waiting.
    *
    * Note that it is illegal to call this function while holding any lock
    * that is acquired by a CPU-hotplug notifier.  It is also illegal to call
    * synchronize_srcu_expedited() from the corresponding SRCU read-side
    * critical section; doing so will result in deadlock.  However, it is
    * perfectly legal to call synchronize_srcu_expedited() on one srcu_struct
    * from some other srcu_struct's read-side critical section, as long as
    * the resulting graph of srcu_structs is acyclic.
    */
   void synchronize_srcu_expedited(struct srcu_struct *sp)
   {
           __synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);
   }
   EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
   
   /**
    * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
    */
   void srcu_barrier(struct srcu_struct *sp)
   {
           synchronize_srcu(sp);
   }
   EXPORT_SYMBOL_GPL(srcu_barrier);
   
   /**
    * srcu_batches_completed - return batches completed.
    * @sp: srcu_struct on which to report batch completion.
    *
    * Report the number of batches, correlated with, but not necessarily
    * precisely the same as, the number of grace periods that have elapsed.
    */
   long srcu_batches_completed(struct srcu_struct *sp)
   {
           return sp->completed;
   }
   EXPORT_SYMBOL_GPL(srcu_batches_completed);
   
   #define SRCU_CALLBACK_BATCH     10
   #define SRCU_INTERVAL           1
   
   /*
    * Move any new SRCU callbacks to the first stage of the SRCU grace
    * period pipeline.
    */
   static void srcu_collect_new(struct srcu_struct *sp)
   {
           if (!rcu_batch_empty(&sp->batch_queue)) {
                   spin_lock_irq(&sp->queue_lock);
                   rcu_batch_move(&sp->batch_check0, &sp->batch_queue);
                   spin_unlock_irq(&sp->queue_lock);
           }
   }
   
   /*
    * Core SRCU state machine.  Advance callbacks from ->batch_check0 to
    * ->batch_check1 and then to ->batch_done as readers drain.
    */
   static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
   {
           int idx = 1 ^ (sp->completed & 1);
   
           /*
            * Because readers might be delayed for an extended period after
            * fetching ->completed for their index, at any point in time there
            * might well be readers using both idx=0 and idx=1.  We therefore
            * need to wait for readers to clear from both index values before
            * invoking a callback.
            */
   
           if (rcu_batch_empty(&sp->batch_check0) &&
               rcu_batch_empty(&sp->batch_check1))
                   return; /* no callbacks need to be advanced */
   
           if (!try_check_zero(sp, idx, trycount))
                   return; /* failed to advance, will try after SRCU_INTERVAL */
   
           /*
            * The callbacks in ->batch_check1 have already done with their
            * first zero check and flip back when they were enqueued on
            * ->batch_check0 in a previous invocation of srcu_advance_batches().
            * (Presumably try_check_zero() returned false during that
            * invocation, leaving the callbacks stranded on ->batch_check1.)
            * They are therefore ready to invoke, so move them to ->batch_done.
            */
           rcu_batch_move(&sp->batch_done, &sp->batch_check1);
   
           if (rcu_batch_empty(&sp->batch_check0))
                   return; /* no callbacks need to be advanced */
           srcu_flip(sp);
   
           /*
            * The callbacks in ->batch_check0 just finished their
            * first check zero and flip, so move them to ->batch_check1
            * for future checking on the other idx.
            */
           rcu_batch_move(&sp->batch_check1, &sp->batch_check0);
   
           /*
            * SRCU read-side critical sections are normally short, so check
            * at least twice in quick succession after a flip.
            */
           trycount = trycount < 2 ? 2 : trycount;
           if (!try_check_zero(sp, idx^1, trycount))
                   return; /* failed to advance, will try after SRCU_INTERVAL */
   
           /*
            * The callbacks in ->batch_check1 have now waited for all
            * pre-existing readers using both idx values.  They are therefore
            * ready to invoke, so move them to ->batch_done.
            */
           rcu_batch_move(&sp->batch_done, &sp->batch_check1);
   }
   
   /*
    * Invoke a limited number of SRCU callbacks that have passed through
    * their grace period.  If there are more to do, SRCU will reschedule
    * the workqueue.
    */
   static void srcu_invoke_callbacks(struct srcu_struct *sp)
   {
           int i;
           struct rcu_head *head;
   
           for (i = 0; i < SRCU_CALLBACK_BATCH; i++) {
                   head = rcu_batch_dequeue(&sp->batch_done);
                   if (!head)
                           break;
                   local_bh_disable();
                   head->func(head);
                   local_bh_enable();
           }
   }
   
   /*
    * Finished one round of SRCU grace period.  Start another if there are
    * more SRCU callbacks queued, otherwise put SRCU into not-running state.
    */
   static void srcu_reschedule(struct srcu_struct *sp)
   {
           bool pending = true;
   
           if (rcu_batch_empty(&sp->batch_done) &&
               rcu_batch_empty(&sp->batch_check1) &&
               rcu_batch_empty(&sp->batch_check0) &&
               rcu_batch_empty(&sp->batch_queue)) {
                   spin_lock_irq(&sp->queue_lock);
                   if (rcu_batch_empty(&sp->batch_done) &&
                       rcu_batch_empty(&sp->batch_check1) &&
                       rcu_batch_empty(&sp->batch_check0) &&
                       rcu_batch_empty(&sp->batch_queue)) {
                           sp->running = false;
                           pending = false;
                   }
                   spin_unlock_irq(&sp->queue_lock);
           }
   
           if (pending)
                   schedule_delayed_work(&sp->work, SRCU_INTERVAL);
   }
   
   /*
    * This is the work-queue function that handles SRCU grace periods.
    */
   void process_srcu(struct work_struct *work)
   {
           struct srcu_struct *sp;
   
           sp = container_of(work, struct srcu_struct, work.work);
   
           srcu_collect_new(sp);
           srcu_advance_batches(sp, 1);
           srcu_invoke_callbacks(sp);
           srcu_reschedule(sp);
   }
   EXPORT_SYMBOL_GPL(process_srcu);

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