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c072a388d5
Because the adaptive synchronize_srcu_expedited() approach has worked very well in testing, remove the kernel parameter and replace it by a C-preprocessor macro. If someone finds problems with this approach, a more complex and aggressively adaptive approach might be required. Longer term, SRCU will be merged with the other RCU implementations, at which point synchronize_srcu_expedited() will be event driven, just as synchronize_sched_expedited() currently is. At that point, there will be no need for this adaptive approach. Reported-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
316 lines
10 KiB
C
316 lines
10 KiB
C
/*
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* Sleepable Read-Copy Update mechanism for mutual exclusion.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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*
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* Copyright (C) IBM Corporation, 2006
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*
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* Author: Paul McKenney <paulmck@us.ibm.com>
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*
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* For detailed explanation of Read-Copy Update mechanism see -
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* Documentation/RCU/ *.txt
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*
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*/
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#include <linux/module.h>
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#include <linux/mutex.h>
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#include <linux/percpu.h>
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#include <linux/preempt.h>
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#include <linux/rcupdate.h>
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#include <linux/sched.h>
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#include <linux/smp.h>
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#include <linux/delay.h>
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#include <linux/srcu.h>
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static int init_srcu_struct_fields(struct srcu_struct *sp)
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{
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sp->completed = 0;
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mutex_init(&sp->mutex);
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sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
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return sp->per_cpu_ref ? 0 : -ENOMEM;
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}
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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int __init_srcu_struct(struct srcu_struct *sp, const char *name,
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struct lock_class_key *key)
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{
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/* Don't re-initialize a lock while it is held. */
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debug_check_no_locks_freed((void *)sp, sizeof(*sp));
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lockdep_init_map(&sp->dep_map, name, key, 0);
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return init_srcu_struct_fields(sp);
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}
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EXPORT_SYMBOL_GPL(__init_srcu_struct);
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#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
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/**
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* init_srcu_struct - initialize a sleep-RCU structure
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* @sp: structure to initialize.
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*
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* Must invoke this on a given srcu_struct before passing that srcu_struct
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* to any other function. Each srcu_struct represents a separate domain
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* of SRCU protection.
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*/
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int init_srcu_struct(struct srcu_struct *sp)
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{
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return init_srcu_struct_fields(sp);
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}
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EXPORT_SYMBOL_GPL(init_srcu_struct);
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#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
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/*
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* srcu_readers_active_idx -- returns approximate number of readers
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* active on the specified rank of per-CPU counters.
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*/
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static int srcu_readers_active_idx(struct srcu_struct *sp, int idx)
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{
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int cpu;
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int sum;
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sum = 0;
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for_each_possible_cpu(cpu)
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sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];
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return sum;
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}
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/**
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* srcu_readers_active - returns approximate number of readers.
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* @sp: which srcu_struct to count active readers (holding srcu_read_lock).
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*
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* Note that this is not an atomic primitive, and can therefore suffer
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* severe errors when invoked on an active srcu_struct. That said, it
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* can be useful as an error check at cleanup time.
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*/
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static int srcu_readers_active(struct srcu_struct *sp)
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{
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return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1);
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}
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/**
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* cleanup_srcu_struct - deconstruct a sleep-RCU structure
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* @sp: structure to clean up.
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*
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* Must invoke this after you are finished using a given srcu_struct that
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* was initialized via init_srcu_struct(), else you leak memory.
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*/
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void cleanup_srcu_struct(struct srcu_struct *sp)
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{
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int sum;
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sum = srcu_readers_active(sp);
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WARN_ON(sum); /* Leakage unless caller handles error. */
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if (sum != 0)
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return;
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free_percpu(sp->per_cpu_ref);
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sp->per_cpu_ref = NULL;
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}
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EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
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/*
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* Counts the new reader in the appropriate per-CPU element of the
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* srcu_struct. Must be called from process context.
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* Returns an index that must be passed to the matching srcu_read_unlock().
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*/
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int __srcu_read_lock(struct srcu_struct *sp)
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{
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int idx;
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preempt_disable();
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idx = sp->completed & 0x1;
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barrier(); /* ensure compiler looks -once- at sp->completed. */
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per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
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srcu_barrier(); /* ensure compiler won't misorder critical section. */
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preempt_enable();
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return idx;
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}
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EXPORT_SYMBOL_GPL(__srcu_read_lock);
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/*
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* Removes the count for the old reader from the appropriate per-CPU
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* element of the srcu_struct. Note that this may well be a different
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* CPU than that which was incremented by the corresponding srcu_read_lock().
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* Must be called from process context.
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*/
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void __srcu_read_unlock(struct srcu_struct *sp, int idx)
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{
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preempt_disable();
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srcu_barrier(); /* ensure compiler won't misorder critical section. */
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per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;
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preempt_enable();
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}
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EXPORT_SYMBOL_GPL(__srcu_read_unlock);
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/*
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* We use an adaptive strategy for synchronize_srcu() and especially for
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* synchronize_srcu_expedited(). We spin for a fixed time period
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* (defined below) to allow SRCU readers to exit their read-side critical
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* sections. If there are still some readers after 10 microseconds,
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* we repeatedly block for 1-millisecond time periods. This approach
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* has done well in testing, so there is no need for a config parameter.
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*/
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#define SYNCHRONIZE_SRCU_READER_DELAY 10
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/*
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* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
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*/
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static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
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{
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int idx;
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idx = sp->completed;
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mutex_lock(&sp->mutex);
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/*
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* Check to see if someone else did the work for us while we were
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* waiting to acquire the lock. We need -two- advances of
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* the counter, not just one. If there was but one, we might have
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* shown up -after- our helper's first synchronize_sched(), thus
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* having failed to prevent CPU-reordering races with concurrent
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* srcu_read_unlock()s on other CPUs (see comment below). So we
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* either (1) wait for two or (2) supply the second ourselves.
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*/
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if ((sp->completed - idx) >= 2) {
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mutex_unlock(&sp->mutex);
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return;
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}
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sync_func(); /* Force memory barrier on all CPUs. */
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/*
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* The preceding synchronize_sched() ensures that any CPU that
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* sees the new value of sp->completed will also see any preceding
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* changes to data structures made by this CPU. This prevents
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* some other CPU from reordering the accesses in its SRCU
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* read-side critical section to precede the corresponding
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* srcu_read_lock() -- ensuring that such references will in
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* fact be protected.
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*
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* So it is now safe to do the flip.
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*/
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idx = sp->completed & 0x1;
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sp->completed++;
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sync_func(); /* Force memory barrier on all CPUs. */
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/*
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* At this point, because of the preceding synchronize_sched(),
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* all srcu_read_lock() calls using the old counters have completed.
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* Their corresponding critical sections might well be still
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* executing, but the srcu_read_lock() primitives themselves
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* will have finished executing. We initially give readers
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* an arbitrarily chosen 10 microseconds to get out of their
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* SRCU read-side critical sections, then loop waiting 1/HZ
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* seconds per iteration. The 10-microsecond value has done
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* very well in testing.
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*/
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if (srcu_readers_active_idx(sp, idx))
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udelay(SYNCHRONIZE_SRCU_READER_DELAY);
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while (srcu_readers_active_idx(sp, idx))
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schedule_timeout_interruptible(1);
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sync_func(); /* Force memory barrier on all CPUs. */
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/*
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* The preceding synchronize_sched() forces all srcu_read_unlock()
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* primitives that were executing concurrently with the preceding
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* for_each_possible_cpu() loop to have completed by this point.
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* More importantly, it also forces the corresponding SRCU read-side
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* critical sections to have also completed, and the corresponding
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* references to SRCU-protected data items to be dropped.
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*
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* Note:
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*
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* Despite what you might think at first glance, the
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* preceding synchronize_sched() -must- be within the
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* critical section ended by the following mutex_unlock().
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* Otherwise, a task taking the early exit can race
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* with a srcu_read_unlock(), which might have executed
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* just before the preceding srcu_readers_active() check,
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* and whose CPU might have reordered the srcu_read_unlock()
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* with the preceding critical section. In this case, there
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* is nothing preventing the synchronize_sched() task that is
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* taking the early exit from freeing a data structure that
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* is still being referenced (out of order) by the task
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* doing the srcu_read_unlock().
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*
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* Alternatively, the comparison with "2" on the early exit
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* could be changed to "3", but this increases synchronize_srcu()
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* latency for bulk loads. So the current code is preferred.
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*/
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mutex_unlock(&sp->mutex);
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}
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/**
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* synchronize_srcu - wait for prior SRCU read-side critical-section completion
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* @sp: srcu_struct with which to synchronize.
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*
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* Flip the completed counter, and wait for the old count to drain to zero.
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* As with classic RCU, the updater must use some separate means of
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* synchronizing concurrent updates. Can block; must be called from
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* process context.
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*
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* Note that it is illegal to call synchronize_srcu() from the corresponding
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* SRCU read-side critical section; doing so will result in deadlock.
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* However, it is perfectly legal to call synchronize_srcu() on one
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* srcu_struct from some other srcu_struct's read-side critical section.
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*/
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void synchronize_srcu(struct srcu_struct *sp)
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{
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__synchronize_srcu(sp, synchronize_sched);
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}
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EXPORT_SYMBOL_GPL(synchronize_srcu);
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/**
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* synchronize_srcu_expedited - like synchronize_srcu, but less patient
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* @sp: srcu_struct with which to synchronize.
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*
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* Flip the completed counter, and wait for the old count to drain to zero.
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* As with classic RCU, the updater must use some separate means of
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* synchronizing concurrent updates. Can block; must be called from
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* process context.
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*
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* Note that it is illegal to call synchronize_srcu_expedited()
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* from the corresponding SRCU read-side critical section; doing so
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* will result in deadlock. However, it is perfectly legal to call
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* synchronize_srcu_expedited() on one srcu_struct from some other
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* srcu_struct's read-side critical section.
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*/
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void synchronize_srcu_expedited(struct srcu_struct *sp)
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{
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__synchronize_srcu(sp, synchronize_sched_expedited);
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}
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EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
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/**
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* srcu_batches_completed - return batches completed.
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* @sp: srcu_struct on which to report batch completion.
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*
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* Report the number of batches, correlated with, but not necessarily
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* precisely the same as, the number of grace periods that have elapsed.
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*/
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long srcu_batches_completed(struct srcu_struct *sp)
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{
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return sp->completed;
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}
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EXPORT_SYMBOL_GPL(srcu_batches_completed);
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