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![Paul E. McKenney](/assets/img/avatar_default.png)
The expedited RCU primitives can be quite useful, but they have some high costs as well. This commit updates and creates docbook comments calling out the costs, and updates the RCU documentation as well. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
328 lines
11 KiB
C
328 lines
11 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/export.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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rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
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!lock_is_held(&rcu_bh_lock_map) &&
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!lock_is_held(&rcu_lock_map) &&
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!lock_is_held(&rcu_sched_lock_map),
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"Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section");
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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 - Brute-force SRCU grace period
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* @sp: srcu_struct with which to synchronize.
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*
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* Wait for an SRCU grace period to elapse, but use a "big hammer"
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* approach to force the grace period to end quickly. This consumes
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* significant time on all CPUs and is unfriendly to real-time workloads,
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* so is thus not recommended for any sort of common-case code. In fact,
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* if you are using synchronize_srcu_expedited() in a loop, please
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* restructure your code to batch your updates, and then use a single
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* synchronize_srcu() instead.
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*
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* Note that it is illegal to call this function while holding any lock
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* that is acquired by a CPU-hotplug notifier. And yes, it is also illegal
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* to call this function from a CPU-hotplug notifier. Failing to observe
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* these restriction will result in deadlock. It is also illegal to call
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* synchronize_srcu_expedited() from the corresponding SRCU read-side
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* critical section; doing so will result in deadlock. However, it is
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* perfectly legal to call synchronize_srcu_expedited() on one srcu_struct
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* from some other srcu_struct's read-side critical section, as long as
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* the resulting graph of srcu_structs is acyclic.
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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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