/* * 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 * * Author: Paul McKenney <paulmck@us.ibm.com> * * For detailed explanation of Read-Copy Update mechanism see - * 		Documentation/RCU/ *.txt * */ #include <linux/module.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/srcu.h> static int init_srcu_struct_fields(struct srcu_struct *sp){	sp->completed = 0;	mutex_init(&sp->mutex);	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 */ /* * srcu_readers_active_idx -- returns approximate number of readers *	active on the specified rank of per-CPU counters. */ static int srcu_readers_active_idx(struct srcu_struct *sp, int idx){	int cpu;	int sum; 	sum = 0;	for_each_possible_cpu(cpu)		sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];	return sum;} /** * 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){	return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1);} /** * 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 = sp->completed & 0x1;	barrier();  /* ensure compiler looks -once- at sp->completed. */	per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;	srcu_barrier();  /* ensure compiler won't misorder critical section. */	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();	srcu_barrier();  /* ensure compiler won't misorder critical section. */	per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;	preempt_enable();}EXPORT_SYMBOL_GPL(__srcu_read_unlock); /* * Helper function for synchronize_srcu() and synchronize_srcu_expedited(). */static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void)){	int idx; 	idx = sp->completed;	mutex_lock(&sp->mutex); 	/*	 * Check to see if someone else did the work for us while we were	 * waiting to acquire the lock.  We need -two- advances of	 * the counter, not just one.  If there was but one, we might have	 * shown up -after- our helper's first synchronize_sched(), thus	 * having failed to prevent CPU-reordering races with concurrent	 * srcu_read_unlock()s on other CPUs (see comment below).  So we	 * either (1) wait for two or (2) supply the second ourselves.	 */ 	if ((sp->completed - idx) >= 2) {		mutex_unlock(&sp->mutex);		return;	} 	sync_func();  /* Force memory barrier on all CPUs. */ 	/*	 * The preceding synchronize_sched() ensures that any CPU that	 * sees the new value of sp->completed will also see any preceding	 * changes to data structures made by this CPU.  This prevents	 * some other CPU from reordering the accesses in its SRCU	 * read-side critical section to precede the corresponding	 * srcu_read_lock() -- ensuring that such references will in	 * fact be protected.	 *	 * So it is now safe to do the flip.	 */ 	idx = sp->completed & 0x1;	sp->completed++; 	sync_func();  /* Force memory barrier on all CPUs. */ 	/*	 * At this point, because of the preceding synchronize_sched(),	 * all srcu_read_lock() calls using the old counters have completed.	 * Their corresponding critical sections might well be still	 * executing, but the srcu_read_lock() primitives themselves	 * will have finished executing.	 */ 	while (srcu_readers_active_idx(sp, idx))		schedule_timeout_interruptible(1); 	sync_func();  /* Force memory barrier on all CPUs. */ 	/*	 * The preceding synchronize_sched() forces all srcu_read_unlock()	 * primitives that were executing concurrently with the preceding	 * for_each_possible_cpu() loop to have completed by this point.	 * More importantly, it also forces the corresponding SRCU read-side	 * critical sections to have also completed, and the corresponding	 * references to SRCU-protected data items to be dropped.	 *	 * Note:	 *	 *	Despite what you might think at first glance, the	 *	preceding synchronize_sched() -must- be within the	 *	critical section ended by the following mutex_unlock().	 *	Otherwise, a task taking the early exit can race	 *	with a srcu_read_unlock(), which might have executed	 *	just before the preceding srcu_readers_active() check,	 *	and whose CPU might have reordered the srcu_read_unlock()	 *	with the preceding critical section.  In this case, there	 *	is nothing preventing the synchronize_sched() task that is	 *	taking the early exit from freeing a data structure that	 *	is still being referenced (out of order) by the task	 *	doing the srcu_read_unlock().	 *	 *	Alternatively, the comparison with "2" on the early exit	 *	could be changed to "3", but this increases synchronize_srcu()	 *	latency for bulk loads.  So the current code is preferred.	 */ 	mutex_unlock(&sp->mutex);} /** * 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, synchronize_sched);}EXPORT_SYMBOL_GPL(synchronize_srcu); /** * synchronize_srcu_expedited - like synchronize_srcu, but less patient * @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_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. */void synchronize_srcu_expedited(struct srcu_struct *sp){	__synchronize_srcu(sp, synchronize_sched_expedited);}EXPORT_SYMBOL_GPL(synchronize_srcu_expedited); /** * 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);