/* * 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/slab.h> #include <linux/smp.h> #include <linux/srcu.h> /** * 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) { sp->completed = 0; mutex_init(&sp->mutex); sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array); return (sp->per_cpu_ref ? 0 : -ENOMEM); } /* * 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; } /** * srcu_read_lock - register a new reader for an SRCU-protected structure. * @sp: srcu_struct in which to register the new reader. * * 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; } /** * srcu_read_unlock - unregister a old reader from an SRCU-protected structure. * @sp: srcu_struct in which to unregister the old reader. * @idx: return value from corresponding 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(); } /** * 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 synchornize_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) { 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; } synchronize_sched(); /* 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++; synchronize_sched(); /* 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); synchronize_sched(); /* 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); } /** * 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(init_srcu_struct); EXPORT_SYMBOL_GPL(cleanup_srcu_struct); EXPORT_SYMBOL_GPL(srcu_read_lock); EXPORT_SYMBOL_GPL(srcu_read_unlock); EXPORT_SYMBOL_GPL(synchronize_srcu); EXPORT_SYMBOL_GPL(srcu_batches_completed);