| /* |
| * 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/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> |
| |
| 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); |
| |
| /* |
| * 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 SYNCHRONIZE_SRCU_READER_DELAY 10 |
| |
| /* |
| * Helper function for synchronize_srcu() and synchronize_srcu_expedited(). |
| */ |
| static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void)) |
| { |
| int idx; |
| |
| 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"); |
| |
| 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. We initially give readers |
| * an arbitrarily chosen 10 microseconds to get out of their |
| * SRCU read-side critical sections, then loop waiting 1/HZ |
| * seconds per iteration. The 10-microsecond value has done |
| * very well in testing. |
| */ |
| |
| if (srcu_readers_active_idx(sp, idx)) |
| udelay(SYNCHRONIZE_SRCU_READER_DELAY); |
| 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); |