linux/kernel/srcu.c
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   1/*
   2 * Sleepable Read-Copy Update mechanism for mutual exclusion.
   3 *
   4 * This program is free software; you can redistribute it and/or modify
   5 * it under the terms of the GNU General Public License as published by
   6 * the Free Software Foundation; either version 2 of the License, or
   7 * (at your option) any later version.
   8 *
   9 * This program is distributed in the hope that it will be useful,
  10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12 * GNU General Public License for more details.
  13 *
  14 * You should have received a copy of the GNU General Public License
  15 * along with this program; if not, write to the Free Software
  16 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  17 *
  18 * Copyright (C) IBM Corporation, 2006
  19 *
  20 * Author: Paul McKenney <paulmck@us.ibm.com>
  21 *
  22 * For detailed explanation of Read-Copy Update mechanism see -
  23 *              Documentation/RCU/ *.txt
  24 *
  25 */
  26
  27#include <linux/module.h>
  28#include <linux/mutex.h>
  29#include <linux/percpu.h>
  30#include <linux/preempt.h>
  31#include <linux/rcupdate.h>
  32#include <linux/sched.h>
  33#include <linux/smp.h>
  34#include <linux/srcu.h>
  35
  36static int init_srcu_struct_fields(struct srcu_struct *sp)
  37{
  38        sp->completed = 0;
  39        mutex_init(&sp->mutex);
  40        sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
  41        return sp->per_cpu_ref ? 0 : -ENOMEM;
  42}
  43
  44#ifdef CONFIG_DEBUG_LOCK_ALLOC
  45
  46int __init_srcu_struct(struct srcu_struct *sp, const char *name,
  47                       struct lock_class_key *key)
  48{
  49        /* Don't re-initialize a lock while it is held. */
  50        debug_check_no_locks_freed((void *)sp, sizeof(*sp));
  51        lockdep_init_map(&sp->dep_map, name, key, 0);
  52        return init_srcu_struct_fields(sp);
  53}
  54EXPORT_SYMBOL_GPL(__init_srcu_struct);
  55
  56#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  57
  58/**
  59 * init_srcu_struct - initialize a sleep-RCU structure
  60 * @sp: structure to initialize.
  61 *
  62 * Must invoke this on a given srcu_struct before passing that srcu_struct
  63 * to any other function.  Each srcu_struct represents a separate domain
  64 * of SRCU protection.
  65 */
  66int init_srcu_struct(struct srcu_struct *sp)
  67{
  68        return init_srcu_struct_fields(sp);
  69}
  70EXPORT_SYMBOL_GPL(init_srcu_struct);
  71
  72#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  73
  74/*
  75 * srcu_readers_active_idx -- returns approximate number of readers
  76 *      active on the specified rank of per-CPU counters.
  77 */
  78
  79static int srcu_readers_active_idx(struct srcu_struct *sp, int idx)
  80{
  81        int cpu;
  82        int sum;
  83
  84        sum = 0;
  85        for_each_possible_cpu(cpu)
  86                sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];
  87        return sum;
  88}
  89
  90/**
  91 * srcu_readers_active - returns approximate number of readers.
  92 * @sp: which srcu_struct to count active readers (holding srcu_read_lock).
  93 *
  94 * Note that this is not an atomic primitive, and can therefore suffer
  95 * severe errors when invoked on an active srcu_struct.  That said, it
  96 * can be useful as an error check at cleanup time.
  97 */
  98static int srcu_readers_active(struct srcu_struct *sp)
  99{
 100        return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1);
 101}
 102
 103/**
 104 * cleanup_srcu_struct - deconstruct a sleep-RCU structure
 105 * @sp: structure to clean up.
 106 *
 107 * Must invoke this after you are finished using a given srcu_struct that
 108 * was initialized via init_srcu_struct(), else you leak memory.
 109 */
 110void cleanup_srcu_struct(struct srcu_struct *sp)
 111{
 112        int sum;
 113
 114        sum = srcu_readers_active(sp);
 115        WARN_ON(sum);  /* Leakage unless caller handles error. */
 116        if (sum != 0)
 117                return;
 118        free_percpu(sp->per_cpu_ref);
 119        sp->per_cpu_ref = NULL;
 120}
 121EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
 122
 123/*
 124 * Counts the new reader in the appropriate per-CPU element of the
 125 * srcu_struct.  Must be called from process context.
 126 * Returns an index that must be passed to the matching srcu_read_unlock().
 127 */
 128int __srcu_read_lock(struct srcu_struct *sp)
 129{
 130        int idx;
 131
 132        preempt_disable();
 133        idx = sp->completed & 0x1;
 134        barrier();  /* ensure compiler looks -once- at sp->completed. */
 135        per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
 136        srcu_barrier();  /* ensure compiler won't misorder critical section. */
 137        preempt_enable();
 138        return idx;
 139}
 140EXPORT_SYMBOL_GPL(__srcu_read_lock);
 141
 142/*
 143 * Removes the count for the old reader from the appropriate per-CPU
 144 * element of the srcu_struct.  Note that this may well be a different
 145 * CPU than that which was incremented by the corresponding srcu_read_lock().
 146 * Must be called from process context.
 147 */
 148void __srcu_read_unlock(struct srcu_struct *sp, int idx)
 149{
 150        preempt_disable();
 151        srcu_barrier();  /* ensure compiler won't misorder critical section. */
 152        per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;
 153        preempt_enable();
 154}
 155EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 156
 157/*
 158 * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
 159 */
 160static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
 161{
 162        int idx;
 163
 164        idx = sp->completed;
 165        mutex_lock(&sp->mutex);
 166
 167        /*
 168         * Check to see if someone else did the work for us while we were
 169         * waiting to acquire the lock.  We need -two- advances of
 170         * the counter, not just one.  If there was but one, we might have
 171         * shown up -after- our helper's first synchronize_sched(), thus
 172         * having failed to prevent CPU-reordering races with concurrent
 173         * srcu_read_unlock()s on other CPUs (see comment below).  So we
 174         * either (1) wait for two or (2) supply the second ourselves.
 175         */
 176
 177        if ((sp->completed - idx) >= 2) {
 178                mutex_unlock(&sp->mutex);
 179                return;
 180        }
 181
 182        sync_func();  /* Force memory barrier on all CPUs. */
 183
 184        /*
 185         * The preceding synchronize_sched() ensures that any CPU that
 186         * sees the new value of sp->completed will also see any preceding
 187         * changes to data structures made by this CPU.  This prevents
 188         * some other CPU from reordering the accesses in its SRCU
 189         * read-side critical section to precede the corresponding
 190         * srcu_read_lock() -- ensuring that such references will in
 191         * fact be protected.
 192         *
 193         * So it is now safe to do the flip.
 194         */
 195
 196        idx = sp->completed & 0x1;
 197        sp->completed++;
 198
 199        sync_func();  /* Force memory barrier on all CPUs. */
 200
 201        /*
 202         * At this point, because of the preceding synchronize_sched(),
 203         * all srcu_read_lock() calls using the old counters have completed.
 204         * Their corresponding critical sections might well be still
 205         * executing, but the srcu_read_lock() primitives themselves
 206         * will have finished executing.
 207         */
 208
 209        while (srcu_readers_active_idx(sp, idx))
 210                schedule_timeout_interruptible(1);
 211
 212        sync_func();  /* Force memory barrier on all CPUs. */
 213
 214        /*
 215         * The preceding synchronize_sched() forces all srcu_read_unlock()
 216         * primitives that were executing concurrently with the preceding
 217         * for_each_possible_cpu() loop to have completed by this point.
 218         * More importantly, it also forces the corresponding SRCU read-side
 219         * critical sections to have also completed, and the corresponding
 220         * references to SRCU-protected data items to be dropped.
 221         *
 222         * Note:
 223         *
 224         *      Despite what you might think at first glance, the
 225         *      preceding synchronize_sched() -must- be within the
 226         *      critical section ended by the following mutex_unlock().
 227         *      Otherwise, a task taking the early exit can race
 228         *      with a srcu_read_unlock(), which might have executed
 229         *      just before the preceding srcu_readers_active() check,
 230         *      and whose CPU might have reordered the srcu_read_unlock()
 231         *      with the preceding critical section.  In this case, there
 232         *      is nothing preventing the synchronize_sched() task that is
 233         *      taking the early exit from freeing a data structure that
 234         *      is still being referenced (out of order) by the task
 235         *      doing the srcu_read_unlock().
 236         *
 237         *      Alternatively, the comparison with "2" on the early exit
 238         *      could be changed to "3", but this increases synchronize_srcu()
 239         *      latency for bulk loads.  So the current code is preferred.
 240         */
 241
 242        mutex_unlock(&sp->mutex);
 243}
 244
 245/**
 246 * synchronize_srcu - wait for prior SRCU read-side critical-section completion
 247 * @sp: srcu_struct with which to synchronize.
 248 *
 249 * Flip the completed counter, and wait for the old count to drain to zero.
 250 * As with classic RCU, the updater must use some separate means of
 251 * synchronizing concurrent updates.  Can block; must be called from
 252 * process context.
 253 *
 254 * Note that it is illegal to call synchronize_srcu() from the corresponding
 255 * SRCU read-side critical section; doing so will result in deadlock.
 256 * However, it is perfectly legal to call synchronize_srcu() on one
 257 * srcu_struct from some other srcu_struct's read-side critical section.
 258 */
 259void synchronize_srcu(struct srcu_struct *sp)
 260{
 261        __synchronize_srcu(sp, synchronize_sched);
 262}
 263EXPORT_SYMBOL_GPL(synchronize_srcu);
 264
 265/**
 266 * synchronize_srcu_expedited - like synchronize_srcu, but less patient
 267 * @sp: srcu_struct with which to synchronize.
 268 *
 269 * Flip the completed counter, and wait for the old count to drain to zero.
 270 * As with classic RCU, the updater must use some separate means of
 271 * synchronizing concurrent updates.  Can block; must be called from
 272 * process context.
 273 *
 274 * Note that it is illegal to call synchronize_srcu_expedited()
 275 * from the corresponding SRCU read-side critical section; doing so
 276 * will result in deadlock.  However, it is perfectly legal to call
 277 * synchronize_srcu_expedited() on one srcu_struct from some other
 278 * srcu_struct's read-side critical section.
 279 */
 280void synchronize_srcu_expedited(struct srcu_struct *sp)
 281{
 282        __synchronize_srcu(sp, synchronize_sched_expedited);
 283}
 284EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
 285
 286/**
 287 * srcu_batches_completed - return batches completed.
 288 * @sp: srcu_struct on which to report batch completion.
 289 *
 290 * Report the number of batches, correlated with, but not necessarily
 291 * precisely the same as, the number of grace periods that have elapsed.
 292 */
 293
 294long srcu_batches_completed(struct srcu_struct *sp)
 295{
 296        return sp->completed;
 297}
 298EXPORT_SYMBOL_GPL(srcu_batches_completed);
 299