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/delay.h>
  35#include <linux/srcu.h>
  36
  37static int init_srcu_struct_fields(struct srcu_struct *sp)
  38{
  39        sp->completed = 0;
  40        mutex_init(&sp->mutex);
  41        sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
  42        return sp->per_cpu_ref ? 0 : -ENOMEM;
  43}
  44
  45#ifdef CONFIG_DEBUG_LOCK_ALLOC
  46
  47int __init_srcu_struct(struct srcu_struct *sp, const char *name,
  48                       struct lock_class_key *key)
  49{
  50        /* Don't re-initialize a lock while it is held. */
  51        debug_check_no_locks_freed((void *)sp, sizeof(*sp));
  52        lockdep_init_map(&sp->dep_map, name, key, 0);
  53        return init_srcu_struct_fields(sp);
  54}
  55EXPORT_SYMBOL_GPL(__init_srcu_struct);
  56
  57#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  58
  59/**
  60 * init_srcu_struct - initialize a sleep-RCU structure
  61 * @sp: structure to initialize.
  62 *
  63 * Must invoke this on a given srcu_struct before passing that srcu_struct
  64 * to any other function.  Each srcu_struct represents a separate domain
  65 * of SRCU protection.
  66 */
  67int init_srcu_struct(struct srcu_struct *sp)
  68{
  69        return init_srcu_struct_fields(sp);
  70}
  71EXPORT_SYMBOL_GPL(init_srcu_struct);
  72
  73#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  74
  75/*
  76 * srcu_readers_active_idx -- returns approximate number of readers
  77 *      active on the specified rank of per-CPU counters.
  78 */
  79
  80static int srcu_readers_active_idx(struct srcu_struct *sp, int idx)
  81{
  82        int cpu;
  83        int sum;
  84
  85        sum = 0;
  86        for_each_possible_cpu(cpu)
  87                sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];
  88        return sum;
  89}
  90
  91/**
  92 * srcu_readers_active - returns approximate number of readers.
  93 * @sp: which srcu_struct to count active readers (holding srcu_read_lock).
  94 *
  95 * Note that this is not an atomic primitive, and can therefore suffer
  96 * severe errors when invoked on an active srcu_struct.  That said, it
  97 * can be useful as an error check at cleanup time.
  98 */
  99static int srcu_readers_active(struct srcu_struct *sp)
 100{
 101        return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1);
 102}
 103
 104/**
 105 * cleanup_srcu_struct - deconstruct a sleep-RCU structure
 106 * @sp: structure to clean up.
 107 *
 108 * Must invoke this after you are finished using a given srcu_struct that
 109 * was initialized via init_srcu_struct(), else you leak memory.
 110 */
 111void cleanup_srcu_struct(struct srcu_struct *sp)
 112{
 113        int sum;
 114
 115        sum = srcu_readers_active(sp);
 116        WARN_ON(sum);  /* Leakage unless caller handles error. */
 117        if (sum != 0)
 118                return;
 119        free_percpu(sp->per_cpu_ref);
 120        sp->per_cpu_ref = NULL;
 121}
 122EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
 123
 124/*
 125 * Counts the new reader in the appropriate per-CPU element of the
 126 * srcu_struct.  Must be called from process context.
 127 * Returns an index that must be passed to the matching srcu_read_unlock().
 128 */
 129int __srcu_read_lock(struct srcu_struct *sp)
 130{
 131        int idx;
 132
 133        preempt_disable();
 134        idx = sp->completed & 0x1;
 135        barrier();  /* ensure compiler looks -once- at sp->completed. */
 136        per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
 137        srcu_barrier();  /* ensure compiler won't misorder critical section. */
 138        preempt_enable();
 139        return idx;
 140}
 141EXPORT_SYMBOL_GPL(__srcu_read_lock);
 142
 143/*
 144 * Removes the count for the old reader from the appropriate per-CPU
 145 * element of the srcu_struct.  Note that this may well be a different
 146 * CPU than that which was incremented by the corresponding srcu_read_lock().
 147 * Must be called from process context.
 148 */
 149void __srcu_read_unlock(struct srcu_struct *sp, int idx)
 150{
 151        preempt_disable();
 152        srcu_barrier();  /* ensure compiler won't misorder critical section. */
 153        per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;
 154        preempt_enable();
 155}
 156EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 157
 158/*
 159 * We use an adaptive strategy for synchronize_srcu() and especially for
 160 * synchronize_srcu_expedited().  We spin for a fixed time period
 161 * (defined below) to allow SRCU readers to exit their read-side critical
 162 * sections.  If there are still some readers after 10 microseconds,
 163 * we repeatedly block for 1-millisecond time periods.  This approach
 164 * has done well in testing, so there is no need for a config parameter.
 165 */
 166#define SYNCHRONIZE_SRCU_READER_DELAY 10
 167
 168/*
 169 * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
 170 */
 171static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
 172{
 173        int idx;
 174
 175        idx = sp->completed;
 176        mutex_lock(&sp->mutex);
 177
 178        /*
 179         * Check to see if someone else did the work for us while we were
 180         * waiting to acquire the lock.  We need -two- advances of
 181         * the counter, not just one.  If there was but one, we might have
 182         * shown up -after- our helper's first synchronize_sched(), thus
 183         * having failed to prevent CPU-reordering races with concurrent
 184         * srcu_read_unlock()s on other CPUs (see comment below).  So we
 185         * either (1) wait for two or (2) supply the second ourselves.
 186         */
 187
 188        if ((sp->completed - idx) >= 2) {
 189                mutex_unlock(&sp->mutex);
 190                return;
 191        }
 192
 193        sync_func();  /* Force memory barrier on all CPUs. */
 194
 195        /*
 196         * The preceding synchronize_sched() ensures that any CPU that
 197         * sees the new value of sp->completed will also see any preceding
 198         * changes to data structures made by this CPU.  This prevents
 199         * some other CPU from reordering the accesses in its SRCU
 200         * read-side critical section to precede the corresponding
 201         * srcu_read_lock() -- ensuring that such references will in
 202         * fact be protected.
 203         *
 204         * So it is now safe to do the flip.
 205         */
 206
 207        idx = sp->completed & 0x1;
 208        sp->completed++;
 209
 210        sync_func();  /* Force memory barrier on all CPUs. */
 211
 212        /*
 213         * At this point, because of the preceding synchronize_sched(),
 214         * all srcu_read_lock() calls using the old counters have completed.
 215         * Their corresponding critical sections might well be still
 216         * executing, but the srcu_read_lock() primitives themselves
 217         * will have finished executing.  We initially give readers
 218         * an arbitrarily chosen 10 microseconds to get out of their
 219         * SRCU read-side critical sections, then loop waiting 1/HZ
 220         * seconds per iteration.  The 10-microsecond value has done
 221         * very well in testing.
 222         */
 223
 224        if (srcu_readers_active_idx(sp, idx))
 225                udelay(SYNCHRONIZE_SRCU_READER_DELAY);
 226        while (srcu_readers_active_idx(sp, idx))
 227                schedule_timeout_interruptible(1);
 228
 229        sync_func();  /* Force memory barrier on all CPUs. */
 230
 231        /*
 232         * The preceding synchronize_sched() forces all srcu_read_unlock()
 233         * primitives that were executing concurrently with the preceding
 234         * for_each_possible_cpu() loop to have completed by this point.
 235         * More importantly, it also forces the corresponding SRCU read-side
 236         * critical sections to have also completed, and the corresponding
 237         * references to SRCU-protected data items to be dropped.
 238         *
 239         * Note:
 240         *
 241         *      Despite what you might think at first glance, the
 242         *      preceding synchronize_sched() -must- be within the
 243         *      critical section ended by the following mutex_unlock().
 244         *      Otherwise, a task taking the early exit can race
 245         *      with a srcu_read_unlock(), which might have executed
 246         *      just before the preceding srcu_readers_active() check,
 247         *      and whose CPU might have reordered the srcu_read_unlock()
 248         *      with the preceding critical section.  In this case, there
 249         *      is nothing preventing the synchronize_sched() task that is
 250         *      taking the early exit from freeing a data structure that
 251         *      is still being referenced (out of order) by the task
 252         *      doing the srcu_read_unlock().
 253         *
 254         *      Alternatively, the comparison with "2" on the early exit
 255         *      could be changed to "3", but this increases synchronize_srcu()
 256         *      latency for bulk loads.  So the current code is preferred.
 257         */
 258
 259        mutex_unlock(&sp->mutex);
 260}
 261
 262/**
 263 * synchronize_srcu - wait for prior SRCU read-side critical-section completion
 264 * @sp: srcu_struct with which to synchronize.
 265 *
 266 * Flip the completed counter, and wait for the old count to drain to zero.
 267 * As with classic RCU, the updater must use some separate means of
 268 * synchronizing concurrent updates.  Can block; must be called from
 269 * process context.
 270 *
 271 * Note that it is illegal to call synchronize_srcu() from the corresponding
 272 * SRCU read-side critical section; doing so will result in deadlock.
 273 * However, it is perfectly legal to call synchronize_srcu() on one
 274 * srcu_struct from some other srcu_struct's read-side critical section.
 275 */
 276void synchronize_srcu(struct srcu_struct *sp)
 277{
 278        __synchronize_srcu(sp, synchronize_sched);
 279}
 280EXPORT_SYMBOL_GPL(synchronize_srcu);
 281
 282/**
 283 * synchronize_srcu_expedited - like synchronize_srcu, but less patient
 284 * @sp: srcu_struct with which to synchronize.
 285 *
 286 * Flip the completed counter, and wait for the old count to drain to zero.
 287 * As with classic RCU, the updater must use some separate means of
 288 * synchronizing concurrent updates.  Can block; must be called from
 289 * process context.
 290 *
 291 * Note that it is illegal to call synchronize_srcu_expedited()
 292 * from the corresponding SRCU read-side critical section; doing so
 293 * will result in deadlock.  However, it is perfectly legal to call
 294 * synchronize_srcu_expedited() on one srcu_struct from some other
 295 * srcu_struct's read-side critical section.
 296 */
 297void synchronize_srcu_expedited(struct srcu_struct *sp)
 298{
 299        __synchronize_srcu(sp, synchronize_sched_expedited);
 300}
 301EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
 302
 303/**
 304 * srcu_batches_completed - return batches completed.
 305 * @sp: srcu_struct on which to report batch completion.
 306 *
 307 * Report the number of batches, correlated with, but not necessarily
 308 * precisely the same as, the number of grace periods that have elapsed.
 309 */
 310
 311long srcu_batches_completed(struct srcu_struct *sp)
 312{
 313        return sp->completed;
 314}
 315EXPORT_SYMBOL_GPL(srcu_batches_completed);
 316
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