kernel/srcu.c

   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
  36 static 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
  46 int __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 }
  54 EXPORT_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  */
  66 int init_srcu_struct(struct srcu_struct *sp)
  67 {
  68         return init_srcu_struct_fields(sp);
  69 }
  70 EXPORT_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
  79 static 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  */
  98 static 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  */
 110 void 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 }
 121 EXPORT_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  */
 128 int __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 }
 140 EXPORT_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  */
 148 void __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 }
 155 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 156
 157 /*
 158  * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
 159  */
 160 static 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  */
 259 void synchronize_srcu(struct srcu_struct *sp)
 260 {
 261         __synchronize_srcu(sp, synchronize_sched);
 262 }
 263 EXPORT_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  */
 280 void synchronize_srcu_expedited(struct srcu_struct *sp)
 281 {
 282         __synchronize_srcu(sp, synchronize_sched_expedited);
 283 }
 284 EXPORT_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
 294 long srcu_batches_completed(struct srcu_struct *sp)
 295 {
 296         return sp->completed;
 297 }
 298 EXPORT_SYMBOL_GPL(srcu_batches_completed);