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