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