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