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/delay.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         /* 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 }
  55 EXPORT_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  */
  67 int init_srcu_struct(struct srcu_struct *sp)
  68 {
  69         return init_srcu_struct_fields(sp);
  70 }
  71 EXPORT_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
  80 static 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  */
  99 static 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  */
 111 void 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 }
 122 EXPORT_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  */
 129 int __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 }
 141 EXPORT_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  */
 149 void __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 }
 156 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 157
 158 /*
 159  * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
 160  */
 161 static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
 162 {
 163         int idx;
 164
 165         idx = sp->completed;
 166         mutex_lock(&sp->mutex);
 167
 168         /*
 169          * Check to see if someone else did the work for us while we were
 170          * waiting to acquire the lock.  We need -two- advances of
 171          * the counter, not just one.  If there was but one, we might have
 172          * shown up -after- our helper's first synchronize_sched(), thus
 173          * having failed to prevent CPU-reordering races with concurrent
 174          * srcu_read_unlock()s on other CPUs (see comment below).  So we
 175          * either (1) wait for two or (2) supply the second ourselves.
 176          */
 177
 178         if ((sp->completed - idx) >= 2) {
 179                 mutex_unlock(&sp->mutex);
 180                 return;
 181         }
 182
 183         sync_func();  /* Force memory barrier on all CPUs. */
 184
 185         /*
 186          * The preceding synchronize_sched() ensures that any CPU that
 187          * sees the new value of sp->completed will also see any preceding
 188          * changes to data structures made by this CPU.  This prevents
 189          * some other CPU from reordering the accesses in its SRCU
 190          * read-side critical section to precede the corresponding
 191          * srcu_read_lock() -- ensuring that such references will in
 192          * fact be protected.
 193          *
 194          * So it is now safe to do the flip.
 195          */
 196
 197         idx = sp->completed & 0x1;
 198         sp->completed++;
 199
 200         sync_func();  /* Force memory barrier on all CPUs. */
 201
 202         /*
 203          * At this point, because of the preceding synchronize_sched(),
 204          * all srcu_read_lock() calls using the old counters have completed.
 205          * Their corresponding critical sections might well be still
 206          * executing, but the srcu_read_lock() primitives themselves
 207          * will have finished executing.  We initially give readers
 208          * an arbitrarily chosen 10 microseconds to get out of their
 209          * SRCU read-side critical sections, then loop waiting 1/HZ
 210          * seconds per iteration.
 211          */
 212
 213         if (srcu_readers_active_idx(sp, idx))
 214                 udelay(CONFIG_SRCU_SYNCHRONIZE_DELAY);
 215         while (srcu_readers_active_idx(sp, idx))
 216                 schedule_timeout_interruptible(1);
 217
 218         sync_func();  /* Force memory barrier on all CPUs. */
 219
 220         /*
 221          * The preceding synchronize_sched() forces all srcu_read_unlock()
 222          * primitives that were executing concurrently with the preceding
 223          * for_each_possible_cpu() loop to have completed by this point.
 224          * More importantly, it also forces the corresponding SRCU read-side
 225          * critical sections to have also completed, and the corresponding
 226          * references to SRCU-protected data items to be dropped.
 227          *
 228          * Note:
 229          *
 230          *      Despite what you might think at first glance, the
 231          *      preceding synchronize_sched() -must- be within the
 232          *      critical section ended by the following mutex_unlock().
 233          *      Otherwise, a task taking the early exit can race
 234          *      with a srcu_read_unlock(), which might have executed
 235          *      just before the preceding srcu_readers_active() check,
 236          *      and whose CPU might have reordered the srcu_read_unlock()
 237          *      with the preceding critical section.  In this case, there
 238          *      is nothing preventing the synchronize_sched() task that is
 239          *      taking the early exit from freeing a data structure that
 240          *      is still being referenced (out of order) by the task
 241          *      doing the srcu_read_unlock().
 242          *
 243          *      Alternatively, the comparison with "2" on the early exit
 244          *      could be changed to "3", but this increases synchronize_srcu()
 245          *      latency for bulk loads.  So the current code is preferred.
 246          */
 247
 248         mutex_unlock(&sp->mutex);
 249 }
 250
 251 /**
 252  * synchronize_srcu - wait for prior SRCU read-side critical-section completion
 253  * @sp: srcu_struct with which to synchronize.
 254  *
 255  * Flip the completed counter, and wait for the old count to drain to zero.
 256  * As with classic RCU, the updater must use some separate means of
 257  * synchronizing concurrent updates.  Can block; must be called from
 258  * process context.
 259  *
 260  * Note that it is illegal to call synchronize_srcu() from the corresponding
 261  * SRCU read-side critical section; doing so will result in deadlock.
 262  * However, it is perfectly legal to call synchronize_srcu() on one
 263  * srcu_struct from some other srcu_struct's read-side critical section.
 264  */
 265 void synchronize_srcu(struct srcu_struct *sp)
 266 {
 267         __synchronize_srcu(sp, synchronize_sched);
 268 }
 269 EXPORT_SYMBOL_GPL(synchronize_srcu);
 270
 271 /**
 272  * synchronize_srcu_expedited - like synchronize_srcu, but less patient
 273  * @sp: srcu_struct with which to synchronize.
 274  *
 275  * Flip the completed counter, and wait for the old count to drain to zero.
 276  * As with classic RCU, the updater must use some separate means of
 277  * synchronizing concurrent updates.  Can block; must be called from
 278  * process context.
 279  *
 280  * Note that it is illegal to call synchronize_srcu_expedited()
 281  * from the corresponding SRCU read-side critical section; doing so
 282  * will result in deadlock.  However, it is perfectly legal to call
 283  * synchronize_srcu_expedited() on one srcu_struct from some other
 284  * srcu_struct's read-side critical section.
 285  */
 286 void synchronize_srcu_expedited(struct srcu_struct *sp)
 287 {
 288         __synchronize_srcu(sp, synchronize_sched_expedited);
 289 }
 290 EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
 291
 292 /**
 293  * srcu_batches_completed - return batches completed.
 294  * @sp: srcu_struct on which to report batch completion.
 295  *
 296  * Report the number of batches, correlated with, but not necessarily
 297  * precisely the same as, the number of grace periods that have elapsed.
 298  */
 299
 300 long srcu_batches_completed(struct srcu_struct *sp)
 301 {
 302         return sp->completed;
 303 }
 304 EXPORT_SYMBOL_GPL(srcu_batches_completed);