fs/btrfs/locking.c

   1 /*
   2  * Copyright (C) 2008 Oracle.  All rights reserved.
   3  *
   4  * This program is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU General Public
   6  * License v2 as published by the Free Software Foundation.
   7  *
   8  * This program is distributed in the hope that it will be useful,
   9  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  10  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  11  * General Public License for more details.
  12  *
  13  * You should have received a copy of the GNU General Public
  14  * License along with this program; if not, write to the
  15  * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  16  * Boston, MA 021110-1307, USA.
  17  */
  18 #include <linux/sched.h>
  19 #include <linux/gfp.h>
  20 #include <linux/pagemap.h>
  21 #include <linux/spinlock.h>
  22 #include <linux/page-flags.h>
  23 #include <asm/bug.h>
  24 #include "ctree.h"
  25 #include "extent_io.h"
  26 #include "locking.h"
  27
  28 /*
  29  * btrfs_header_level() isn't free, so don't call it when lockdep isn't
  30  * on
  31  */
  32 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  33 static inline void spin_nested(struct extent_buffer *eb)
  34 {
  35         spin_lock_nested(&eb->lock, BTRFS_MAX_LEVEL - btrfs_header_level(eb));
  36 }
  37 #else
  38 static inline void spin_nested(struct extent_buffer *eb)
  39 {
  40         spin_lock(&eb->lock);
  41 }
  42 #endif
  43
  44 /*
  45  * Setting a lock to blocking will drop the spinlock and set the
  46  * flag that forces other procs who want the lock to wait.  After
  47  * this you can safely schedule with the lock held.
  48  */
  49 void btrfs_set_lock_blocking(struct extent_buffer *eb)
  50 {
  51         if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) {
  52                 set_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags);
  53                 spin_unlock(&eb->lock);
  54         }
  55         /* exit with the spin lock released and the bit set */
  56 }
  57
  58 /*
  59  * clearing the blocking flag will take the spinlock again.
  60  * After this you can't safely schedule
  61  */
  62 void btrfs_clear_lock_blocking(struct extent_buffer *eb)
  63 {
  64         if (test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) {
  65                 spin_nested(eb);
  66                 clear_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags);
  67                 smp_mb__after_clear_bit();
  68         }
  69         /* exit with the spin lock held */
  70 }
  71
  72 /*
  73  * unfortunately, many of the places that currently set a lock to blocking
  74  * don't end up blocking for every long, and often they don't block
  75  * at all.  For a dbench 50 run, if we don't spin one the blocking bit
  76  * at all, the context switch rate can jump up to 400,000/sec or more.
  77  *
  78  * So, we're still stuck with this crummy spin on the blocking bit,
  79  * at least until the most common causes of the short blocks
  80  * can be dealt with.
  81  */
  82 static int btrfs_spin_on_block(struct extent_buffer *eb)
  83 {
  84         int i;
  85         for (i = 0; i < 512; i++) {
  86                 cpu_relax();
  87                 if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
  88                         return 1;
  89                 if (need_resched())
  90                         break;
  91         }
  92         return 0;
  93 }
  94
  95 /*
  96  * This is somewhat different from trylock.  It will take the
  97  * spinlock but if it finds the lock is set to blocking, it will
  98  * return without the lock held.
  99  *
 100  * returns 1 if it was able to take the lock and zero otherwise
 101  *
 102  * After this call, scheduling is not safe without first calling
 103  * btrfs_set_lock_blocking()
 104  */
 105 int btrfs_try_spin_lock(struct extent_buffer *eb)
 106 {
 107         int i;
 108
 109         spin_nested(eb);
 110         if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
 111                 return 1;
 112         spin_unlock(&eb->lock);
 113
 114         /* spin for a bit on the BLOCKING flag */
 115         for (i = 0; i < 2; i++) {
 116                 if (!btrfs_spin_on_block(eb))
 117                         break;
 118
 119                 spin_nested(eb);
 120                 if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
 121                         return 1;
 122                 spin_unlock(&eb->lock);
 123         }
 124         return 0;
 125 }
 126
 127 /*
 128  * the autoremove wake function will return 0 if it tried to wake up
 129  * a process that was already awake, which means that process won't
 130  * count as an exclusive wakeup.  The waitq code will continue waking
 131  * procs until it finds one that was actually sleeping.
 132  *
 133  * For btrfs, this isn't quite what we want.  We want a single proc
 134  * to be notified that the lock is ready for taking.  If that proc
 135  * already happen to be awake, great, it will loop around and try for
 136  * the lock.
 137  *
 138  * So, btrfs_wake_function always returns 1, even when the proc that we
 139  * tried to wake up was already awake.
 140  */
 141 static int btrfs_wake_function(wait_queue_t *wait, unsigned mode,
 142                                int sync, void *key)
 143 {
 144         autoremove_wake_function(wait, mode, sync, key);
 145         return 1;
 146 }
 147
 148 /*
 149  * returns with the extent buffer spinlocked.
 150  *
 151  * This will spin and/or wait as required to take the lock, and then
 152  * return with the spinlock held.
 153  *
 154  * After this call, scheduling is not safe without first calling
 155  * btrfs_set_lock_blocking()
 156  */
 157 int btrfs_tree_lock(struct extent_buffer *eb)
 158 {
 159         DEFINE_WAIT(wait);
 160         wait.func = btrfs_wake_function;
 161
 162         while(1) {
 163                 spin_nested(eb);
 164
 165                 /* nobody is blocking, exit with the spinlock held */
 166                 if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
 167                         return 0;
 168
 169                 /*
 170                  * we have the spinlock, but the real owner is blocking.
 171                  * wait for them
 172                  */
 173                 spin_unlock(&eb->lock);
 174
 175                 /*
 176                  * spin for a bit, and if the blocking flag goes away,
 177                  * loop around
 178                  */
 179                 if (btrfs_spin_on_block(eb))
 180                         continue;
 181
 182                 prepare_to_wait_exclusive(&eb->lock_wq, &wait,
 183                                           TASK_UNINTERRUPTIBLE);
 184
 185                 if (test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
 186                         schedule();
 187
 188                 finish_wait(&eb->lock_wq, &wait);
 189         }
 190         return 0;
 191 }
 192
 193 /*
 194  * Very quick trylock, this does not spin or schedule.  It returns
 195  * 1 with the spinlock held if it was able to take the lock, or it
 196  * returns zero if it was unable to take the lock.
 197  *
 198  * After this call, scheduling is not safe without first calling
 199  * btrfs_set_lock_blocking()
 200  */
 201 int btrfs_try_tree_lock(struct extent_buffer *eb)
 202 {
 203         if (spin_trylock(&eb->lock)) {
 204                 if (test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) {
 205                         /*
 206                          * we've got the spinlock, but the real owner is
 207                          * blocking.  Drop the spinlock and return failure
 208                          */
 209                         spin_unlock(&eb->lock);
 210                         return 0;
 211                 }
 212                 return 1;
 213         }
 214         /* someone else has the spinlock giveup */
 215         return 0;
 216 }
 217
 218 int btrfs_tree_unlock(struct extent_buffer *eb)
 219 {
 220         /*
 221          * if we were a blocking owner, we don't have the spinlock held
 222          * just clear the bit and look for waiters
 223          */
 224         if (test_and_clear_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
 225                 smp_mb__after_clear_bit();
 226         else
 227                 spin_unlock(&eb->lock);
 228
 229         if (waitqueue_active(&eb->lock_wq))
 230                 wake_up(&eb->lock_wq);
 231         return 0;
 232 }
 233
 234 int btrfs_tree_locked(struct extent_buffer *eb)
 235 {
 236         return test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags) ||
 237                         spin_is_locked(&eb->lock);
 238 }