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/pagemap.h>
  20 #include <linux/spinlock.h>
  21 #include <linux/page-flags.h>
  22 #include <asm/bug.h>
  23 #include "ctree.h"
  24 #include "extent_io.h"
  25 #include "locking.h"
  26
  27 static inline void spin_nested(struct extent_buffer *eb)
  28 {
  29         spin_lock(&eb->lock);
  30 }
  31
  32 /*
  33  * Setting a lock to blocking will drop the spinlock and set the
  34  * flag that forces other procs who want the lock to wait.  After
  35  * this you can safely schedule with the lock held.
  36  */
  37 void btrfs_set_lock_blocking(struct extent_buffer *eb)
  38 {
  39         if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) {
  40                 set_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags);
  41                 spin_unlock(&eb->lock);
  42         }
  43         /* exit with the spin lock released and the bit set */
  44 }
  45
  46 /*
  47  * clearing the blocking flag will take the spinlock again.
  48  * After this you can't safely schedule
  49  */
  50 void btrfs_clear_lock_blocking(struct extent_buffer *eb)
  51 {
  52         if (test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) {
  53                 spin_nested(eb);
  54                 clear_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags);
  55                 smp_mb__after_clear_bit();
  56         }
  57         /* exit with the spin lock held */
  58 }
  59
  60 /*
  61  * unfortunately, many of the places that currently set a lock to blocking
  62  * don't end up blocking for very long, and often they don't block
  63  * at all.  For a dbench 50 run, if we don't spin on the blocking bit
  64  * at all, the context switch rate can jump up to 400,000/sec or more.
  65  *
  66  * So, we're still stuck with this crummy spin on the blocking bit,
  67  * at least until the most common causes of the short blocks
  68  * can be dealt with.
  69  */
  70 static int btrfs_spin_on_block(struct extent_buffer *eb)
  71 {
  72         int i;
  73
  74         for (i = 0; i < 512; i++) {
  75                 if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
  76                         return 1;
  77                 if (need_resched())
  78                         break;
  79                 cpu_relax();
  80         }
  81         return 0;
  82 }
  83
  84 /*
  85  * This is somewhat different from trylock.  It will take the
  86  * spinlock but if it finds the lock is set to blocking, it will
  87  * return without the lock held.
  88  *
  89  * returns 1 if it was able to take the lock and zero otherwise
  90  *
  91  * After this call, scheduling is not safe without first calling
  92  * btrfs_set_lock_blocking()
  93  */
  94 int btrfs_try_spin_lock(struct extent_buffer *eb)
  95 {
  96         int i;
  97
  98         if (btrfs_spin_on_block(eb)) {
  99                 spin_nested(eb);
 100                 if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
 101                         return 1;
 102                 spin_unlock(&eb->lock);
 103         }
 104         /* spin for a bit on the BLOCKING flag */
 105         for (i = 0; i < 2; i++) {
 106                 cpu_relax();
 107                 if (!btrfs_spin_on_block(eb))
 108                         break;
 109
 110                 spin_nested(eb);
 111                 if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
 112                         return 1;
 113                 spin_unlock(&eb->lock);
 114         }
 115         return 0;
 116 }
 117
 118 /*
 119  * the autoremove wake function will return 0 if it tried to wake up
 120  * a process that was already awake, which means that process won't
 121  * count as an exclusive wakeup.  The waitq code will continue waking
 122  * procs until it finds one that was actually sleeping.
 123  *
 124  * For btrfs, this isn't quite what we want.  We want a single proc
 125  * to be notified that the lock is ready for taking.  If that proc
 126  * already happen to be awake, great, it will loop around and try for
 127  * the lock.
 128  *
 129  * So, btrfs_wake_function always returns 1, even when the proc that we
 130  * tried to wake up was already awake.
 131  */
 132 static int btrfs_wake_function(wait_queue_t *wait, unsigned mode,
 133                                int sync, void *key)
 134 {
 135         autoremove_wake_function(wait, mode, sync, key);
 136         return 1;
 137 }
 138
 139 /*
 140  * returns with the extent buffer spinlocked.
 141  *
 142  * This will spin and/or wait as required to take the lock, and then
 143  * return with the spinlock held.
 144  *
 145  * After this call, scheduling is not safe without first calling
 146  * btrfs_set_lock_blocking()
 147  */
 148 int btrfs_tree_lock(struct extent_buffer *eb)
 149 {
 150         DEFINE_WAIT(wait);
 151         wait.func = btrfs_wake_function;
 152
 153         if (!btrfs_spin_on_block(eb))
 154                 goto sleep;
 155
 156         while(1) {
 157                 spin_nested(eb);
 158
 159                 /* nobody is blocking, exit with the spinlock held */
 160                 if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
 161                         return 0;
 162
 163                 /*
 164                  * we have the spinlock, but the real owner is blocking.
 165                  * wait for them
 166                  */
 167                 spin_unlock(&eb->lock);
 168
 169                 /*
 170                  * spin for a bit, and if the blocking flag goes away,
 171                  * loop around
 172                  */
 173                 cpu_relax();
 174                 if (btrfs_spin_on_block(eb))
 175                         continue;
 176 sleep:
 177                 prepare_to_wait_exclusive(&eb->lock_wq, &wait,
 178                                           TASK_UNINTERRUPTIBLE);
 179
 180                 if (test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
 181                         schedule();
 182
 183                 finish_wait(&eb->lock_wq, &wait);
 184         }
 185         return 0;
 186 }
 187
 188 int btrfs_tree_unlock(struct extent_buffer *eb)
 189 {
 190         /*
 191          * if we were a blocking owner, we don't have the spinlock held
 192          * just clear the bit and look for waiters
 193          */
 194         if (test_and_clear_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
 195                 smp_mb__after_clear_bit();
 196         else
 197                 spin_unlock(&eb->lock);
 198
 199         if (waitqueue_active(&eb->lock_wq))
 200                 wake_up(&eb->lock_wq);
 201         return 0;
 202 }
 203
 204 void btrfs_assert_tree_locked(struct extent_buffer *eb)
 205 {
 206         if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
 207                 assert_spin_locked(&eb->lock);
 208 }