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 void btrfs_assert_tree_read_locked(struct extent_buffer *eb);
  28
  29 /*
  30  * if we currently have a spinning reader or writer lock
  31  * (indicated by the rw flag) this will bump the count
  32  * of blocking holders and drop the spinlock.
  33  */
  34 void btrfs_set_lock_blocking_rw(struct extent_buffer *eb, int rw)
  35 {
  36         /*
  37          * no lock is required.  The lock owner may change if
  38          * we have a read lock, but it won't change to or away
  39          * from us.  If we have the write lock, we are the owner
  40          * and it'll never change.
  41          */
  42         if (eb->lock_nested && current->pid == eb->lock_owner)
  43                 return;
  44         if (rw == BTRFS_WRITE_LOCK) {
  45                 if (atomic_read(&eb->blocking_writers) == 0) {
  46                         WARN_ON(atomic_read(&eb->spinning_writers) != 1);
  47                         atomic_dec(&eb->spinning_writers);
  48                         btrfs_assert_tree_locked(eb);
  49                         atomic_inc(&eb->blocking_writers);
  50                         write_unlock(&eb->lock);
  51                 }
  52         } else if (rw == BTRFS_READ_LOCK) {
  53                 btrfs_assert_tree_read_locked(eb);
  54                 atomic_inc(&eb->blocking_readers);
  55                 WARN_ON(atomic_read(&eb->spinning_readers) == 0);
  56                 atomic_dec(&eb->spinning_readers);
  57                 read_unlock(&eb->lock);
  58         }
  59         return;
  60 }
  61
  62 /*
  63  * if we currently have a blocking lock, take the spinlock
  64  * and drop our blocking count
  65  */
  66 void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
  67 {
  68         /*
  69          * no lock is required.  The lock owner may change if
  70          * we have a read lock, but it won't change to or away
  71          * from us.  If we have the write lock, we are the owner
  72          * and it'll never change.
  73          */
  74         if (eb->lock_nested && current->pid == eb->lock_owner)
  75                 return;
  76
  77         if (rw == BTRFS_WRITE_LOCK_BLOCKING) {
  78                 BUG_ON(atomic_read(&eb->blocking_writers) != 1);
  79                 write_lock(&eb->lock);
  80                 WARN_ON(atomic_read(&eb->spinning_writers));
  81                 atomic_inc(&eb->spinning_writers);
  82                 if (atomic_dec_and_test(&eb->blocking_writers) &&
  83                     waitqueue_active(&eb->write_lock_wq))
  84                         wake_up(&eb->write_lock_wq);
  85         } else if (rw == BTRFS_READ_LOCK_BLOCKING) {
  86                 BUG_ON(atomic_read(&eb->blocking_readers) == 0);
  87                 read_lock(&eb->lock);
  88                 atomic_inc(&eb->spinning_readers);
  89                 if (atomic_dec_and_test(&eb->blocking_readers) &&
  90                     waitqueue_active(&eb->read_lock_wq))
  91                         wake_up(&eb->read_lock_wq);
  92         }
  93         return;
  94 }
  95
  96 /*
  97  * take a spinning read lock.  This will wait for any blocking
  98  * writers
  99  */
 100 void btrfs_tree_read_lock(struct extent_buffer *eb)
 101 {
 102 again:
 103         BUG_ON(!atomic_read(&eb->blocking_writers) &&
 104                current->pid == eb->lock_owner);
 105
 106         read_lock(&eb->lock);
 107         if (atomic_read(&eb->blocking_writers) &&
 108             current->pid == eb->lock_owner) {
 109                 /*
 110                  * This extent is already write-locked by our thread. We allow
 111                  * an additional read lock to be added because it's for the same
 112                  * thread. btrfs_find_all_roots() depends on this as it may be
 113                  * called on a partly (write-)locked tree.
 114                  */
 115                 BUG_ON(eb->lock_nested);
 116                 eb->lock_nested = 1;
 117                 read_unlock(&eb->lock);
 118                 return;
 119         }
 120         if (atomic_read(&eb->blocking_writers)) {
 121                 read_unlock(&eb->lock);
 122                 wait_event(eb->write_lock_wq,
 123                            atomic_read(&eb->blocking_writers) == 0);
 124                 goto again;
 125         }
 126         atomic_inc(&eb->read_locks);
 127         atomic_inc(&eb->spinning_readers);
 128 }
 129
 130 /*
 131  * returns 1 if we get the read lock and 0 if we don't
 132  * this won't wait for blocking writers
 133  */
 134 int btrfs_try_tree_read_lock(struct extent_buffer *eb)
 135 {
 136         if (atomic_read(&eb->blocking_writers))
 137                 return 0;
 138
 139         if (!read_trylock(&eb->lock))
 140                 return 0;
 141
 142         if (atomic_read(&eb->blocking_writers)) {
 143                 read_unlock(&eb->lock);
 144                 return 0;
 145         }
 146         atomic_inc(&eb->read_locks);
 147         atomic_inc(&eb->spinning_readers);
 148         return 1;
 149 }
 150
 151 /*
 152  * returns 1 if we get the read lock and 0 if we don't
 153  * this won't wait for blocking writers or readers
 154  */
 155 int btrfs_try_tree_write_lock(struct extent_buffer *eb)
 156 {
 157         if (atomic_read(&eb->blocking_writers) ||
 158             atomic_read(&eb->blocking_readers))
 159                 return 0;
 160
 161         if (!write_trylock(&eb->lock))
 162                 return 0;
 163
 164         if (atomic_read(&eb->blocking_writers) ||
 165             atomic_read(&eb->blocking_readers)) {
 166                 write_unlock(&eb->lock);
 167                 return 0;
 168         }
 169         atomic_inc(&eb->write_locks);
 170         atomic_inc(&eb->spinning_writers);
 171         eb->lock_owner = current->pid;
 172         return 1;
 173 }
 174
 175 /*
 176  * drop a spinning read lock
 177  */
 178 void btrfs_tree_read_unlock(struct extent_buffer *eb)
 179 {
 180         /*
 181          * if we're nested, we have the write lock.  No new locking
 182          * is needed as long as we are the lock owner.
 183          * The write unlock will do a barrier for us, and the lock_nested
 184          * field only matters to the lock owner.
 185          */
 186         if (eb->lock_nested && current->pid == eb->lock_owner) {
 187                 eb->lock_nested = 0;
 188                 return;
 189         }
 190         btrfs_assert_tree_read_locked(eb);
 191         WARN_ON(atomic_read(&eb->spinning_readers) == 0);
 192         atomic_dec(&eb->spinning_readers);
 193         atomic_dec(&eb->read_locks);
 194         read_unlock(&eb->lock);
 195 }
 196
 197 /*
 198  * drop a blocking read lock
 199  */
 200 void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
 201 {
 202         /*
 203          * if we're nested, we have the write lock.  No new locking
 204          * is needed as long as we are the lock owner.
 205          * The write unlock will do a barrier for us, and the lock_nested
 206          * field only matters to the lock owner.
 207          */
 208         if (eb->lock_nested && current->pid == eb->lock_owner) {
 209                 eb->lock_nested = 0;
 210                 return;
 211         }
 212         btrfs_assert_tree_read_locked(eb);
 213         WARN_ON(atomic_read(&eb->blocking_readers) == 0);
 214         if (atomic_dec_and_test(&eb->blocking_readers) &&
 215             waitqueue_active(&eb->read_lock_wq))
 216                 wake_up(&eb->read_lock_wq);
 217         atomic_dec(&eb->read_locks);
 218 }
 219
 220 /*
 221  * take a spinning write lock.  This will wait for both
 222  * blocking readers or writers
 223  */
 224 void btrfs_tree_lock(struct extent_buffer *eb)
 225 {
 226 again:
 227         wait_event(eb->read_lock_wq, atomic_read(&eb->blocking_readers) == 0);
 228         wait_event(eb->write_lock_wq, atomic_read(&eb->blocking_writers) == 0);
 229         write_lock(&eb->lock);
 230         if (atomic_read(&eb->blocking_readers)) {
 231                 write_unlock(&eb->lock);
 232                 wait_event(eb->read_lock_wq,
 233                            atomic_read(&eb->blocking_readers) == 0);
 234                 goto again;
 235         }
 236         if (atomic_read(&eb->blocking_writers)) {
 237                 write_unlock(&eb->lock);
 238                 wait_event(eb->write_lock_wq,
 239                            atomic_read(&eb->blocking_writers) == 0);
 240                 goto again;
 241         }
 242         WARN_ON(atomic_read(&eb->spinning_writers));
 243         atomic_inc(&eb->spinning_writers);
 244         atomic_inc(&eb->write_locks);
 245         eb->lock_owner = current->pid;
 246 }
 247
 248 /*
 249  * drop a spinning or a blocking write lock.
 250  */
 251 void btrfs_tree_unlock(struct extent_buffer *eb)
 252 {
 253         int blockers = atomic_read(&eb->blocking_writers);
 254
 255         BUG_ON(blockers > 1);
 256
 257         btrfs_assert_tree_locked(eb);
 258         eb->lock_owner = 0;
 259         atomic_dec(&eb->write_locks);
 260
 261         if (blockers) {
 262                 WARN_ON(atomic_read(&eb->spinning_writers));
 263                 atomic_dec(&eb->blocking_writers);
 264                 smp_mb();
 265                 if (waitqueue_active(&eb->write_lock_wq))
 266                         wake_up(&eb->write_lock_wq);
 267         } else {
 268                 WARN_ON(atomic_read(&eb->spinning_writers) != 1);
 269                 atomic_dec(&eb->spinning_writers);
 270                 write_unlock(&eb->lock);
 271         }
 272 }
 273
 274 void btrfs_assert_tree_locked(struct extent_buffer *eb)
 275 {
 276         BUG_ON(!atomic_read(&eb->write_locks));
 277 }
 278
 279 static void btrfs_assert_tree_read_locked(struct extent_buffer *eb)
 280 {
 281         BUG_ON(!atomic_read(&eb->read_locks));
 282 }