kernel/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                 /*
  83                  * atomic_dec_and_test implies a barrier for waitqueue_active
  84                  */
  85                 if (atomic_dec_and_test(&eb->blocking_writers) &&
  86                     waitqueue_active(&eb->write_lock_wq))
  87                         wake_up(&eb->write_lock_wq);
  88         } else if (rw == BTRFS_READ_LOCK_BLOCKING) {
  89                 BUG_ON(atomic_read(&eb->blocking_readers) == 0);
  90                 read_lock(&eb->lock);
  91                 atomic_inc(&eb->spinning_readers);
  92                 /*
  93                  * atomic_dec_and_test implies a barrier for waitqueue_active
  94                  */
  95                 if (atomic_dec_and_test(&eb->blocking_readers) &&
  96                     waitqueue_active(&eb->read_lock_wq))
  97                         wake_up(&eb->read_lock_wq);
  98         }
  99         return;
 100 }
 101
 102 /*
 103  * take a spinning read lock.  This will wait for any blocking
 104  * writers
 105  */
 106 void btrfs_tree_read_lock(struct extent_buffer *eb)
 107 {
 108 again:
 109         BUG_ON(!atomic_read(&eb->blocking_writers) &&
 110                current->pid == eb->lock_owner);
 111
 112         read_lock(&eb->lock);
 113         if (atomic_read(&eb->blocking_writers) &&
 114             current->pid == eb->lock_owner) {
 115                 /*
 116                  * This extent is already write-locked by our thread. We allow
 117                  * an additional read lock to be added because it's for the same
 118                  * thread. btrfs_find_all_roots() depends on this as it may be
 119                  * called on a partly (write-)locked tree.
 120                  */
 121                 BUG_ON(eb->lock_nested);
 122                 eb->lock_nested = 1;
 123                 read_unlock(&eb->lock);
 124                 return;
 125         }
 126         if (atomic_read(&eb->blocking_writers)) {
 127                 read_unlock(&eb->lock);
 128                 wait_event(eb->write_lock_wq,
 129                            atomic_read(&eb->blocking_writers) == 0);
 130                 goto again;
 131         }
 132         atomic_inc(&eb->read_locks);
 133         atomic_inc(&eb->spinning_readers);
 134 }
 135
 136 /*
 137  * take a spinning read lock.
 138  * returns 1 if we get the read lock and 0 if we don't
 139  * this won't wait for blocking writers
 140  */
 141 int btrfs_tree_read_lock_atomic(struct extent_buffer *eb)
 142 {
 143         if (atomic_read(&eb->blocking_writers))
 144                 return 0;
 145
 146         read_lock(&eb->lock);
 147         if (atomic_read(&eb->blocking_writers)) {
 148                 read_unlock(&eb->lock);
 149                 return 0;
 150         }
 151         atomic_inc(&eb->read_locks);
 152         atomic_inc(&eb->spinning_readers);
 153         return 1;
 154 }
 155
 156 /*
 157  * returns 1 if we get the read lock and 0 if we don't
 158  * this won't wait for blocking writers
 159  */
 160 int btrfs_try_tree_read_lock(struct extent_buffer *eb)
 161 {
 162         if (atomic_read(&eb->blocking_writers))
 163                 return 0;
 164
 165         if (!read_trylock(&eb->lock))
 166                 return 0;
 167
 168         if (atomic_read(&eb->blocking_writers)) {
 169                 read_unlock(&eb->lock);
 170                 return 0;
 171         }
 172         atomic_inc(&eb->read_locks);
 173         atomic_inc(&eb->spinning_readers);
 174         return 1;
 175 }
 176
 177 /*
 178  * returns 1 if we get the read lock and 0 if we don't
 179  * this won't wait for blocking writers or readers
 180  */
 181 int btrfs_try_tree_write_lock(struct extent_buffer *eb)
 182 {
 183         if (atomic_read(&eb->blocking_writers) ||
 184             atomic_read(&eb->blocking_readers))
 185                 return 0;
 186
 187         write_lock(&eb->lock);
 188         if (atomic_read(&eb->blocking_writers) ||
 189             atomic_read(&eb->blocking_readers)) {
 190                 write_unlock(&eb->lock);
 191                 return 0;
 192         }
 193         atomic_inc(&eb->write_locks);
 194         atomic_inc(&eb->spinning_writers);
 195         eb->lock_owner = current->pid;
 196         return 1;
 197 }
 198
 199 /*
 200  * drop a spinning read lock
 201  */
 202 void btrfs_tree_read_unlock(struct extent_buffer *eb)
 203 {
 204         /*
 205          * if we're nested, we have the write lock.  No new locking
 206          * is needed as long as we are the lock owner.
 207          * The write unlock will do a barrier for us, and the lock_nested
 208          * field only matters to the lock owner.
 209          */
 210         if (eb->lock_nested && current->pid == eb->lock_owner) {
 211                 eb->lock_nested = 0;
 212                 return;
 213         }
 214         btrfs_assert_tree_read_locked(eb);
 215         WARN_ON(atomic_read(&eb->spinning_readers) == 0);
 216         atomic_dec(&eb->spinning_readers);
 217         atomic_dec(&eb->read_locks);
 218         read_unlock(&eb->lock);
 219 }
 220
 221 /*
 222  * drop a blocking read lock
 223  */
 224 void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
 225 {
 226         /*
 227          * if we're nested, we have the write lock.  No new locking
 228          * is needed as long as we are the lock owner.
 229          * The write unlock will do a barrier for us, and the lock_nested
 230          * field only matters to the lock owner.
 231          */
 232         if (eb->lock_nested && current->pid == eb->lock_owner) {
 233                 eb->lock_nested = 0;
 234                 return;
 235         }
 236         btrfs_assert_tree_read_locked(eb);
 237         WARN_ON(atomic_read(&eb->blocking_readers) == 0);
 238         /*
 239          * atomic_dec_and_test implies a barrier for waitqueue_active
 240          */
 241         if (atomic_dec_and_test(&eb->blocking_readers) &&
 242             waitqueue_active(&eb->read_lock_wq))
 243                 wake_up(&eb->read_lock_wq);
 244         atomic_dec(&eb->read_locks);
 245 }
 246
 247 /*
 248  * take a spinning write lock.  This will wait for both
 249  * blocking readers or writers
 250  */
 251 void btrfs_tree_lock(struct extent_buffer *eb)
 252 {
 253         WARN_ON(eb->lock_owner == current->pid);
 254 again:
 255         wait_event(eb->read_lock_wq, atomic_read(&eb->blocking_readers) == 0);
 256         wait_event(eb->write_lock_wq, atomic_read(&eb->blocking_writers) == 0);
 257         write_lock(&eb->lock);
 258         if (atomic_read(&eb->blocking_readers)) {
 259                 write_unlock(&eb->lock);
 260                 wait_event(eb->read_lock_wq,
 261                            atomic_read(&eb->blocking_readers) == 0);
 262                 goto again;
 263         }
 264         if (atomic_read(&eb->blocking_writers)) {
 265                 write_unlock(&eb->lock);
 266                 wait_event(eb->write_lock_wq,
 267                            atomic_read(&eb->blocking_writers) == 0);
 268                 goto again;
 269         }
 270         WARN_ON(atomic_read(&eb->spinning_writers));
 271         atomic_inc(&eb->spinning_writers);
 272         atomic_inc(&eb->write_locks);
 273         eb->lock_owner = current->pid;
 274 }
 275
 276 /*
 277  * drop a spinning or a blocking write lock.
 278  */
 279 void btrfs_tree_unlock(struct extent_buffer *eb)
 280 {
 281         int blockers = atomic_read(&eb->blocking_writers);
 282
 283         BUG_ON(blockers > 1);
 284
 285         btrfs_assert_tree_locked(eb);
 286         eb->lock_owner = 0;
 287         atomic_dec(&eb->write_locks);
 288
 289         if (blockers) {
 290                 WARN_ON(atomic_read(&eb->spinning_writers));
 291                 atomic_dec(&eb->blocking_writers);
 292                 /*
 293                  * Make sure counter is updated before we wake up waiters.
 294                  */
 295                 smp_mb();
 296                 if (waitqueue_active(&eb->write_lock_wq))
 297                         wake_up(&eb->write_lock_wq);
 298         } else {
 299                 WARN_ON(atomic_read(&eb->spinning_writers) != 1);
 300                 atomic_dec(&eb->spinning_writers);
 301                 write_unlock(&eb->lock);
 302         }
 303 }
 304
 305 void btrfs_assert_tree_locked(struct extent_buffer *eb)
 306 {
 307         BUG_ON(!atomic_read(&eb->write_locks));
 308 }
 309
 310 static void btrfs_assert_tree_read_locked(struct extent_buffer *eb)
 311 {
 312         BUG_ON(!atomic_read(&eb->read_locks));
 313 }