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                 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  * take a spinning read lock.
 132  * returns 1 if we get the read lock and 0 if we don't
 133  * this won't wait for blocking writers
 134  */
 135 int btrfs_tree_read_lock_atomic(struct extent_buffer *eb)
 136 {
 137         if (atomic_read(&eb->blocking_writers))
 138                 return 0;
 139
 140         read_lock(&eb->lock);
 141         if (atomic_read(&eb->blocking_writers)) {
 142                 read_unlock(&eb->lock);
 143                 return 0;
 144         }
 145         atomic_inc(&eb->read_locks);
 146         atomic_inc(&eb->spinning_readers);
 147         return 1;
 148 }
 149
 150 /*
 151  * returns 1 if we get the read lock and 0 if we don't
 152  * this won't wait for blocking writers
 153  */
 154 int btrfs_try_tree_read_lock(struct extent_buffer *eb)
 155 {
 156         if (atomic_read(&eb->blocking_writers))
 157                 return 0;
 158
 159         if (!read_trylock(&eb->lock))
 160                 return 0;
 161
 162         if (atomic_read(&eb->blocking_writers)) {
 163                 read_unlock(&eb->lock);
 164                 return 0;
 165         }
 166         atomic_inc(&eb->read_locks);
 167         atomic_inc(&eb->spinning_readers);
 168         return 1;
 169 }
 170
 171 /*
 172  * returns 1 if we get the read lock and 0 if we don't
 173  * this won't wait for blocking writers or readers
 174  */
 175 int btrfs_try_tree_write_lock(struct extent_buffer *eb)
 176 {
 177         if (atomic_read(&eb->blocking_writers) ||
 178             atomic_read(&eb->blocking_readers))
 179                 return 0;
 180
 181         write_lock(&eb->lock);
 182         if (atomic_read(&eb->blocking_writers) ||
 183             atomic_read(&eb->blocking_readers)) {
 184                 write_unlock(&eb->lock);
 185                 return 0;
 186         }
 187         atomic_inc(&eb->write_locks);
 188         atomic_inc(&eb->spinning_writers);
 189         eb->lock_owner = current->pid;
 190         return 1;
 191 }
 192
 193 /*
 194  * drop a spinning read lock
 195  */
 196 void btrfs_tree_read_unlock(struct extent_buffer *eb)
 197 {
 198         /*
 199          * if we're nested, we have the write lock.  No new locking
 200          * is needed as long as we are the lock owner.
 201          * The write unlock will do a barrier for us, and the lock_nested
 202          * field only matters to the lock owner.
 203          */
 204         if (eb->lock_nested && current->pid == eb->lock_owner) {
 205                 eb->lock_nested = 0;
 206                 return;
 207         }
 208         btrfs_assert_tree_read_locked(eb);
 209         WARN_ON(atomic_read(&eb->spinning_readers) == 0);
 210         atomic_dec(&eb->spinning_readers);
 211         atomic_dec(&eb->read_locks);
 212         read_unlock(&eb->lock);
 213 }
 214
 215 /*
 216  * drop a blocking read lock
 217  */
 218 void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
 219 {
 220         /*
 221          * if we're nested, we have the write lock.  No new locking
 222          * is needed as long as we are the lock owner.
 223          * The write unlock will do a barrier for us, and the lock_nested
 224          * field only matters to the lock owner.
 225          */
 226         if (eb->lock_nested && current->pid == eb->lock_owner) {
 227                 eb->lock_nested = 0;
 228                 return;
 229         }
 230         btrfs_assert_tree_read_locked(eb);
 231         WARN_ON(atomic_read(&eb->blocking_readers) == 0);
 232         if (atomic_dec_and_test(&eb->blocking_readers) &&
 233             waitqueue_active(&eb->read_lock_wq))
 234                 wake_up(&eb->read_lock_wq);
 235         atomic_dec(&eb->read_locks);
 236 }
 237
 238 /*
 239  * take a spinning write lock.  This will wait for both
 240  * blocking readers or writers
 241  */
 242 void btrfs_tree_lock(struct extent_buffer *eb)
 243 {
 244 again:
 245         wait_event(eb->read_lock_wq, atomic_read(&eb->blocking_readers) == 0);
 246         wait_event(eb->write_lock_wq, atomic_read(&eb->blocking_writers) == 0);
 247         write_lock(&eb->lock);
 248         if (atomic_read(&eb->blocking_readers)) {
 249                 write_unlock(&eb->lock);
 250                 wait_event(eb->read_lock_wq,
 251                            atomic_read(&eb->blocking_readers) == 0);
 252                 goto again;
 253         }
 254         if (atomic_read(&eb->blocking_writers)) {
 255                 write_unlock(&eb->lock);
 256                 wait_event(eb->write_lock_wq,
 257                            atomic_read(&eb->blocking_writers) == 0);
 258                 goto again;
 259         }
 260         WARN_ON(atomic_read(&eb->spinning_writers));
 261         atomic_inc(&eb->spinning_writers);
 262         atomic_inc(&eb->write_locks);
 263         eb->lock_owner = current->pid;
 264 }
 265
 266 /*
 267  * drop a spinning or a blocking write lock.
 268  */
 269 void btrfs_tree_unlock(struct extent_buffer *eb)
 270 {
 271         int blockers = atomic_read(&eb->blocking_writers);
 272
 273         BUG_ON(blockers > 1);
 274
 275         btrfs_assert_tree_locked(eb);
 276         eb->lock_owner = 0;
 277         atomic_dec(&eb->write_locks);
 278
 279         if (blockers) {
 280                 WARN_ON(atomic_read(&eb->spinning_writers));
 281                 atomic_dec(&eb->blocking_writers);
 282                 smp_mb();
 283                 if (waitqueue_active(&eb->write_lock_wq))
 284                         wake_up(&eb->write_lock_wq);
 285         } else {
 286                 WARN_ON(atomic_read(&eb->spinning_writers) != 1);
 287                 atomic_dec(&eb->spinning_writers);
 288                 write_unlock(&eb->lock);
 289         }
 290 }
 291
 292 void btrfs_assert_tree_locked(struct extent_buffer *eb)
 293 {
 294         BUG_ON(!atomic_read(&eb->write_locks));
 295 }
 296
 297 static void btrfs_assert_tree_read_locked(struct extent_buffer *eb)
 298 {
 299         BUG_ON(!atomic_read(&eb->read_locks));
 300 }