fs/btrfs/locking.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Copyright (C) 2008 Oracle.  All rights reserved.
   4  */
   5
   6 #include <linux/sched.h>
   7 #include <linux/pagemap.h>
   8 #include <linux/spinlock.h>
   9 #include <linux/page-flags.h>
  10 #include <asm/bug.h>
  11 #include "ctree.h"
  12 #include "extent_io.h"
  13 #include "locking.h"
  14
  15 static void btrfs_assert_tree_read_locked(struct extent_buffer *eb);
  16
  17 /*
  18  * if we currently have a spinning reader or writer lock
  19  * (indicated by the rw flag) this will bump the count
  20  * of blocking holders and drop the spinlock.
  21  */
  22 void btrfs_set_lock_blocking_rw(struct extent_buffer *eb, int rw)
  23 {
  24         /*
  25          * no lock is required.  The lock owner may change if
  26          * we have a read lock, but it won't change to or away
  27          * from us.  If we have the write lock, we are the owner
  28          * and it'll never change.
  29          */
  30         if (eb->lock_nested && current->pid == eb->lock_owner)
  31                 return;
  32         if (rw == BTRFS_WRITE_LOCK) {
  33                 if (atomic_read(&eb->blocking_writers) == 0) {
  34                         WARN_ON(atomic_read(&eb->spinning_writers) != 1);
  35                         atomic_dec(&eb->spinning_writers);
  36                         btrfs_assert_tree_locked(eb);
  37                         atomic_inc(&eb->blocking_writers);
  38                         write_unlock(&eb->lock);
  39                 }
  40         } else if (rw == BTRFS_READ_LOCK) {
  41                 btrfs_assert_tree_read_locked(eb);
  42                 atomic_inc(&eb->blocking_readers);
  43                 WARN_ON(atomic_read(&eb->spinning_readers) == 0);
  44                 atomic_dec(&eb->spinning_readers);
  45                 read_unlock(&eb->lock);
  46         }
  47 }
  48
  49 /*
  50  * if we currently have a blocking lock, take the spinlock
  51  * and drop our blocking count
  52  */
  53 void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
  54 {
  55         /*
  56          * no lock is required.  The lock owner may change if
  57          * we have a read lock, but it won't change to or away
  58          * from us.  If we have the write lock, we are the owner
  59          * and it'll never change.
  60          */
  61         if (eb->lock_nested && current->pid == eb->lock_owner)
  62                 return;
  63
  64         if (rw == BTRFS_WRITE_LOCK_BLOCKING) {
  65                 BUG_ON(atomic_read(&eb->blocking_writers) != 1);
  66                 write_lock(&eb->lock);
  67                 WARN_ON(atomic_read(&eb->spinning_writers));
  68                 atomic_inc(&eb->spinning_writers);
  69                 /*
  70                  * atomic_dec_and_test implies a barrier for waitqueue_active
  71                  */
  72                 if (atomic_dec_and_test(&eb->blocking_writers) &&
  73                     waitqueue_active(&eb->write_lock_wq))
  74                         wake_up(&eb->write_lock_wq);
  75         } else if (rw == BTRFS_READ_LOCK_BLOCKING) {
  76                 BUG_ON(atomic_read(&eb->blocking_readers) == 0);
  77                 read_lock(&eb->lock);
  78                 atomic_inc(&eb->spinning_readers);
  79                 /*
  80                  * atomic_dec_and_test implies a barrier for waitqueue_active
  81                  */
  82                 if (atomic_dec_and_test(&eb->blocking_readers) &&
  83                     waitqueue_active(&eb->read_lock_wq))
  84                         wake_up(&eb->read_lock_wq);
  85         }
  86 }
  87
  88 /*
  89  * take a spinning read lock.  This will wait for any blocking
  90  * writers
  91  */
  92 void btrfs_tree_read_lock(struct extent_buffer *eb)
  93 {
  94 again:
  95         BUG_ON(!atomic_read(&eb->blocking_writers) &&
  96                current->pid == eb->lock_owner);
  97
  98         read_lock(&eb->lock);
  99         if (atomic_read(&eb->blocking_writers) &&
 100             current->pid == eb->lock_owner) {
 101                 /*
 102                  * This extent is already write-locked by our thread. We allow
 103                  * an additional read lock to be added because it's for the same
 104                  * thread. btrfs_find_all_roots() depends on this as it may be
 105                  * called on a partly (write-)locked tree.
 106                  */
 107                 BUG_ON(eb->lock_nested);
 108                 eb->lock_nested = 1;
 109                 read_unlock(&eb->lock);
 110                 return;
 111         }
 112         if (atomic_read(&eb->blocking_writers)) {
 113                 read_unlock(&eb->lock);
 114                 wait_event(eb->write_lock_wq,
 115                            atomic_read(&eb->blocking_writers) == 0);
 116                 goto again;
 117         }
 118         atomic_inc(&eb->read_locks);
 119         atomic_inc(&eb->spinning_readers);
 120 }
 121
 122 /*
 123  * take a spinning read lock.
 124  * returns 1 if we get the read lock and 0 if we don't
 125  * this won't wait for blocking writers
 126  */
 127 int btrfs_tree_read_lock_atomic(struct extent_buffer *eb)
 128 {
 129         if (atomic_read(&eb->blocking_writers))
 130                 return 0;
 131
 132         read_lock(&eb->lock);
 133         if (atomic_read(&eb->blocking_writers)) {
 134                 read_unlock(&eb->lock);
 135                 return 0;
 136         }
 137         atomic_inc(&eb->read_locks);
 138         atomic_inc(&eb->spinning_readers);
 139         return 1;
 140 }
 141
 142 /*
 143  * returns 1 if we get the read lock and 0 if we don't
 144  * this won't wait for blocking writers
 145  */
 146 int btrfs_try_tree_read_lock(struct extent_buffer *eb)
 147 {
 148         if (atomic_read(&eb->blocking_writers))
 149                 return 0;
 150
 151         if (!read_trylock(&eb->lock))
 152                 return 0;
 153
 154         if (atomic_read(&eb->blocking_writers)) {
 155                 read_unlock(&eb->lock);
 156                 return 0;
 157         }
 158         atomic_inc(&eb->read_locks);
 159         atomic_inc(&eb->spinning_readers);
 160         return 1;
 161 }
 162
 163 /*
 164  * returns 1 if we get the read lock and 0 if we don't
 165  * this won't wait for blocking writers or readers
 166  */
 167 int btrfs_try_tree_write_lock(struct extent_buffer *eb)
 168 {
 169         if (atomic_read(&eb->blocking_writers) ||
 170             atomic_read(&eb->blocking_readers))
 171                 return 0;
 172
 173         write_lock(&eb->lock);
 174         if (atomic_read(&eb->blocking_writers) ||
 175             atomic_read(&eb->blocking_readers)) {
 176                 write_unlock(&eb->lock);
 177                 return 0;
 178         }
 179         atomic_inc(&eb->write_locks);
 180         atomic_inc(&eb->spinning_writers);
 181         eb->lock_owner = current->pid;
 182         return 1;
 183 }
 184
 185 /*
 186  * drop a spinning read lock
 187  */
 188 void btrfs_tree_read_unlock(struct extent_buffer *eb)
 189 {
 190         /*
 191          * if we're nested, we have the write lock.  No new locking
 192          * is needed as long as we are the lock owner.
 193          * The write unlock will do a barrier for us, and the lock_nested
 194          * field only matters to the lock owner.
 195          */
 196         if (eb->lock_nested && current->pid == eb->lock_owner) {
 197                 eb->lock_nested = 0;
 198                 return;
 199         }
 200         btrfs_assert_tree_read_locked(eb);
 201         WARN_ON(atomic_read(&eb->spinning_readers) == 0);
 202         atomic_dec(&eb->spinning_readers);
 203         atomic_dec(&eb->read_locks);
 204         read_unlock(&eb->lock);
 205 }
 206
 207 /*
 208  * drop a blocking read lock
 209  */
 210 void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
 211 {
 212         /*
 213          * if we're nested, we have the write lock.  No new locking
 214          * is needed as long as we are the lock owner.
 215          * The write unlock will do a barrier for us, and the lock_nested
 216          * field only matters to the lock owner.
 217          */
 218         if (eb->lock_nested && current->pid == eb->lock_owner) {
 219                 eb->lock_nested = 0;
 220                 return;
 221         }
 222         btrfs_assert_tree_read_locked(eb);
 223         WARN_ON(atomic_read(&eb->blocking_readers) == 0);
 224         /*
 225          * atomic_dec_and_test implies a barrier for waitqueue_active
 226          */
 227         if (atomic_dec_and_test(&eb->blocking_readers) &&
 228             waitqueue_active(&eb->read_lock_wq))
 229                 wake_up(&eb->read_lock_wq);
 230         atomic_dec(&eb->read_locks);
 231 }
 232
 233 /*
 234  * take a spinning write lock.  This will wait for both
 235  * blocking readers or writers
 236  */
 237 void btrfs_tree_lock(struct extent_buffer *eb)
 238 {
 239         WARN_ON(eb->lock_owner == current->pid);
 240 again:
 241         wait_event(eb->read_lock_wq, atomic_read(&eb->blocking_readers) == 0);
 242         wait_event(eb->write_lock_wq, atomic_read(&eb->blocking_writers) == 0);
 243         write_lock(&eb->lock);
 244         if (atomic_read(&eb->blocking_readers)) {
 245                 write_unlock(&eb->lock);
 246                 wait_event(eb->read_lock_wq,
 247                            atomic_read(&eb->blocking_readers) == 0);
 248                 goto again;
 249         }
 250         if (atomic_read(&eb->blocking_writers)) {
 251                 write_unlock(&eb->lock);
 252                 wait_event(eb->write_lock_wq,
 253                            atomic_read(&eb->blocking_writers) == 0);
 254                 goto again;
 255         }
 256         WARN_ON(atomic_read(&eb->spinning_writers));
 257         atomic_inc(&eb->spinning_writers);
 258         atomic_inc(&eb->write_locks);
 259         eb->lock_owner = current->pid;
 260 }
 261
 262 /*
 263  * drop a spinning or a blocking write lock.
 264  */
 265 void btrfs_tree_unlock(struct extent_buffer *eb)
 266 {
 267         int blockers = atomic_read(&eb->blocking_writers);
 268
 269         BUG_ON(blockers > 1);
 270
 271         btrfs_assert_tree_locked(eb);
 272         eb->lock_owner = 0;
 273         atomic_dec(&eb->write_locks);
 274
 275         if (blockers) {
 276                 WARN_ON(atomic_read(&eb->spinning_writers));
 277                 atomic_dec(&eb->blocking_writers);
 278                 /*
 279                  * Make sure counter is updated before we wake up waiters.
 280                  */
 281                 smp_mb__after_atomic();
 282                 if (waitqueue_active(&eb->write_lock_wq))
 283                         wake_up(&eb->write_lock_wq);
 284         } else {
 285                 WARN_ON(atomic_read(&eb->spinning_writers) != 1);
 286                 atomic_dec(&eb->spinning_writers);
 287                 write_unlock(&eb->lock);
 288         }
 289 }
 290
 291 void btrfs_assert_tree_locked(struct extent_buffer *eb)
 292 {
 293         BUG_ON(!atomic_read(&eb->write_locks));
 294 }
 295
 296 static void btrfs_assert_tree_read_locked(struct extent_buffer *eb)
 297 {
 298         BUG_ON(!atomic_read(&eb->read_locks));
 299 }