linux/fs/btrfs/locking.c
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   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
  27void 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 */
  34void btrfs_set_lock_blocking_rw(struct extent_buffer *eb, int rw)
  35{
  36        if (eb->lock_nested) {
  37                read_lock(&eb->lock);
  38                if (eb->lock_nested && current->pid == eb->lock_owner) {
  39                        read_unlock(&eb->lock);
  40                        return;
  41                }
  42                read_unlock(&eb->lock);
  43        }
  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 */
  66void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
  67{
  68        if (eb->lock_nested) {
  69                read_lock(&eb->lock);
  70                if (eb->lock_nested && current->pid == eb->lock_owner) {
  71                        read_unlock(&eb->lock);
  72                        return;
  73                }
  74                read_unlock(&eb->lock);
  75        }
  76        if (rw == BTRFS_WRITE_LOCK_BLOCKING) {
  77                BUG_ON(atomic_read(&eb->blocking_writers) != 1);
  78                write_lock(&eb->lock);
  79                WARN_ON(atomic_read(&eb->spinning_writers));
  80                atomic_inc(&eb->spinning_writers);
  81                if (atomic_dec_and_test(&eb->blocking_writers) &&
  82                    waitqueue_active(&eb->write_lock_wq))
  83                        wake_up(&eb->write_lock_wq);
  84        } else if (rw == BTRFS_READ_LOCK_BLOCKING) {
  85                BUG_ON(atomic_read(&eb->blocking_readers) == 0);
  86                read_lock(&eb->lock);
  87                atomic_inc(&eb->spinning_readers);
  88                if (atomic_dec_and_test(&eb->blocking_readers) &&
  89                    waitqueue_active(&eb->read_lock_wq))
  90                        wake_up(&eb->read_lock_wq);
  91        }
  92        return;
  93}
  94
  95/*
  96 * take a spinning read lock.  This will wait for any blocking
  97 * writers
  98 */
  99void btrfs_tree_read_lock(struct extent_buffer *eb)
 100{
 101again:
 102        read_lock(&eb->lock);
 103        if (atomic_read(&eb->blocking_writers) &&
 104            current->pid == eb->lock_owner) {
 105                /*
 106                 * This extent is already write-locked by our thread. We allow
 107                 * an additional read lock to be added because it's for the same
 108                 * thread. btrfs_find_all_roots() depends on this as it may be
 109                 * called on a partly (write-)locked tree.
 110                 */
 111                BUG_ON(eb->lock_nested);
 112                eb->lock_nested = 1;
 113                read_unlock(&eb->lock);
 114                return;
 115        }
 116        if (atomic_read(&eb->blocking_writers)) {
 117                read_unlock(&eb->lock);
 118                wait_event(eb->write_lock_wq,
 119                           atomic_read(&eb->blocking_writers) == 0);
 120                goto again;
 121        }
 122        atomic_inc(&eb->read_locks);
 123        atomic_inc(&eb->spinning_readers);
 124}
 125
 126/*
 127 * returns 1 if we get the read lock and 0 if we don't
 128 * this won't wait for blocking writers
 129 */
 130int btrfs_try_tree_read_lock(struct extent_buffer *eb)
 131{
 132        if (atomic_read(&eb->blocking_writers))
 133                return 0;
 134
 135        read_lock(&eb->lock);
 136        if (atomic_read(&eb->blocking_writers)) {
 137                read_unlock(&eb->lock);
 138                return 0;
 139        }
 140        atomic_inc(&eb->read_locks);
 141        atomic_inc(&eb->spinning_readers);
 142        return 1;
 143}
 144
 145/*
 146 * returns 1 if we get the read lock and 0 if we don't
 147 * this won't wait for blocking writers or readers
 148 */
 149int btrfs_try_tree_write_lock(struct extent_buffer *eb)
 150{
 151        if (atomic_read(&eb->blocking_writers) ||
 152            atomic_read(&eb->blocking_readers))
 153                return 0;
 154        write_lock(&eb->lock);
 155        if (atomic_read(&eb->blocking_writers) ||
 156            atomic_read(&eb->blocking_readers)) {
 157                write_unlock(&eb->lock);
 158                return 0;
 159        }
 160        atomic_inc(&eb->write_locks);
 161        atomic_inc(&eb->spinning_writers);
 162        eb->lock_owner = current->pid;
 163        return 1;
 164}
 165
 166/*
 167 * drop a spinning read lock
 168 */
 169void btrfs_tree_read_unlock(struct extent_buffer *eb)
 170{
 171        if (eb->lock_nested) {
 172                read_lock(&eb->lock);
 173                if (eb->lock_nested && current->pid == eb->lock_owner) {
 174                        eb->lock_nested = 0;
 175                        read_unlock(&eb->lock);
 176                        return;
 177                }
 178                read_unlock(&eb->lock);
 179        }
 180        btrfs_assert_tree_read_locked(eb);
 181        WARN_ON(atomic_read(&eb->spinning_readers) == 0);
 182        atomic_dec(&eb->spinning_readers);
 183        atomic_dec(&eb->read_locks);
 184        read_unlock(&eb->lock);
 185}
 186
 187/*
 188 * drop a blocking read lock
 189 */
 190void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
 191{
 192        if (eb->lock_nested) {
 193                read_lock(&eb->lock);
 194                if (eb->lock_nested && current->pid == eb->lock_owner) {
 195                        eb->lock_nested = 0;
 196                        read_unlock(&eb->lock);
 197                        return;
 198                }
 199                read_unlock(&eb->lock);
 200        }
 201        btrfs_assert_tree_read_locked(eb);
 202        WARN_ON(atomic_read(&eb->blocking_readers) == 0);
 203        if (atomic_dec_and_test(&eb->blocking_readers) &&
 204            waitqueue_active(&eb->read_lock_wq))
 205                wake_up(&eb->read_lock_wq);
 206        atomic_dec(&eb->read_locks);
 207}
 208
 209/*
 210 * take a spinning write lock.  This will wait for both
 211 * blocking readers or writers
 212 */
 213void btrfs_tree_lock(struct extent_buffer *eb)
 214{
 215again:
 216        wait_event(eb->read_lock_wq, atomic_read(&eb->blocking_readers) == 0);
 217        wait_event(eb->write_lock_wq, atomic_read(&eb->blocking_writers) == 0);
 218        write_lock(&eb->lock);
 219        if (atomic_read(&eb->blocking_readers)) {
 220                write_unlock(&eb->lock);
 221                wait_event(eb->read_lock_wq,
 222                           atomic_read(&eb->blocking_readers) == 0);
 223                goto again;
 224        }
 225        if (atomic_read(&eb->blocking_writers)) {
 226                write_unlock(&eb->lock);
 227                wait_event(eb->write_lock_wq,
 228                           atomic_read(&eb->blocking_writers) == 0);
 229                goto again;
 230        }
 231        WARN_ON(atomic_read(&eb->spinning_writers));
 232        atomic_inc(&eb->spinning_writers);
 233        atomic_inc(&eb->write_locks);
 234        eb->lock_owner = current->pid;
 235}
 236
 237/*
 238 * drop a spinning or a blocking write lock.
 239 */
 240void btrfs_tree_unlock(struct extent_buffer *eb)
 241{
 242        int blockers = atomic_read(&eb->blocking_writers);
 243
 244        BUG_ON(blockers > 1);
 245
 246        btrfs_assert_tree_locked(eb);
 247        atomic_dec(&eb->write_locks);
 248
 249        if (blockers) {
 250                WARN_ON(atomic_read(&eb->spinning_writers));
 251                atomic_dec(&eb->blocking_writers);
 252                smp_mb();
 253                if (waitqueue_active(&eb->write_lock_wq))
 254                        wake_up(&eb->write_lock_wq);
 255        } else {
 256                WARN_ON(atomic_read(&eb->spinning_writers) != 1);
 257                atomic_dec(&eb->spinning_writers);
 258                write_unlock(&eb->lock);
 259        }
 260}
 261
 262void btrfs_assert_tree_locked(struct extent_buffer *eb)
 263{
 264        BUG_ON(!atomic_read(&eb->write_locks));
 265}
 266
 267void btrfs_assert_tree_read_locked(struct extent_buffer *eb)
 268{
 269        BUG_ON(!atomic_read(&eb->read_locks));
 270}
 271