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/gfp.h>
  20 #include <linux/pagemap.h>
  21 #include <linux/spinlock.h>
  22 #include <linux/page-flags.h>
  23 #include <asm/bug.h>
  24 #include "ctree.h"
  25 #include "extent_io.h"
  26 #include "locking.h"
  27
  28 /*
  29  * locks the per buffer mutex in an extent buffer.  This uses adaptive locks
  30  * and the spin is not tuned very extensively.  The spinning does make a big
  31  * difference in almost every workload, but spinning for the right amount of
  32  * time needs some help.
  33  *
  34  * In general, we want to spin as long as the lock holder is doing btree
  35  * searches, and we should give up if they are in more expensive code.
  36  */
  37
  38 int btrfs_tree_lock(struct extent_buffer *eb)
  39 {
  40         int i;
  41
  42         if (mutex_trylock(&eb->mutex))
  43                 return 0;
  44         for (i = 0; i < 512; i++) {
  45                 cpu_relax();
  46                 if (mutex_trylock(&eb->mutex))
  47                         return 0;
  48         }
  49         cpu_relax();
  50         mutex_lock_nested(&eb->mutex, BTRFS_MAX_LEVEL - btrfs_header_level(eb));
  51         return 0;
  52 }
  53
  54 int btrfs_try_tree_lock(struct extent_buffer *eb)
  55 {
  56         return mutex_trylock(&eb->mutex);
  57 }
  58
  59 int btrfs_tree_unlock(struct extent_buffer *eb)
  60 {
  61         mutex_unlock(&eb->mutex);
  62         return 0;
  63 }
  64
  65 int btrfs_tree_locked(struct extent_buffer *eb)
  66 {
  67         return mutex_is_locked(&eb->mutex);
  68 }
  69
  70 /*
  71  * btrfs_search_slot uses this to decide if it should drop its locks
  72  * before doing something expensive like allocating free blocks for cow.
  73  */
  74 int btrfs_path_lock_waiting(struct btrfs_path *path, int level)
  75 {
  76         int i;
  77         struct extent_buffer *eb;
  78         for (i = level; i <= level + 1 && i < BTRFS_MAX_LEVEL; i++) {
  79                 eb = path->nodes[i];
  80                 if (!eb)
  81                         break;
  82                 smp_mb();
  83                 if (!list_empty(&eb->mutex.wait_list))
  84                         return 1;
  85         }
  86         return 0;
  87 }
  88