Commit ed55b6ac authored Mar 26, 2014 by Jeff Mahoney Committed by Chris Mason Apr 07, 2014

btrfs: fix lockdep warning with reclaim lock inversion



When encountering memory pressure, testers have run into the following
lockdep warning. It was caused by __link_block_group calling kobject_add
with the groups_sem held. kobject_add calls kvasprintf with GFP_KERNEL,
which gets us into reclaim context. The kobject doesn't actually need
to be added under the lock -- it just needs to ensure that it's only
added for the first block group to be linked.

=========================================================
[ INFO: possible irq lock inversion dependency detected ]
3.14.0-rc8-default #1 Not tainted
---------------------------------------------------------
kswapd0/169 just changed the state of lock:
 (&delayed_node->mutex){+.+.-.}, at: [<ffffffffa018baea>] __btrfs_release_delayed_node+0x3a/0x200 [btrfs]
but this lock took another, RECLAIM_FS-unsafe lock in the past:
 (&found->groups_sem){+++++.}

and interrupts could create inverse lock ordering between them.

other info that might help us debug this:
 Possible interrupt unsafe locking scenario:
       CPU0                    CPU1
       ----                    ----
  lock(&found->groups_sem);
                               local_irq_disable();
                               lock(&delayed_node->mutex);
                               lock(&found->groups_sem);
  <Interrupt>
    lock(&delayed_node->mutex);

 *** DEADLOCK ***
2 locks held by kswapd0/169:
 #0:  (shrinker_rwsem){++++..}, at: [<ffffffff81159e8a>] shrink_slab+0x3a/0x160
 #1:  (&type->s_umount_key#27){++++..}, at: [<ffffffff811bac6f>] grab_super_passive+0x3f/0x90

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

parent 3f8a18cc

fs/btrfs/extent-tree.c

+7 −3

Original line number	Diff line number	Diff line
		@@ -8337,9 +8337,15 @@ static void __link_block_group(struct btrfs_space_info *space_info,
		struct btrfs_block_group_cache *cache)
		{
		int index = get_block_group_index(cache);
		bool first = false;

		down_write(&space_info->groups_sem);
		if (list_empty(&space_info->block_groups[index])) {
		if (list_empty(&space_info->block_groups[index]))
		first = true;
		list_add_tail(&cache->list, &space_info->block_groups[index]);
		up_write(&space_info->groups_sem);

		if (first) {
		struct kobject *kobj = &space_info->block_group_kobjs[index];
		int ret;

		@@ -8351,8 +8357,6 @@ static void __link_block_group(struct btrfs_space_info *space_info,
		kobject_put(&space_info->kobj);
		}
		}
		list_add_tail(&cache->list, &space_info->block_groups[index]);
		up_write(&space_info->groups_sem);
		}

		static struct btrfs_block_group_cache *

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