From: Qu Wenruo Date: Wed, 20 Aug 2014 16:10:15 +0800 Subject: [PATCH] btrfs: Fix a deadlock in btrfs_dev_replace_finishing() (cherry picked from commit 12b894cb288d57292b01cf158177b6d5c89a6272) btrfs-transacion:5657 [stack snip] btrfs_bio_map() btrfs_bio_counter_inc_blocked() percpu_counter_inc(&fs_info->bio_counter) ###bio_counter > 0(A) __btrfs_bio_map() btrfs_dev_replace_lock() mutex_lock(dev_replace->lock) ###wait mutex(B) btrfs:32612 [stack snip] btrfs_dev_replace_start() btrfs_dev_replace_lock() mutex_lock(dev_replace->lock) ###hold mutex(B) btrfs_dev_replace_finishing() btrfs_rm_dev_replace_blocked() wait until percpu_counter_sum == 0 ###wait on bio_counter(A) This bug can be triggered quite easily by the following test script: http://pastebin.com/MQmb37Cy This patch will fix the ABBA problem by calling btrfs_dev_replace_unlock() before btrfs_rm_dev_replace_blocked(). The consistency of btrfs devices list and their superblocks is protected by device_list_mutex, not btrfs_dev_replace_lock/unlock(). So it is safe the move btrfs_dev_replace_unlock() before btrfs_rm_dev_replace_blocked(). Reported-by: Zhao Lei Signed-off-by: Qu Wenruo Cc: Stefan Behrens Signed-off-by: Chris Mason --- fs/btrfs/dev-replace.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c index eea26e1b2fda..d738ff8ab81c 100644 --- a/fs/btrfs/dev-replace.c +++ b/fs/btrfs/dev-replace.c @@ -567,6 +567,8 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info, btrfs_kobj_rm_device(fs_info, src_device); btrfs_kobj_add_device(fs_info, tgt_device); + btrfs_dev_replace_unlock(dev_replace); + btrfs_rm_dev_replace_blocked(fs_info); btrfs_rm_dev_replace_srcdev(fs_info, src_device); @@ -580,7 +582,6 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info, * superblock is scratched out so that it is no longer marked to * belong to this filesystem. */ - btrfs_dev_replace_unlock(dev_replace); mutex_unlock(&root->fs_info->fs_devices->device_list_mutex); mutex_unlock(&root->fs_info->chunk_mutex); -- 1.9.3