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    Subject[PATCH 3.16 002/127] btrfs: Fix a deadlock in btrfs_dev_replace_finishing()
    Date
    3.16-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Qu Wenruo <quwenruo@cn.fujitsu.com>

    commit 12b894cb288d57292b01cf158177b6d5c89a6272 upstream.

    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 <zhaolei@cn.fujitsu.com>
    Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
    Cc: Stefan Behrens <sbehrens@giantdisaster.de>
    Signed-off-by: Chris Mason <clm@fb.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/btrfs/dev-replace.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    --- a/fs/btrfs/dev-replace.c
    +++ b/fs/btrfs/dev-replace.c
    @@ -567,6 +567,8 @@ static int btrfs_dev_replace_finishing(s
    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(s
    * 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);




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