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    Subject[PATCH 5.19 041/207] btrfs: fix hang during unmount when stopping block group reclaim worker
    Date
    From: Filipe Manana <fdmanana@suse.com>

    commit 8a1f1e3d1eecf9d2359a2709e276743a67e145db upstream.

    During early unmount, at close_ctree(), we try to stop the block group
    reclaim task with cancel_work_sync(), but that may hang if the block group
    reclaim task is currently at btrfs_relocate_block_group() waiting for the
    flag BTRFS_FS_UNFINISHED_DROPS to be cleared from fs_info->flags. During
    unmount we only clear that flag later, after trying to stop the block
    group reclaim task.

    Fix that by clearing BTRFS_FS_UNFINISHED_DROPS before trying to stop the
    block group reclaim task and after setting BTRFS_FS_CLOSING_START, so that
    if the reclaim task is waiting on that bit, it will stop immediately after
    being woken, because it sees the filesystem is closing (with a call to
    btrfs_fs_closing()), and then returns immediately with -EINTR.

    Fixes: 31e70e527806c5 ("btrfs: fix hang during unmount when block group reclaim task is running")
    CC: stable@vger.kernel.org # 5.15+
    Reviewed-by: Josef Bacik <josef@toxicpanda.com>
    Signed-off-by: Filipe Manana <fdmanana@suse.com>
    Signed-off-by: David Sterba <dsterba@suse.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/btrfs/disk-io.c | 17 +++++++++++------
    1 file changed, 11 insertions(+), 6 deletions(-)

    --- a/fs/btrfs/disk-io.c
    +++ b/fs/btrfs/disk-io.c
    @@ -4587,6 +4587,17 @@ void __cold close_ctree(struct btrfs_fs_
    set_bit(BTRFS_FS_CLOSING_START, &fs_info->flags);

    /*
    + * If we had UNFINISHED_DROPS we could still be processing them, so
    + * clear that bit and wake up relocation so it can stop.
    + * We must do this before stopping the block group reclaim task, because
    + * at btrfs_relocate_block_group() we wait for this bit, and after the
    + * wait we stop with -EINTR if btrfs_fs_closing() returns non-zero - we
    + * have just set BTRFS_FS_CLOSING_START, so btrfs_fs_closing() will
    + * return 1.
    + */
    + btrfs_wake_unfinished_drop(fs_info);
    +
    + /*
    * We may have the reclaim task running and relocating a data block group,
    * in which case it may create delayed iputs. So stop it before we park
    * the cleaner kthread otherwise we can get new delayed iputs after
    @@ -4604,12 +4615,6 @@ void __cold close_ctree(struct btrfs_fs_
    */
    kthread_park(fs_info->cleaner_kthread);

    - /*
    - * If we had UNFINISHED_DROPS we could still be processing them, so
    - * clear that bit and wake up relocation so it can stop.
    - */
    - btrfs_wake_unfinished_drop(fs_info);
    -
    /* wait for the qgroup rescan worker to stop */
    btrfs_qgroup_wait_for_completion(fs_info, false);


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