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    Subject[PATCH] loop: fput() called in loop_clr_fd() may cause bd_mutex recursive locking
    Unmonting mounted with `-o loop' block device causes recursive
    bd_mutex locking. fput() calls blkdev_put() for bdev that issued
    disk->fops->release() (loop_clr_fd()) call:

    [23044.654647] umount/24442 is trying to acquire lock:
    [23044.654652] (&bdev->bd_mutex){+.+.+.}, at: [<ffffffff81144311>] blkdev_put+0x1f/0x131
    [23044.654670]
    [23044.654672] but task is already holding lock:
    [23044.654677] (&bdev->bd_mutex){+.+.+.}, at: [<ffffffff811441a1>] __blkdev_put+0x33/0x184
    [23044.654690]
    [23044.654692] other info that might help us debug this:
    [23044.654697] Possible unsafe locking scenario:
    [23044.654727]
    [23044.654731] 1 lock held by umount/24442:
    [23044.654735] #0: (&bdev->bd_mutex){+.+.+.}, at: [<ffffffff811441a1>] __blkdev_put+0x33/0x184
    [23044.654748]
    [23044.654762] Call Trace:
    [23044.654773] [<ffffffff81075611>] __lock_acquire+0x15bf/0x1659
    [23044.654784] [<ffffffff8114b3e3>] ? inotify_free_group_priv+0x4f/0x4f
    [23044.654792] [<ffffffff81144311>] ? blkdev_put+0x1f/0x131
    [23044.654799] [<ffffffff81075c6a>] lock_acquire+0x138/0x1b3
    [23044.654807] [<ffffffff81144311>] ? blkdev_put+0x1f/0x131
    [23044.654814] [<ffffffff81144311>] ? blkdev_put+0x1f/0x131
    [23044.654824] [<ffffffff8147ce67>] mutex_lock_nested+0x5e/0x325
    [23044.654831] [<ffffffff81144311>] ? blkdev_put+0x1f/0x131
    [23044.654838] [<ffffffff81149963>] ? fsnotify+0x441/0x459
    [23044.654846] [<ffffffff81144311>] blkdev_put+0x1f/0x131
    [23044.654853] [<ffffffff81144443>] blkdev_close+0x20/0x22
    [23044.654863] [<ffffffff81116b21>] fput+0x117/0x1cf
    [23044.654874] [<ffffffffa016eb71>] loop_clr_fd+0x1f2/0x201 [loop]
    [23044.654882] [<ffffffffa016f861>] lo_release+0x40/0x6f [loop]
    [23044.654890] [<ffffffff81144244>] __blkdev_put+0xd6/0x184
    [23044.654898] [<ffffffff8114441a>] blkdev_put+0x128/0x131
    [23044.654906] [<ffffffff8111704e>] kill_block_super+0x60/0x65
    [23044.654914] [<ffffffff81117366>] deactivate_locked_super+0x32/0x63
    [23044.654922] [<ffffffff81117cc9>] deactivate_super+0x3a/0x3e
    [23044.654931] [<ffffffff8112fc5d>] mntput_no_expire+0xbf/0xc4
    [23044.654939] [<ffffffff811309c7>] sys_umount+0x2c5/0x2f3
    [23044.654949] [<ffffffff81484b12>] system_call_fastpath+0x16/0x1b


    Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

    ---

    diff --git a/drivers/block/loop.c b/drivers/block/loop.c
    index 1e888c9..b004779 100644
    --- a/drivers/block/loop.c
    +++ b/drivers/block/loop.c
    @@ -1028,6 +1028,15 @@ static int loop_clr_fd(struct loop_device *lo)
    * lock dependency possibility warning as fput can take
    * bd_mutex which is usually taken before lo_ctl_mutex.
    */
    + /*
    + * Need to put file f_op, otherwise fput() may cause
    + * recursive locking on bd_mutex, calling blkdev_put()
    + * for bdev that issued disk->fops->release() call.
    + */
    + if (bdev && bdev == bdev->bd_contains) {
    + fops_put(filp->f_op);
    + filp->f_op = NULL;
    + }
    fput(filp);
    return 0;
    }



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