Re: INFO: task hung in __blkdev_get

[Dmitry Vyukov]

On Tue, Apr 10, 2018 at 12:55 PM, Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
> Hello.
>
> Since syzbot is reporting so many hung up bug which involves /dev/loopX ,
> is it possible to "temporarily" apply below patch for testing under syzbot

Unfortunately it's not possible, for full explanation please see:
https://github.com/google/syzkaller/blob/master/docs/syzbot.md#no-custom-patches


> ( after "block/loop: fix deadlock after loop_set_status" and
> "loop: fix LOOP_GET_STATUS lock imbalance" in linux-block.git#for-linus
> are merged into linux.git )?
>
> I haven't got a smoking gun by lockdep, but I noticed that
>
>   [upstream] INFO: task hung in lo_open (2)
>   https://syzkaller.appspot.com/bug?id=1f93b57f496d969efb9fb24167f6f9de5ee068fd
>
> contained "lo->lo_ctl_mutex => bdev->bd_mutex" locking order
>
>   2 locks held by syz-executor6/15084:
>    #0:  (&lo->lo_ctl_mutex/1){+.+.}, at: [<00000000cc154b8d>] lo_ioctl+0x8b/0x1b70 drivers/block/loop.c:1355
>    #1:  (&bdev->bd_mutex){+.+.}, at: [<0000000058b7c5b5>] blkdev_reread_part+0x1e/0x40 block/ioctl.c:192
>
> while commit f028f3b2f987ebc6 ("loop: fix circular locking in loop_clr_fd()")
> says that
>
>   * Calling fput holding lo_ctl_mutex triggers a circular
>   * lock dependency possibility warning as fput can take
>   * bd_mutex which is usually taken before lo_ctl_mutex.
>
> which implies that the locking order should be "bdev->bd_mutex => lo->lo_ctl_mutex"
> and also says that use of "_nested" version might mask some other real bugs
> which could be the bug syzbot is frequently reporting as hung tasks...
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 264abaa..5559b15 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1360,7 +1360,7 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
>         struct loop_device *lo = bdev->bd_disk->private_data;
>         int err;
>
> -       err = mutex_lock_killable_nested(&lo->lo_ctl_mutex, 1);
> +       err = mutex_lock_killable(&lo->lo_ctl_mutex);
>         if (err)
>                 goto out_unlocked;
>
>