Re: INFO: task hung in sync_blockdev

[Andi Kleen]

>  #0:  (&bdev->bd_mutex){+.+.}, at: [<0000000040269370>]
> __blkdev_put+0xbc/0x7f0 fs/block_dev.c:1757
> 1 lock held by blkid/19199:
>  #0:  (&bdev->bd_mutex){+.+.}, at: [<00000000b4dcaa18>]
> __blkdev_get+0x158/0x10e0 fs/block_dev.c:1439
>  #1:  (&ldata->atomic_read_lock){+.+.}, at: [<0000000033edf9f2>]
> n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
> 1 lock held by syz-executor5/19330:
>  #0:  (&bdev->bd_mutex){+.+.}, at: [<00000000b4dcaa18>]
> __blkdev_get+0x158/0x10e0 fs/block_dev.c:1439
> 1 lock held by syz-executor5/19331:
>  #0:  (&bdev->bd_mutex){+.+.}, at: [<00000000b4dcaa18>]
> __blkdev_get+0x158/0x10e0 fs/block_dev.c:1439

It seems multiple processes deadlocked on the bd_mutex. 
Unfortunately there's no backtrace for the lock acquisitions,
so it's hard to see the exact sequence.

It seems lockdep is already active, so it's likely not
just an ordering violation, but something else.

-Andi