linux-kernel - Re: [linux-next / tty] possible circular locking dependency detected

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Date:   Sat, 3 Jun 2017 22:50:19 +0200
From:   Vegard Nossum <vegard.nossum@...cle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Jiri Slaby <jslaby@...e.com>, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [linux-next / tty] possible circular locking dependency detected

On 06/03/17 11:34, Greg Kroah-Hartman wrote:
> On Mon, May 29, 2017 at 12:43:39PM +0200, Vegard Nossum wrote:
>> On 05/22/17 12:27, Vegard Nossum wrote:
>>> On 05/22/17 12:24, Greg Kroah-Hartman wrote:
>>>> On Mon, May 22, 2017 at 04:39:43PM +0900, Sergey Senozhatsky wrote:
>>>>> Hello,
>>>>>
>>>>> [ 1274.378287] ======================================================
>>>>> [ 1274.378289] WARNING: possible circular locking dependency detected
>>>>> [ 1274.378290]
>>>>> 4.12.0-rc1-next-20170522-dbg-00007-gc09b2ab28b74-dirty #1317 Not
>>>>> tainted
>>>>> [ 1274.378291] ------------------------------------------------------
>>>>> [ 1274.378293] kworker/u8:5/111 is trying to acquire lock:
>>>>> [ 1274.378294]  (&buf->lock){+.+...}, at: [<ffffffff812f2831>]
>>>>> tty_buffer_flush+0x34/0x88
>>>>> [ 1274.378300]
>>>>>                  but task is already holding lock:
>>>>> [ 1274.378301]  (&o_tty->termios_rwsem/1){++++..}, at:
>>>>> [<ffffffff812ee5c7>] isig+0x47/0xd2
>>>>> [ 1274.378307]
>>>>>                  which lock already depends on the new lock.
>>>>>
>>>
>>>> Any hint as to what you were doing when this happened?
>>>>
>>>> Does this also show up in 4.11?
>>>
>>> It's my patch "tty: fix port buffer locking" :-/
>>>
>>> At a glance, looks related to pty taking the lock on the other side in a
>>> different order. I'll have a closer look.
>>
>> I can reproduce the lockdep report locally on v4.12-rc3. Looking at it now.
> 
> Any ideas?  Or should I just revert the original patch?

I think we must revert it for now, as I can easily reproduce not just
the lockdep warning but actual hangs. It seems I missed some code paths
when I worked the original patch.

I'm working on a fix.


Vegard