linux-kernel - Re: [PATCH 3/3] tty_lock: Localise the lock

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Message-ID: <CA+1xoqdi1Jdkrjtj9bmnDfs4Ok=o=AsmpP=MY0V8cDXjcSNm1w@mail.gmail.com>
Date:	Tue, 8 May 2012 20:12:37 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org, Jiri Slaby <jirislaby@...il.com>
Subject: Re: [PATCH 3/3] tty_lock: Localise the lock

On Mon, May 7, 2012 at 11:04 PM, Jiri Slaby <jslaby@...e.cz> wrote:
> On 05/07/2012 07:00 PM, Sasha Levin wrote:
>>> So whatever your trace is showing, that's not the bug. Something more
>>> complicated would appear to be afoot.
>>
>> Oddly enough, tty != tty->link, but the lockdep warning triggers.
>>
>> Any idea why it might happen?
>
> I think so, both locks are the same lockdep class. So lockdep thinks it
> is the same lock. However this is a false positive. I guess we need
> mutex_lock_nested...

It looks like it causes an actual deadlock, and hung_tasks screams if
left alone for a bit, so it doesn't look like a lockdep issue.
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