linux-kernel - Re: [PATCH] tty: tty_mutex: fix lockdep warning in tty_lock

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Message-ID: <20120525144713.6946df1e@bob.linux.org.uk>
Date:	Fri, 25 May 2012 14:47:13 +0100
From:	Alan Cox <alan@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ming Lei <ming.lei@...onical.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] tty: tty_mutex: fix lockdep warning in
 tty_lock_pair(v1)

> Having looked at the source I still don't see how it could possibly
> work,.. So the problem with tty_release() -> tty_ldisc_release() is
> that tty_ldisc_release() does an unlock/lock of tty.

Yes it should do the pair, see the patch I posted restructing it, and
the second one restructing it right.

> However your tty_lock_pair() can still result in tty being subclass 1,
> see your else branch, nested case.
> 
> That said, how is this not a real deadlock? If you rely on tty pointer
> ordering to avoid deadlocks, you always need to lock them in the same
> order. The unlock+lock in ldisc_release violates that.

Which was a bug.

Alan
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