linux-kernel - Re: 3.4+ tty lockdep trace

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Message-ID: <20120525120216.45381ec6@bob.linux.org.uk>
Date:	Fri, 25 May 2012 12:02:16 +0100
From:	Alan Cox <alan@...ux.intel.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	Ming Lei <tom.leiming@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: 3.4+ tty lockdep trace

> Applying Ming's patch over Alan's 2 patches from yesterday, I'm still
> seeing two lockdep warnings. Full trace attached.

In that code path we've just succesfully done

	tty_lock_pair(tty, o_tty)

(no warning issued)

We've then called into tty_ldisc_release which has done

	tty_unlock_pair(tty, o_tty);

and then

	tty_lock_pair(tty, o_tty);

which can't error unless our locking hosed

and at that point we then then do a recursive

	tty_ldisc_release(o_tty, NULL)

which embarrassingly already has a comment above it I put there saying


        /* This will need doing differently if we need to lock */


Let me go rewrite that particular routine to make sense with the
locking in place.

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