linux-kernel - Re: [bug] WARNING: at drivers/char/tty_io.c:1266 tty

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Message-ID: <20090616111316.6b3bb078@lxorguk.ukuu.org.uk>
Date:	Tue, 16 Jun 2009 11:13:16 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	Vegard Nossum <vegard.nossum@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [bug] WARNING: at drivers/char/tty_io.c:1266
 tty_open+0x1ea/0x388()

> I have applied your patch from yesterday (attached further below for 
> reference) and the SLAB corruption has not triggered - instead i'm 
> now getting this warning, after 96 reboots

That one is interesting btw - however its not a new bug. The WARN_ON()
was added in the new patches to catch cases where the tty open/close
locking was broken and see if all the ldisc related ones were nailed.

Apparently on a very SMP box they are not. It's not however a new bug -
just the result of checking for the problem.

+        WARN_ON(!test_bit(TTY_LDISC, &tty->flags));

..

which means that someone cleared the ldisc behind our back despite us
holding tty_mutex. That would suggest a hangup/reopen race which
shouldn't be too hard to find.

Dunno what you feed your SMP box but its very useful 8)

Alan
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