linux-kernel - Re: [PATCH] Fix race/oops in tty layer after BKL pushdown

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Message-ID: <20080807204439.1f8de826@lxorguk.ukuu.org.uk>
Date:	Thu, 7 Aug 2008 20:44:39 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Olaf Schnapper <os@...ibm.com>
Subject: Re: [PATCH] Fix race/oops in tty layer after BKL pushdown

> It seems that tty was already free in disassocate_ctty when it tries
> to dereference tty->driver.
> 
> After moving the lock_kernel before the mutex_unlock, I can no longer
> reproduce the problem.
> 
> Please review and consider to apply:

This doesn't help as the BKL doesn't protect tty here - we don't have
refcounting on the ttys yet so this bug (which goes back forever) simply
becomes a different sized race if you swap the lock ordering.

Given tty_vhangup just fires off a wait queue which is killed on the tty
destruction you should be able for now at least to move the mutex_unlock
to after the call to tty_vhangup().

Does that also fix the problem ?

Alan
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