linux-kernel - Re: [2.6.33-rc5] starting emacs makes lockdep warning

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Message-ID: <20100126065426.GG19799@ZenIV.linux.org.uk>
Date:	Tue, 26 Jan 2010 06:54:26 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Am??rico Wang <xiyou.wangcong@...il.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tavis Ormandy <taviso@...gle.com>,
	Jeff Dike <jdike@...toit.com>, Julien Tinnes <jln@...gle.com>,
	Matt Mackall <mpm@...enic.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [2.6.33-rc5] starting emacs makes lockdep warning

On Tue, Jan 26, 2010 at 02:24:30PM +0800, Am??rico Wang wrote:
> On Tue, Jan 26, 2010 at 2:07 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> > On Tue, Jan 26, 2010 at 02:01:12PM +0800, Am??rico Wang wrote:
> >
> >> I agree, it seems that patch is useless, since we already
> >> do lock_kernel() before calling __f_setown()...
> >
> > What's to prevent pid from being freed under us? ??BKL won't...
> >
> 
> Hmm, I don't fully understand the race here. If it is used to protect
> 'pid' which we get from 'tty->pgrp' or 'current', in the former case,
> it is protected by 'tty->ctrl_lock', in the later case, it doesn't need
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Without that patch it isn't.

> the lock. So it's not necessary to protect 'pid' by 'tty->crtl_lock'.
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