linux-kernel - Re: [2.6.33-rc5] tty: possible irq lock inversion dependency in tty

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Date:	Mon, 8 Feb 2010 00:06:57 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Américo Wang <xiyou.wangcong@...il.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	gregkh@...e.de, taviso@...gle.com, viro@...IV.linux.org.uk,
	linux-kernel@...r.kernel.org, ebiederm@...ssion.com,
	alan@...rguk.ukuu.org.uk, jdike@...toit.com, jln@...gle.com,
	mpm@...enic.com
Subject: Re: [2.6.33-rc5] tty: possible irq lock inversion dependency in
	tty_fasync

On Sat, Feb 06, 2010 at 10:59:56PM -0800, Linus Torvalds wrote:
>
>
>On Sun, 7 Feb 2010, Américo Wang wrote:
>> 
>> We already fixed this, a better fix:
>
>No we didn't.
>
>> http://lkml.org/lkml/2010/1/26/338
>> 
>> I sent a same fix with Greg's.
>
>We already did that. You didn't read Tetsuo's email carefully.
>
>Let's quote the important parts:
>
>  "is not yet fixed as of 2.6.33-rc7."
>
>and also the _second_ lockdep complaint he quotes, which starts out with
>
>  [   81.651199] =========================================================
>  [   81.651199] [ INFO: possible irq lock inversion dependency detected ]
>  [   81.651199] 2.6.33-rc7 #11
>  [   81.651199] ---------------------------------------------------------
>
>(note the -rc7 there).
>


Ah, I was just mislead by the subject... sigh


>The problem? Look at f_getown: it does
>
>	read_lock(&filp->f_owner.lock);
>
>ie it holds f_owner without interrupts disabled. Now an interrupt comes 
>in, and takes 'siglock' because it ends up sending a signal (timer, SIGIO, 
>whatever). So you have a f_owner -> siglock ordering.
>
>But we _also_ have a siglock -> ctrl_lock -> f_owner ordering, in that 
>problematic tty_fasync() thing. So we have a ABBA deadlock situation.
>
>Yes, it's hard (practically impossible) to trigger, because you have to 
>get an interrupt just at the right point with all the right processes, but 
>lockdep seems to be entirely correct. 
>
>So it is simply _wrong_ to take f_owner while we hold ctrl_lock.
>
>Which is why I suggest just reverting both the original problematic commit 
>_and_ the commit you point to, and just fix the race with that pid_get/put 
>pair instead. As per my patch.
>

Good analysis, this seems better for me too.

Thank you!
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