linux-kernel - Re: [PATCH/RFC] signal races/bugs, losing TIF

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Message-ID: <Pine.LNX.4.64.0706050839350.23304@alien.or.mcafeemobile.com>
Date:	Tue, 5 Jun 2007 08:52:07 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH/RFC] signal races/bugs, losing TIF_SIGPENDING and other
 woes

On Tue, 5 Jun 2007, Benjamin Herrenschmidt wrote:

> We don't actually call recalc_sigpending_tsk() when sending a signal to
> some other task, we just set the flag... so I need to recheck my theory
> here about recalc_sigpending_tsk being called for somebody else...
> Something is doing it somewhere it seems (we are losing the
> TIF_SIGPENDING bit) but I'll need to cook up a repro case to track
> exactly where.
>> 
>  - I still think there's something wrong with dequeue_signal() being
> potentially called with a task different than current by signalfd, since
> __dequeue_signal() (among others) mucks around with current regardless.
> I'd love to just make signalfd's read() only do anything if current ==
> ctx->tsk and remove the task argument from dequeue_signal... that would
> fix it nicely too no ?

I agree with Ben that recalc_sigpending_tsk() on tsk!=current is a bad 
idea. I think, since dequeue_signal() is called from user context, we 
could just have only recalc_sigpending(), that acts on current, and then 
inside dequeue_signal we have:

	if (tsk == current)
		recalc_sigpending();

What can happen, is that a task may notice TIF_SIGPENDING and not find a 
signal once it calls dequeue_signal(), but this is fine as far as signalfd 
goes. This should be OK in general, no?


- Davide


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