linux-kernel - Re: signalfd API issues (was Re: [PATCH/RFC] signal races/bugs, losing TIF

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Date:	Wed, 6 Jun 2007 15:36:30 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nicholas Miell <nmiell@...cast.net>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: signalfd API issues (was Re: [PATCH/RFC] signal races/bugs,
 losing TIF_SIGPENDING and other woes)

On Wed, 6 Jun 2007, Benjamin Herrenschmidt wrote:

> On Tue, 2007-06-05 at 20:37 -0700, Linus Torvalds wrote:
> 
> > I agree that it would be a limitation, but it would be a sane one.
> > 
> > How about we try to live with that limitation, if only to avoid the issue 
> > of having the private signals being stolen by anybody else. If we actually 
> > find a real-live use-case where that is bad in the future, we can re-visit 
> > the issue - it's always easier to _expand_ semantics later than it is to 
> > restrict them, so I think this thread is a good argument for starting it 
> > out in a more restricted form before people start depending on semantics 
> > that can be nasty..
> 
> Here's a patch. Let me know if I missed something.
> 
> Fix races with signalfd and TIF_SIGPENDING
> 
> We must never clear TIF_SIGPENDING for another task. This patch
> ensures that by preventing recalc_sigpending_tsk() from clearing
> that bit if the target task is not current.
> 
> In addition we also prevent __dequeue_signal() from calling the
> DRM notifier thingy when stealing signals from another task via
> signalfd.
> 
> Finally, we only dequeue shared signals when called from another
> task (via signalfd), we leave private signals alone.

Makes sense...



> Signed-off-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>

Acked-by: Davide Libenzi <davidel@...ilserver.org>



- Davide


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