linux-kernel - Re: Fix signalfd interaction with thread-private signals

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Message-ID: <20070620111415.GA91@tv-sign.ru>
Date:	Wed, 20 Jun 2007 15:14:15 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	Nicholas Miell <nmiell@...cast.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Fix signalfd interaction with thread-private signals

On 06/20, Benjamin Herrenschmidt wrote:
>
> On Tue, 2007-06-19 at 18:06 +0400, Oleg Nesterov wrote:
> > On 06/19, Benjamin Herrenschmidt wrote:
> > >
> > > On Tue, 2007-06-19 at 13:14 +0400, Oleg Nesterov wrote:
> > > 
> > > > The commited "Fix signalfd interaction with thread-private signals"
> > > > (commit caec4e8dc85e0644ec24aeb36285e1ba02da58cc) doesn't implement
> > > > this.
> > > 
> > > Indeed, if you want what Davide described, you need to also change
> > > signalfd side. The patch I did merely prevents another thread from
> > > dequeuing somebody else private signals.
> > 
> > Yes I see, but why do we need this change? Yes, we can dequeue SIGSEGV
> > from another thread. Just don't do it if you have a handler for SIGSEGV?
> 
> Well, for such synchronous signals, it's a fairly stupid idea,
> especially since you can't predict who will get it. Signals such as SEGV
> are forced-in, which means they are force-unblocked. Thus, you can't
> know for sure whome of signalfd or the target thread will get it first,
> depending on who catches the siglock first I suppose. In one case,
> you'll manage to steal it, in the other, you'll thread will be killed.

Yes. As I said, I think this falls into the "just don't do that" category.
But nothing bad happens from the kernel POV.

Also, suppose that some thread does

	for (;;)
		signal(SIGSEGV, SIG_IGN);

Now we have the same situation. do_sigaction() can steal SIGSEGV from
another thread.


Perhaps, the proposed behaviour

	> Multiple threads can wait on the signalfd. Each one will dequeue either
	> its own private signals (tsk->pending) or the process shared ones
	> (tsk->signal->shared_pending).

is more convenient, I can't judge. If we implement this, sys_signalfd(-1) in
essence means "attach to the thread group, not current". This also makes sense.
But what we have now (with this patch applied) is a bit strange, imho.

Oleg.

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