Re: Fix signalfd interaction with thread-private signals

From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Davide Libenzi <davidel@xmailserver.org>,
	Nicholas Miell <nmiell@comcast.net>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Fix signalfd interaction with thread-private signals
Date: Wed, 20 Jun 2007 09:24:33 +1000	[thread overview]
Message-ID: <1182295473.26853.386.camel@localhost.localdomain> (raw)
In-Reply-To: <20070619140646.GB27343@tv-sign.ru>

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.

Ben.