Subject: Re: [PATCH/RFC] signal races/bugs, losing TIF_SIGPENDING and other
	woes
From: Nicholas Miell <nmiell@comcast.net>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Davide Libenzi <davidel@xmailserver.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Linux Kernel list <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Paul Mackerras <paulus@samba.org>
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Date: Tue, 05 Jun 2007 16:51:02 -0700
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On Tue, 2007-06-05 at 17:27 +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2007-06-04 at 23:09 -0700, Nicholas Miell wrote:
> > signalfd() doesn't deliver thread-targeted signals to the wrong
> > threads,
> > does it?
> > 
> > Hmm.
> > 
> > It looks like reading from a signalfd will give you either
> > process-global signals or the thread-specific signals that are
> > targeted
> > towards the thread that originally created the signalfd (regardless of
> > which thread actually calls read()).
> > 
> > Which is weird, to say the least. Definitely needs to be noted in the
> > man page, which doesn't seem to exist yet.
> > 
> > Is there a reason why signalfd() doesn't behave like regular signals
> > in
> > this regard? 
> 
> It's worse than that ... by being able to call dequeue_signal from the
> contxt of another thread than the one dequeuing from.
> 
> Ben.

Yes, that's certainly wrong, but that's an implementation issue. I was
more concerned about the design of the API.

Naively, I would expect a reads on a signalfd to return either process
signals or thread signals targeted towards the thread doing the read.

What it actually does (delivering process signals or thread signals
targeted towards the thread that created the signalfd) is weird.

For one, it means you can't create a single signalfd, stick it in an
epoll set, and then wait on that set from multiple threads.

-- 
Nicholas Miell <nmiell@comcast.net>

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