Re: Fix signalfd interaction with thread-private signals

Benjamin Herrenschmidt
2007-06-19T23:24:57Z

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.