linux-kernel - Re: [RFC] TIF_NOTIFY_RESUME, arch/*/*/*signal*.c and all such

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Date:	Wed, 25 Apr 2012 15:32:09 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Russell King <linux@....linux.org.uk>,
	Tejun Heo <tj@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	Roland McGrath <roland@...k.frob.com>
Subject: Re: [RFC] TIF_NOTIFY_RESUME, arch/*/*/*signal*.c and all such

On 04/25, Oleg Nesterov wrote:
>
> On 04/25, Al Viro wrote:
> >
> > On Wed, Apr 25, 2012 at 02:37:46PM +0200, Oleg Nesterov wrote:
> > > As for sys_sigsuspend() and this race in particular:
> > >
> > > > Arrival of a signal that has userland handler
> > > > and that isn't blocked by the mask given to sigsuspend() should terminate
> > > > sigsuspend().
> > >
> > > Yes. But note that do_signal() restores the old sigmask. This means that
> > > the signal we get after the first do_signal() was not blocked before
> > > sigsuspend() was called. So, to some extent, we can pretend that the
> > > handler was executed before sigsuspend() and it was never restarted.
> >
> > Signal might have already arrived by the time we restore sigmask.
>
> Yes, and it sets TIF_SIGPENDING, but unless I missed something this doesn't
> matter.
>
> > So no,
> > it might have been blocked prior to sigsuspend().
>
> If it was not blocked,
        ^^^^^^^
I meant, If it WAS blocked.

> then the next do_signal()->get_signal_to_deliver()
> returns 0 and clears TIF_SIGPENDING. After that we finally re-enter
> sys_sigsuspend() and (assuming it unblocks this sig) notice this pending
> signal again and return -EINTR eventually.
>
> Oleg.

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