Re: [tip:locking/core] sched/wait: Fix signal handling in bit wait helpers

From: Peter Zijlstra <peterz@infradead.org>
To: Paul Turner <pjt@google.com>
Cc: NeilBrown <nfbrown@novell.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Mike Galbraith <efault@gmx.de>, Ingo Molnar <mingo@kernel.org>,
	Peter Anvin <hpa@zytor.com>,
	vladimir.murzin@arm.com, linux-tip-commits@vger.kernel.org,
	jstancek@redhat.com, Oleg Nesterov <oleg@redhat.com>
Subject: Re: [tip:locking/core] sched/wait: Fix signal handling in bit wait helpers
Date: Fri, 11 Dec 2015 12:39:59 +0100	[thread overview]
Message-ID: <20151211113959.GI6356@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <CAPM31R+ohAB3w+wTWj08LfM9ePP8tfyW-Vie5Uef-RwCu-b4sw@mail.gmail.com>

On Fri, Dec 11, 2015 at 03:30:33AM -0800, Paul Turner wrote:

> > Blergh, all I've managed to far is to confuse myself further. Even
> > something like the original (+- the EINTR) should work when we consider
> > the looping, even when mixed with an occasional spurious wakeup.
> >
> >
> > int bit_wait()
> > {
> >         if (signal_pending_state(current->state, current))
> >                 return -EINTR;
> >         schedule();
> > }

So I asked Vladimir to test that (simply changing the return from 1 to
-EINTR) and it made his fail much less likely but it still failed in the
same way.

So I'm fairly sure I'm still missing something :/

> Hugh asked me about this after seeing a crash, here's another exciting
> way in which the current code breaks -- this one actually quite
> serious:

Yep, this got reported by Jan and I did kick myself for that.

> Peter's proposed follow-up above looks strictly more correct.  We need
> to evaluate the potential existence of a signal, *after* we return
> from schedule, but in the context of the state which we previously
> _entered_ schedule() on.
> 
> Reviewed-by: Paul Turner <pjt@google.com>

Right, its maybe a bit overkill, but at this point I'm a tad
conservative/paranoid.

Vladimir, Jan could you both please that patch?

 lkml.kernel.org/r/20151208104712.GJ6356@twins.programming.kicks-ass.net

Thanks!