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

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Date:	Fri, 11 Dec 2015 12:39:59 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Turner <pjt@...gle.com>
Cc:	NeilBrown <nfbrown@...ell.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...nel.org>,
	Peter Anvin <hpa@...or.com>, vladimir.murzin@....com,
	linux-tip-commits@...r.kernel.org, jstancek@...hat.com,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [tip:locking/core] sched/wait: Fix signal handling in bit wait
 helpers

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@...gle.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@...ns.programming.kicks-ass.net


Thanks!


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