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

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Message-ID: <566B0E76.9020704@arm.com>
Date:	Fri, 11 Dec 2015 17:57:10 +0000
From:	Vladimir Murzin <vladimir.murzin@....com>
To:	Peter Zijlstra <peterz@...radead.org>, 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>, 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 11/12/15 11:39, Peter Zijlstra wrote:
> 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
> 

By this time my test has been run ~500 times without any stalls. I'll
keep running overnight (just in case), but I think that patch can be
marked as tested.

Cheers
Vladimir

> 
> Thanks!
> 
> 

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