Date: Mon, 8 Jun 2015 17:49:18 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: umgwanakikbuti@gmail.com, mingo@elte.hu, ktkhai@parallels.com,
        rostedt@goodmis.org, tglx@linutronix.de, juri.lelli@gmail.com,
        pang.xunlei@linaro.org, wanpeng.li@linux.intel.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 08/14] hrtimer: Allow hrtimer::function() to free the
	timer
Message-ID: <20150608154918.GA23743@redhat.com>
References: <20150605084836.364306429@infradead.org> <20150605085205.723058588@infradead.org> <20150607223317.GA5193@redhat.com> <20150608091417.GM19282@twins.programming.kicks-ass.net> <20150608124234.GW18673@twins.programming.kicks-ass.net> <20150608142749.GB13168@redhat.com> <20150608144231.GL3644@twins.programming.kicks-ass.net>
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On 06/08, Peter Zijlstra wrote:
>
> On Mon, Jun 08, 2015 at 04:27:49PM +0200, Oleg Nesterov wrote:
> >
> > Most probably I missed something... I'll try to think more, but perhaps
> > you see a hole immediately?
>
> This is something I proposed earlier; Kirill said:
>
>   lkml.kernel.org/r/2134411433408823@web8j.yandex.ru
>
> Which I read like the below, imagine our timer expires periodically and
> rearms itself:
>
>  acquire
>  cpu_base->running = timer;
>  wmb
>  timer->state = INACTIVE;
>  release
> 				[R] timer->state (== INACTIVE)
>  fn()
>  acquire
>  timer->state = ACTIVE
>  wmb
>  cpu_base->running = NULL
>  release
>
> 				[R] cpu_base->running (== NULL)
>
>  acquire
>  cpu_base->running = timer;
>  wmb
>  timer->state = INACTIVE;
>  release
>
> 				[R] timer->state (== INACTIVE)

Damn yes. Thanks Kirill and Peter.

And I swear, I swear I was thinking about this race yesterday but
forgot this problem today ;)

Yes it seems that we can't avoid the seq counter. But perhaps we
can increment/check it once in run_hrtimer/hrtimer_inactive ...
I'll try to think.

Thanks!

Oleg.

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