linux-kernel - Re: Patch Upstream: cputimer: Cure lock inversion

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Message-ID: <CA+5PVA6GnscQTRbv=r0NZvU4jNGwnr64OG6qAZvGhnjSAGCgrA@mail.gmail.com>
Date:	Wed, 19 Oct 2011 12:03:50 -0400
From:	Josh Boyer <jwboyer@...il.com>
To:	Greg KH <greg@...ah.com>
Cc:	Dave Jones <davej@...hat.com>, Simon Kirby <sim@...tway.ca>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	stable@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Patch Upstream: cputimer: Cure lock inversion

On Wed, Oct 19, 2011 at 12:00 PM, Greg KH <greg@...ah.com> wrote:
> On Wed, Oct 19, 2011 at 11:55:01AM -0400, Josh Boyer wrote:
>> On Wed, Oct 19, 2011 at 11:49 AM, Greg KH <greg@...ah.com> wrote:
>> > On Wed, Oct 19, 2011 at 11:39:14AM -0400, Gregs git-bot wrote:
>> >> commit: bcd5cff7216f9b2de0a148cc355eac199dc6f1cf
>> >> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>> >> Date: Mon, 17 Oct 2011 11:50:30 +0200
>> >> Subject: cputimer: Cure lock inversion
>> >>
>> >> There's a lock inversion between the cputimer->lock and rq->lock;
>> >> notably the two callchains involved are:
>> >>
>> >>  update_rlimit_cpu()
>> >>    sighand->siglock
>> >>    set_process_cpu_timer()
>> >>      cpu_timer_sample_group()
>> >>        thread_group_cputimer()
>> >>          cputimer->lock
>> >>          thread_group_cputime()
>> >>            task_sched_runtime()
>> >>              ->pi_lock
>> >>              rq->lock
>> >>
>> >>  scheduler_tick()
>> >>    rq->lock
>> >>    task_tick_fair()
>> >>      update_curr()
>> >>        account_group_exec()
>> >>          cputimer->lock
>> >>
>> >> Where the first one is enabling a CLOCK_PROCESS_CPUTIME_ID timer, and
>> >> the second one is keeping up-to-date.
>> >>
>> >> This problem was introduced by e8abccb7193 ("posix-cpu-timers: Cure
>> >> SMP accounting oddities").
>> >
>> > There is no such patch in Linus's tree that I can find.  So, what
>> > problem is this really trying to cure here and what kernel did it show
>> > up in?
>>
>> Uh...
>>
>> bcd5cff7216f9b2de0a148cc355eac199dc6f1cf is the upstream commit (post -rc10).
>
> No, I understand that this is the commit I just referenced.
>
> I'm talking about the "This problem was introduced..." line in the
> commit.  I want to find out what was the original problem that this
> patch is fixing, to determine how far back in the -stable series I need
> to backport this to.

Ah, I misunderstood.

> The issue is that there is no e8abccb7193 ("posix-cpu-timers: Cure
> SMP accounting oddities") commit that I can see in Linus's tree right
> now.
>
>> This thread covers the conversation (it's long):
>>
>> http://thread.gmane.org/gmane.linux.kernel/1199406/focus=1204676
>
> Ugh, I'll go dig, but help would be appreciated...

My guess is they meant d670ec13178d0fd868

josh
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