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

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Date:	Wed, 19 Oct 2011 11:55:01 -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 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).

This thread covers the conversation (it's long):

http://thread.gmane.org/gmane.linux.kernel/1199406/focus=1204676

josh
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