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

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Date:	Wed, 19 Oct 2011 08:49:10 -0700
From:	Greg KH <greg@...ah.com>
To:	Dave Jones <davej@...hat.com>, Simon Kirby <sim@...tway.ca>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	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 11:39:14AM -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?

confused,

greg k-h
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