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

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Date:	Wed, 19 Oct 2011 13:01:24 -0700
From:	Greg KH <greg@...ah.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Dave Jones <davej@...hat.com>, Simon Kirby <sim@...tway.ca>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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 09:41:52PM +0200, Thomas Gleixner wrote:
> On Wed, 19 Oct 2011, Greg KH 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?
> 
> Oops. It's in 3.0.7
> 
> commit 249cf808ba1a0d403fe7c476a74b66e2bc0a8e53
> Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date:   Thu Sep 1 12:42:04 2011 +0200
> 
>     posix-cpu-timers: Cure SMP wobbles
>     
>     commit d670ec13178d0fd8680e6742a2bc6e04f28f87d8 upstream.
> 
> and that patch introduced the above deadlock, which is cured by:
> 
> commit bcd5cff7216f9b2de0a148cc355eac199dc6f1cf
> Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date:   Mon Oct 17 11:50:30 2011 +0200
> 
>     cputimer: Cure lock inversion

Thanks for the explanation.  I should have it all straightened out now.

greg k-h
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