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SubjectRe: 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@chello.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@chello.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@chello.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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