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Date: Wed, 7 May 2014 17:29:24 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, hpa@zytor.com,
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        khilman@linaro.org, tglx@linutronix.de, axboe@fb.com,
        linux-tip-commits@vger.kernel.org
Subject: Re: [tip:timers/nohz] nohz: Move full nohz kick to its own IPI
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On Wed, May 07, 2014 at 05:17:35PM +0200, Peter Zijlstra wrote:
> On Mon, May 05, 2014 at 05:34:08PM +0200, Frederic Weisbecker wrote:
> > On Mon, May 05, 2014 at 05:12:28PM +0200, Peter Zijlstra wrote:
> > > > Note the current ordering:
> > > > 
> > > >     cmpxchg(&qsd->pending, 0, 1)       get ipi
> > > >     csd_lock(qsd->csd)                 xchg(&qsd->pending, 1)
> > > >     send ipi                           csd_unlock(qsd->csd)
> > > > 
> > > > 
> > > > So there shouldn't be racing updaters. Also ipi sender shouldn't
> > > > race with ipi receiver, the update shouldn't always eventually see
> > > > the unlock happening.
> > > 
> > > Yeah, I've not spotted how this particular train wreck happens either.
> > > 
> > > The problem is reproduction, it took me 9 hours to confirm I could
> > > reproduce the problem on my machine. So how long to I run it with this
> > > patch reverted to show its gone..
> > 
> > Maybe it could be favoured cpu hotplug. Anyway converting to irq_work should
> > fix it.
> 
> Ingo needs a commit msg for the revert of this patch; do you think you
> have time to look into _why_ this patch is broken and write such a
> thing?

I can try but I need to reproduce it. Do you have any clue on how to do so?
Also which HEAD were you guys using?

Thanks.