linux-kernel - Re: [tip:timers/nohz] nohz: Move full nohz kick to its own IPI

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Message-ID: <20140507151735.GQ30445@twins.programming.kicks-ass.net>
Date:	Wed, 7 May 2014 17:17:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, hpa@...or.com,
	paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	khilman@...aro.org, tglx@...utronix.de, axboe@...com,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/nohz] nohz: Move full nohz kick to its own IPI

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?

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