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

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Date:	Mon, 5 May 2014 17:12:28 +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

> 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..
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