linux-kernel - Re: rcu self-detected stall messages on OMAP3, 4 boards

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Date:	Sat, 22 Sep 2012 22:25:59 +0000 (UTC)
From:	Paul Walmsley <paul@...an.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	"Bruce, Becky" <bbruce@...com>,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	"<linux-kernel@...r.kernel.org>" <linux-kernel@...r.kernel.org>,
	"<linux-omap@...r.kernel.org>" <linux-omap@...r.kernel.org>,
	"<linux-arm-kernel@...ts.infradead.org>" 
	<linux-arm-kernel@...ts.infradead.org>,
	"Hilman, Kevin" <khilman@...com>,
	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	"Hunter, Jon" <jon-hunter@...com>,
	"<snijsure@...d-net.com>" <snijsure@...d-net.com>,
	fweisbec@...il.com
Subject: Re: rcu self-detected stall messages on OMAP3, 4 boards

On Sat, 22 Sep 2012, Paul E. McKenney wrote:

> And here is a patch.  I am still having trouble reproducing the problem,
> but figured that I should avoid serializing things.

Thanks, testing this now on v3.6-rc6.  One question though about the patch 
description:

> All this begs the question of exactly how a callback-free grace period
> gets started in the first place.  This can happen due to the fact that
> CPUs do not necessarily agree on which grace period is in progress.
> If a CPU still believes that the grace period that just completed is
> still ongoing, it will believe that it has callbacks that need to wait
> for another grace period, never mind the fact that the grace period
> that they were waiting for just completed.  This CPU can therefore
> erroneously decide to start a new grace period.

Doesn't this imply that this bug would only affect multi-CPU systems?  

The recent tests here have been on Pandaboard, which is dual-CPU, but my 
recollection is that I also observed the warnings on a single-core 
Beagleboard.  Will re-test.


- Paul
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