linux-kernel - Re: mx6: suspicious RCU usage

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Date:	Tue, 14 Jul 2015 14:31:03 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Fabio Estevam <festevam@...il.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Peter Zijlstra <peterz@...radead.org>, rostedt@...dmis.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: mx6: suspicious RCU usage

On Tue, Jul 14, 2015 at 09:49:09PM +0100, Russell King - ARM Linux wrote:
> On Tue, Jul 14, 2015 at 09:26:00AM -0700, Paul E. McKenney wrote:
> > On Tue, Jul 14, 2015 at 01:06:32PM -0300, Fabio Estevam wrote:
> > > Hi Paul,
> > > 
> > > On Tue, Jul 14, 2015 at 1:00 PM, Paul E. McKenney
> > > <paulmck@...ux.vnet.ibm.com> wrote:
> > > 
> > > > Does this patch help?
> > > >
> > > >         https://lkml.org/lkml/2015/5/12/885
> > > 
> > > I am using an ARM 32-bit machine, so I used this one instead:
> > > https://lkml.org/lkml/2015/5/12/892
> > > 
> > > , and it fixes the problem. Thanks!
> > > 
> > > Feel free to add:
> > > 
> > > Tested-by: Fabio Estevam <fabio.estevam@...escale.com>
> > 
> > Glad it helped!
> > 
> > Russell, did you want me to push this, or would you rather take it?
> 
> If it's the one I'm thinking of (using the generic code) because it
> doesn't actually solve the problem we have.  It may shut up the RCU
> warning, but it doesn't solve the underlying problem - one which is
> caused by the use of atomics (which use the exclusive instructions)
> vs cache line migration between CPUs vs speculative prefetching...
> 
> It's possible right now that _dirty_ cache lines can be migrated to
> the dying CPU, which are then lost on power down - and if we disable
> the caches on the dying CPU, we then can't use exclusives, so atomics
> (and all of the other normal kernel synchronisation mechanisms) are
> out of the question.

OK, that could explain any number of failures.

I will leave this issue in your hands, then.

							Thanx, Paul

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