linux-kernel - Re: [RFC][PATCH] mips: Fix arch_spin

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Date:	Wed, 3 Feb 2016 19:03:08 +0000
From:	Will Deacon <will.deacon@....com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Maciej W. Rozycki" <macro@...tec.com>,
	David Daney <ddaney@...iumnetworks.com>,
	Måns Rullgård <mans@...sr.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] mips: Fix arch_spin_unlock()

On Wed, Feb 03, 2016 at 01:32:10PM +0000, Will Deacon wrote:
> On Wed, Feb 03, 2016 at 09:33:39AM +0100, Ingo Molnar wrote:
> > In fact I'd suggest to test this via a quick runtime hack like this in rcupdate.h:
> > 
> > 	extern int panic_timeout;
> > 
> > 	...
> > 
> > 	if (panic_timeout)
> > 		smp_load_acquire(p);
> > 	else
> > 		typeof(*p) *________p1 = (typeof(*p) *__force)lockless_dereference(p);
> > 
> > (or so)
> 
> So the problem with this is that a LOAD <ctrl> LOAD sequence isn't an
> ordering hazard on ARM, so you're potentially at the mercy of the branch
> predictor as to whether you get an acquire. That's not to say it won't
> be discarded as soon as the conditional is resolved, but it could
> screw up the benchmarking.
> 
> I'd be better off doing some runtime patching, but that's not something
> I can knock up in a couple of minutes (so I'll add it to my list).

... so I actually got that up and running, believe it or not. Filthy stuff.

The good news is that you're right, and I'm now seeing ~1% difference
between the runs with ~0.3% noise for either of them. I still think
that's significant, but it's a lot more reassuring than 4%.

Will