linux-kernel - Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu barrier

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Date:	Tue, 17 Nov 2015 13:22:08 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	gilf@...hip.com, talz@...hip.com, linux-kernel@...r.kernel.org,
	cmetcalf@...hip.com, Noam Camus <noamc@...hip.com>,
	linux-snps-arc@...ts.infradead.org
Subject: Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu
 barrier

On Tue, Nov 17, 2015 at 05:07:38PM +0530, Vineet Gupta wrote:
> > as is ARC-SMP seems to have a _lot_ of superfluous
> > barriers many of which have no explanation yet (I'm thinking of those
> > extra smp_mb()s in the lock primitives).
> 
> Other than the lock primitives can u think of any more.

Not of the top of my head.

> I verified that with llock/scond based spinlocks, those smp_mb() can be safely
> removed.

Good!

> I didn't send that patch over yet as part of puzzle is why removing them
> in EX based locks causes hackbench to jitter on quad core builds. This required
> some perf investigation but that seems to be causing some sort of livelock with
> callgraph profiling which is what I'm debugging currently :-)

So there's two superfluous barriers right; the one before LOCK and the
one after UNLOCK. Does removing either cause the jitter? I'm thinking
that maybe its the smp_mb after UNLOCK that force flushes the store
buffer that helps (MIPS has something similar).

> BTW since we are on the topic we have this loop in stack unwinder which can
> potentially cause RCU stalls, actual lockups etc. I was planning to add the
> following - does that seem fine to you.

Worries me more than anything. How could you get stuck in there?
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