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 12:23:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	Noam Camus <noamc@...hip.com>, linux-snps-arc@...ts.infradead.org,
	linux-kernel@...r.kernel.org, talz@...hip.com, gilf@...hip.com,
	cmetcalf@...hip.com
Subject: Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu
 barrier

On Tue, Nov 17, 2015 at 04:42:49PM +0530, Vineet Gupta wrote:
> On Saturday 07 November 2015 04:22 PM, Noam Camus wrote:
> > From: Tal Zilcer <talz@...hip.com>
> > 
> > In SMT system like we have the generic "sync" is not working with
> > HW threads. The replacement is "schd.rw" instruction that is served
> > as cpu barrier for HW threads.
> 
> As discussed in v2 of this patch, SYNC or some such in __switch_to is completely
> superfluous. We don't need this patch, you may instead wanna submit a patch which
> removes the sync. Also please do that in assembler version of this file as well !

Do test it though; 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).
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