linux-kernel - Re: [PATCH v1 19/20] ARC: [plat-eznps] replace sync with proper cpu barrier

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Message-ID: <20151102092654.GM17308@twins.programming.kicks-ass.net>
Date:	Mon, 2 Nov 2015 10:26:54 +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-snps-arc@...ts.infradead.org>,
	"cmetcalf@...hip.com" <cmetcalf@...hip.com>,
	"gilf@...hip.com" <gilf@...hip.com>,
	"talz@...hip.com" <talz@...hip.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Gilad Ben Yossef <giladb@...hip.com>
Subject: Re: [PATCH v1 19/20] ARC: [plat-eznps] replace sync with proper cpu
 barrier

On Mon, Nov 02, 2015 at 07:48:54AM +0000, Vineet Gupta wrote:
> Since u bring this up - I think we don't need the original SYNC and/or
> SMT thread schedule at all.  The SYNC here is a historic relic at best
> and we can get rid of it per reasoning below:
> 
> In UP context it is obviously useless, why would we want to stall the
> core for all updates to stack memory of t0 to complete before loading
> kernel ode callee registers from t1 stack's memory.
> 
> In SMP, we could have a potential race in which outdoing task could be
> concurrently picked for running, thus the writes to stack here need to
> be visible before the reads from stack on other core. But I think
> since this is the same rq, there would be a taken spinlock and once a
> core gives it up, an smp barrier would come naturally.
> 
> Peter do u concur ?

I'm still somewhat jet-lagged, but I think the below reference should
answer your question:

lkml.kernel.org/r/20150917130125.GL3816@...ns.programming.kicks-ass.net

I (still) need to update that patch and send it out again.

But I think it answers your question; we do not rely on arch code to
provide barriers for the generic code.

Now, if for some reason the arch code has further constraints, then
maybe, but I don't think so.
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