linux-kernel - Re: [PATCH 5/6] net: __this_cpu

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Date:	Wed, 16 Oct 2013 10:46:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Tejun Heo <tj@...nel.org>, akpm@...uxfoundation.org,
	rostedt@...dmis.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH 5/6] net: __this_cpu_inc in route.c

On Tue, Oct 15, 2013 at 12:47:27PM -0500, Christoph Lameter wrote:
> The RT_CACHE_STAT_INC macro triggers the new preemption checks
> for __this_cpu ops
> 
> I do not see any other synchronization that would allow the use
> of a __this_cpu operation here however in commit
> dbd2915ce87e811165da0717f8e159276ebb803e Andrew justifies
> the use of raw_smp_processor_id() here because "we do not care"
> about races.
> 
> So lets use raw_cpu ops here and hope for the best. The use of
> __this_cpu op improves the situation already from what commit
> dbd2915ce87e811165da0717f8e159276ebb803e did since the single instruction
> emitted on x86 does not allow the race to occur anymore. However,
> non x86 platforms could still experience a race here.
> 

Are we sure all !x86 implementations will DTRT in that it will increment
some CPU and not get horribly confused? I suppose it would; but is
that a guarantee given someplace?
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