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Date: Tue, 24 May 2016 15:56:09 -0400
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20160524195609.GA30809@flamenco>
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Subject: Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic
 primitives for RCU atomics
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>, MTTCG Devel <mttcg@greensocs.com>, Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>, QEMU Developers <qemu-devel@nongnu.org>, Sergey Fedorov <serge.fdrv@gmail.com>

On Tue, May 24, 2016 at 09:08:01 +0200, Paolo Bonzini wrote:
> On 23/05/2016 19:09, Emilio G. Cota wrote:
> > PS. And really equating smp_wmb/rmb to release/acquire as we have under
> > #ifdef __ATOMIC is hard to justify, other than to please tsan.
> 
> That only makes a difference on arm64, right?
> 
> 	acquire		release		rmb		wmb
> x86	--		--		--		--
> power	lwsync		lwsync		lwsync		lwsync
> armv7	dmb		dmb		dmb		dmb
> arm64	dmb ishld	dmb ish		dmb ishld	dmb ishst
> ia64	--		--		--		--

Yes. I now see why we're defining rmb/wmb based on acquire/release:
it's quite convenient given that the compiler provides them, and
the (tiny) differences in practice are not worth the trouble of
adding asm for them. So I take back my comment =)

The gains of getting rid of the consume barrier from atomic_rcu_read
are clear though; updated patch to follow.

Thanks,

		Emilio