From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:44147)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <alex.bennee@linaro.org>) id 1b5UXy-00027R-1N
	for qemu-devel@nongnu.org; Wed, 25 May 2016 04:52:10 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <alex.bennee@linaro.org>) id 1b5UXt-0004Fz-UE
	for qemu-devel@nongnu.org; Wed, 25 May 2016 04:52:09 -0400
Received: from mail-wm0-x22a.google.com ([2a00:1450:400c:c09::22a]:36238)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <alex.bennee@linaro.org>) id 1b5UXt-0004Ft-FM
	for qemu-devel@nongnu.org; Wed, 25 May 2016 04:52:05 -0400
Received: by mail-wm0-x22a.google.com with SMTP id n129so169636259wmn.1
	for <qemu-devel@nongnu.org>; Wed, 25 May 2016 01:52:04 -0700 (PDT)
References: <1463863336-28760-1-git-send-email-cota@braap.org>
	<1463863336-28760-2-git-send-email-cota@braap.org>
	<e48bb769-9339-7c3f-6ec4-496609bc2bca@twiddle.net>
	<20160523170912.GA16390@flamenco>
	<ecc59b16-8496-db4d-3df8-cfe1e6b0357a@redhat.com>
	<20160524195609.GA30809@flamenco> <5744B2BD.5000705@gmail.com>
From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
In-reply-to: <5744B2BD.5000705@gmail.com>
Date: Wed, 25 May 2016 09:52:41 +0100
Message-ID: <87y46ycyee.fsf@linaro.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Subject: Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic
 primitives for RCU atomics
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel/>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Sergey Fedorov <serge.fdrv@gmail.com>
Cc: "Emilio G. Cota" <cota@braap.org>, Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <rth@twiddle.net>, MTTCG Devel <mttcg@greensocs.com>, QEMU Developers <qemu-devel@nongnu.org>


Sergey Fedorov <serge.fdrv@gmail.com> writes:

> On 24/05/16 22:56, Emilio G. Cota wrote:
>> 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.
>
> However, maybe it's not such a pain to maintain an optimized version for
> AArch64 in assembly :P

Please don't. The advantage of the builtins is they are known by things
like tsan.

>
> Best,
> Sergey


--
Alex Bennée