From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:52958)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <serge.fdrv@gmail.com>) id 1b5WaR-0003y2-B0
	for qemu-devel@nongnu.org; Wed, 25 May 2016 07:02:52 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <serge.fdrv@gmail.com>) id 1b5WaL-0003E0-II
	for qemu-devel@nongnu.org; Wed, 25 May 2016 07:02:51 -0400
Received: from mail-lf0-x236.google.com ([2a00:1450:4010:c07::236]:36145)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <serge.fdrv@gmail.com>) id 1b5WaL-0003Dw-6W
	for qemu-devel@nongnu.org; Wed, 25 May 2016 07:02:45 -0400
Received: by mail-lf0-x236.google.com with SMTP id e130so16596470lfe.3
	for <qemu-devel@nongnu.org>; Wed, 25 May 2016 04:02:45 -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>
	<87y46ycyee.fsf@linaro.org>
From: Sergey Fedorov <serge.fdrv@gmail.com>
Message-ID: <57458650.8090902@gmail.com>
Date: Wed, 25 May 2016 14:02:40 +0300
MIME-Version: 1.0
In-Reply-To: <87y46ycyee.fsf@linaro.org>
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: =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>
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>

On 25/05/16 11:52, Alex Bennée wrote:
> 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.
>

We can always do:

    #if defined(__aarch64__) && !defined(__SANITIZE_THREAD__)
    /* AArch64 asm variant */
    #else
    /* GCC __atomic variant */
    #endif


Kind regards,
Sergey