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Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for

From: Alex Bennée
Subject: Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics
Date: Wed, 25 May 2016 09:52:41 +0100
User-agent: mu4e 0.9.17; emacs

Sergey Fedorov <address@hidden> 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

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