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

[Alex Bennée]

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