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

[Sergey Fedorov]

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

Best,
Sergey