From: "Emilio G. Cota" <cota@braap.org>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Richard Henderson" <rth@twiddle.net>,
"MTTCG Devel" <mttcg@greensocs.com>,
"Alex Bennée" <alex.bennee@linaro.org>,
"QEMU Developers" <qemu-devel@nongnu.org>,
"Sergey Fedorov" <serge.fdrv@gmail.com>
Subject: Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics
Date: Tue, 24 May 2016 15:56:09 -0400 [thread overview]
Message-ID: <20160524195609.GA30809@flamenco> (raw)
In-Reply-To: <ecc59b16-8496-db4d-3df8-cfe1e6b0357a@redhat.com>
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.
Thanks,
Emilio
next prev parent reply other threads:[~2016-05-24 19:56 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-21 20:42 [Qemu-devel] [PATCH 0/2] atomics: fix small RCU perf. regression + update documentation Emilio G. Cota
2016-05-21 20:42 ` [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics Emilio G. Cota
2016-05-22 7:58 ` Alex Bennée
2016-05-24 18:42 ` Emilio G. Cota
2016-05-23 14:21 ` Paolo Bonzini
2016-05-23 15:55 ` Emilio G. Cota
2016-05-23 16:53 ` Richard Henderson
2016-05-23 17:09 ` Emilio G. Cota
2016-05-24 7:08 ` Paolo Bonzini
2016-05-24 19:56 ` Emilio G. Cota [this message]
2016-05-24 19:59 ` Sergey Fedorov
2016-05-25 8:52 ` Alex Bennée
2016-05-25 11:02 ` Sergey Fedorov
2016-05-21 20:42 ` [Qemu-devel] [PATCH 2/2] docs/atomics: update atomic_read/set comparison with Linux Emilio G. Cota
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