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From: Paolo Bonzini <pbonzini@redhat.com>
To: "Emilio G. Cota" <cota@braap.org>,
	QEMU Developers <qemu-devel@nongnu.org>,
	MTTCG Devel <mttcg@greensocs.com>
Cc: "Alex Bennée" <alex.bennee@linaro.org>,
	"Richard Henderson" <rth@twiddle.net>,
	"Sergey Fedorov" <serge.fdrv@gmail.com>
Subject: Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic primitives for RCU atomics
Date: Mon, 23 May 2016 16:21:36 +0200	[thread overview]
Message-ID: <955e8307-01a5-b2f9-48df-8309bd30c443@redhat.com> (raw)
In-Reply-To: <1463863336-28760-2-git-send-email-cota@braap.org>



On 21/05/2016 22:42, Emilio G. Cota wrote:
> Commit a0aa44b4 ("include/qemu/atomic.h: default to __atomic functions")
> set all atomics to default (on recent GCC versions) to __atomic primitives.
> 
> In the process, the atomic_rcu_read/set were converted to implement
> consume/release semantics, respectively. This is inefficient; for
> correctness and maximum performance we only need an smp_barrier_depends
> for reads, and an smp_wmb for writes. Fix it by using the original
> definition of these two primitives for all compilers.

Indeed most compilers implement consume the same as acquire, which is
inefficient.  However, isn't in practice atomic_thread_fence(release) +
atomic_store(relaxed) the same as atomic_store(release)?

Thanks,

Paolo

  parent reply	other threads:[~2016-05-23 14:21 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 [this message]
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
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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