linux-kernel - Re: [PATCH RFC] kvm: optimize out smp_mb using srcu_read

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Message-ID: <52723AD9.4040902@redhat.com>
Date:	Thu, 31 Oct 2013 12:11:21 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH RFC] kvm: optimize out smp_mb using srcu_read_unlock

Il 31/10/2013 07:47, Gleb Natapov ha scritto:
> This looks dubious to me. All other smp_mb__after_* variants are there
> because some atomic operations have different memory barrier semantics on
> different arches,

It doesn't have to be arches; unlock APIs typically have release
semantics only, but SRCU is stronger.

> but srcu_read_unlock() have the same semantics on all
> arches, so smp_mb__after_srcu_read_unlock() becomes
> smp_mb__after_a_function_that_happens_to_have_mb_now_but_may_not_have_in_the_feature().
> How likely it is that smp_mb() will disappear from srcu_read_unlock()
> (if was added for a reason I guess)?  May be we should change documentation
> to say that srcu_read_unlock() is a memory barrier which will reflect
> the reality.

That would be different from all other unlock APIs.

Paolo
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