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

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Message-ID: <20131031122836.GQ4651@redhat.com>
Date:	Thu, 31 Oct 2013 14:28:36 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Paolo Bonzini <pbonzini@...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

On Thu, Oct 31, 2013 at 12:11:21PM +0100, Paolo Bonzini wrote:
> 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; 
Of course it doesn't, but it is now :)

>                               unlock APIs typically have release
> semantics only, but SRCU is stronger.
> 
Yes the question is if it is by design or implementation detail we should
not rely on.

> > 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.
> 
As long as it is documented... smp_mb__after_srcu_read_unlock() is just
a form of documentation anyway right now. I do not have strong objection
to smp_mb__after_srcu_read_unlock() though, the improvement is impressive
for such a small change.

--
			Gleb.
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