linux-kernel - Re: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier

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Message-ID: <20171205232516-mutt-send-email-mst@kernel.org>
Date:   Tue, 5 Dec 2017 23:42:44 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, Jason Wang <jasowang@...hat.com>,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant
 read_barrier_depends()

On Tue, Dec 05, 2017 at 10:17:35PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 05, 2017 at 10:28:38PM +0200, Michael S. Tsirkin wrote:
> > On Tue, Dec 05, 2017 at 08:57:52PM +0100, Peter Zijlstra wrote:
> > > On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote:
> > > > > > WRITE_ONCE(obj->val, 1);
> > > > > > smp_wmb();
> > > > > > WRITE_ONCE(*foo, obj);
> > > > > 
> > > > > I believe Peter was instead suggesting:
> > > > > 
> > > > > WRITE_ONCE(obj->val, 1);
> > > > > smp_store_release(foo, obj);
> > > > 
> > > > Isn't that more expensive though?
> > > 
> > > Depends on the architecture. The only architecture where it is more
> > > expensive and people actually still care about is ARM I think.
> > 
> > Right. Why should I use the more expensive smp_store_release then?
> 
> Because it makes more sense. Memory ordering is hard enough, don't make
> it harder still if you don't have to.

I suspect I have to -  ptr_ring is a very low level construct used by
netowrking on data path so making it a bit more complicated for
a bit of performance is probably justified.

-- 
MST