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

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Message-ID: <20171205195752.GS3165@worktop.lehotels.local>
Date:   Tue, 5 Dec 2017 20:57:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
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 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.