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

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Date:   Tue, 5 Dec 2017 19:39:46 +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 08:31:20PM +0200, Michael S. Tsirkin wrote:

> Apropos, READ_ONCE is now asymmetrical with WRITE_ONCE.
> 
> I can read a pointer with READ_ONCE and be sure the value
> is sane, but only if I also remember to put in smp_wmb before
> WRITE_ONCE. Otherwise the pointer is ok but no guarantees
> about the data pointed to.

That was already the case on everything except Alpha. And the canonical
match do the data dependency is store_release, not wmb.