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

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Message-ID: <20171205212053-mutt-send-email-mst@kernel.org>
Date:   Tue, 5 Dec 2017 21:24:21 +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 08:17:33PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 05, 2017 at 08:57:46PM +0200, Michael S. Tsirkin wrote:
> 
> > I don't see WRITE_ONCE inserting any barriers, release or
> > write.
> 
> Correct, never claimed there was.
> 
> Just saying that:
> 
> 	obj = READ_ONCE(*foo);
> 	val = READ_ONCE(obj->val);
> 
> Never needs a barrier (except on Alpha and we want to make that go
> away). Simply because a CPU needs to complete the load of @obj before it
> can compute the address &obj->val. Thus the second load _must_ come
> after the first load and we get LOAD-LOAD ordering.
> 
> Alpha messing that up is a royal pain, and Alpha not being an
> active/living architecture is just not worth the pain of keeping this in
> the generic model.
> 

Right. What I am saying is that for writes you need

WRITE_ONCE(obj->val, 1);
smp_wmb();
WRITE_ONCE(*foo, obj);

and this barrier is no longer paired with anything until
you realize there's a dependency barrier within READ_ONCE.

Barrier pairing was a useful tool to check code validity,
maybe there are other, better tools now.


-- 
MST