Date: Tue, 5 Dec 2017 21:51:48 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
        dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
        oleg@redhat.com, Jason Wang <jasowang@redhat.com>, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org
Subject: Re: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant
 read_barrier_depends()
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On Tue, Dec 05, 2017 at 11:33:39AM -0800, Paul E. McKenney wrote:
> On Tue, Dec 05, 2017 at 09:24:21PM +0200, Michael S. Tsirkin wrote:
> > 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);
> 
> I believe Peter was instead suggesting:
> 
> WRITE_ONCE(obj->val, 1);
> smp_store_release(foo, obj);

Isn't that more expensive though?


> > 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.
> 
> There are quite a few people who say that smp_store_release() is
> easier for the tools to analyze than is smp_wmb().  My experience with
> smp_read_barrier_depends() and rcu_dereference() leads me to believe
> that they are correct.
> 
> 							Thanx, Paul

OK, but smp_store_release is still not paired with anything since we
rely on READ_ONCE to include the implicit dpendendency barrier.

-- 
MST