Subject: Re: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier_depends()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 5 Dec 2017 21:24:21 +0200

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