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

On Tue, Dec 05, 2017 at 10:28:38PM +0200, Michael S. Tsirkin wrote:

On Tue, Dec 05, 2017 at 08:57:52PM +0100, Peter Zijlstra wrote:

quoted

On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote:

quoted

quoted

quoted

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.

Right. Why should I use the more expensive smp_store_release then?

Because it makes more sense. Memory ordering is hard enough, don't make
it harder still if you don't have to.