Linux Virtualization - Dec 2017 - [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier_depends()

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

On Tue, Dec 05, 2017 at 08:57:52PM +0100, Peter Zijlstra
wrote:
> On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote:
> > > > 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?

-- 
MST

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