On Sun, May 08, 2016 at 10:36:21PM -0700, Peter Hurley wrote: > On 05/08/2016 09:56 PM, Davidlohr Bueso wrote: > > The field is obviously updated w.o the lock and needs a READ_ONCE > > while waiting for lock holder(s) to go away, just like we do with > > all other ->count accesses. > > This isn't actually fixing a bug because it's passed through > several full barriers which will force reloading from sem->count. > > I think the patch is ok if you want it just for consistency anyway, > but please change $subject and changelog.
Agreed, and note that the READ_ONCE does prohibit load-tearing, while the current code does not. So in that respect the patched code is more strict. But yes, the current code does not allow using a stale value of sem->count() because, as PeterH notes, we've just passed through at least the full memory barrier implied by set_current_state().
