On Wed, May 11, 2016 at 03:04:20PM -0700, Peter Hurley wrote: > > + return !rwsem_is_reader_owned(READ_ONCE(sem->owner)); > > It doesn't make sense to force reload sem->owner here; if sem->owner > is not being reloaded then the loop above will execute forever. > > Arguably, this check should be bumped out to the optimistic spin and > reload/check the owner there? >
Note that barrier() and READ_ONCE() have overlapping but not identical results and the combined use actually makes sense here. Yes, a barrier() anywhere in the loop will force a reload of the variable, _however_ it doesn't force that reload to not suffer from load tearing. Using volatile also forces a reload, but also ensures the load cannot be torn IFF it is of machine word side and naturally aligned. So while the READ_ONCE() here is pointless for forcing the reload; that's already ensured, we still need to make sure the load isn't torn.
