Messages in this thread | | | Date | Tue, 10 Dec 2013 09:43:20 -0800 | From | Josh Triplett <> | Subject | Re: [PATCH v5 tip/core/locking 5/7] Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK |
| |
On Tue, Dec 10, 2013 at 06:25:28PM +0100, Peter Zijlstra wrote: > On Tue, Dec 10, 2013 at 09:12:47AM -0800, Paul E. McKenney wrote: > > > The way I read the above it says that you need > > > smp_mb__after_unlock_lock() when the UNLOCK and LOCK are on the same > > > variable. That doesn't make sense, I thought that was the one case we > > > all agreed on it would indeed be a full barrier without extra trickery. > > > > On x86, sure, but smp_mb__after_unlock_lock() is nothingness on x86 > > anyway. Other architectures might benefit from requiring that the > > smp_mb__after_unlock_lock() be used in this case. > > Confused, UNLOCK X, LOCK X, must always be fully serializing. That's the > entire purpose of the thing. > > The only place you can go play games (and clearly we are going there) is > when the UNLOCK and LOCK are on different variables.
That would certainly be a good assumption to preserve, and it would eliminate most of the need for smp_mb__after_unlock_lock().
- Josh Triplett
| |