On Mon, Jul 13, 2015 at 01:20:32PM -0700, Paul E. McKenney wrote: > On Mon, Jul 13, 2015 at 06:50:29PM +0100, Will Deacon wrote: > > So if I'm following along, smp_mb__after_unlock_lock *does* provide > > transitivity when used with UNLOCK + LOCK, which is stronger than your > > example here. > > Yes, that is indeed the intent. Maybe good to state this explicitly somewhere. > > I don't think we want to make the same guarantee for general RELEASE + > > ACQUIRE, because we'd end up forcing most architectures to implement the > > expensive macro for a case that currently has no users. > > Agreed, smp_mb__after_unlock_lock() makes a limited guarantee. I'm still not seeing how the archs that implement load_acquire and store_release with smp_mb() are a problem. If we look at the inside of the critical section again -- similar argument as before: *A = a smp_mb() store M load N smp_mb() *B = b A and B are fully ordered, and in this case even transitivity is provided. I'm stating that the order of M and N don't matter, only the load/stores that are inside the acquire/release are constrained. IOW, I think smp_mb__after_unlock_lock() already works as advertised with all our acquire/release thingies -- as is stated by the documentation. That said, I'm not aware of anybody but RCU actually using this, so its not used in that capacity. > > In which case, it boils down to the question of how expensive it would > > be to implement an SC UNLOCK operation on PowerPC and whether that justifies > > the existence of a complicated barrier macro that isn't used outside of > > RCU. > > Given that it is either smp_mb() or nothing, I am not seeing the > "complicated" part... The 'complicated' part is that we need think about it; that is we need to realized and remember that UNLOCK+LOCK is a load-store barrier but fails to provide transitivity. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
