On Thu, Nov 28, 2013 at 05:38:53PM +0000, Paul E. McKenney wrote: > On Thu, Nov 28, 2013 at 11:40:59AM +0000, Will Deacon wrote: > > On Wed, Nov 27, 2013 at 05:11:43PM +0000, Paul E. McKenney wrote: > > > And in fact the unlock+lock barrier is all that RCU needs. I guess the > > > question is whether it is worth having two flavors of __after_spinlock(), > > > one that is a full barrier with just the lock, and another that is > > > only guaranteed to be a full barrier with unlock+lock. > > > > I think it's worth distinguishing those cases because, in my mind, one is > > potentially a lot heavier than the other. The risk is that we end up > > producing a set of strangely named barrier abstractions that nobody can > > figure out how to use properly: > > > > > > /* > > * Prevent re-ordering of arbitrary accesses across spin_lock and > > * spin_unlock. > > */ > > mb__after_spin_lock() > > mb__after_spin_unlock() > > > > /* > > * Order spin_lock() vs spin_unlock() > > */ > > mb__between_spin_unlock_lock() /* Horrible name! */ > > > > > > We could potentially replace the first set of barriers with spin_lock_mb() > > and spin_unlock_mb() variants (which would be more efficient than half > > barrier + full barrier), then we only end up with strangely named barrier > > which applies to the non _mb() spinlock routines. > > How about the current mb__before_spinlock() making the acquisition be > a full barrier, and an mb_after_spinlock() making a prior release plus > this acquisition be a full barrier? Hmm, without horrible hacks to keep track of whether we've done an mb__before_spinlock() without a matching spinlock(), that's going to end up with full-barrier + pointless half-barrier (similarly on the release path). > Yes, we might need better names, but I believe that this approach does > what you need. > > Thoughts? I still think we need to draw the distinction between ordering all accesses against a lock and ordering an unlock against a lock. The latter is free for arm64 (STLR => LDAR is ordered) but the former requires a DMB. Not sure I completely got your drift... Will -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>