On Wed, Nov 27, 2013 at 10:16:13AM +0000, Will Deacon wrote: > On Tue, Nov 26, 2013 at 10:51:36PM +0000, Paul E. McKenney wrote: > > On Tue, Nov 26, 2013 at 11:32:25AM -0800, Linus Torvalds wrote: > > > On Tue, Nov 26, 2013 at 11:20 AM, Paul E. McKenney > > > <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > There are several places in RCU that assume unlock+lock is a full > > > > memory barrier, but I would be more than happy to fix them up given > > > > an smp_mb__after_spinlock() and an smp_mb__before_spinunlock(), or > > > > something similar. > > > > > > A "before_spinunlock" would actually be expensive on x86. > > > > Good point, on x86 the typical non-queued spin-lock acquisition path > > has an atomic operation with full memory barrier in any case. I believe > > that this is the case for the other TSO architectures. For the non-TSO > > architectures: > > > > o ARM has an smp_mb() during lock acquisition, so after_spinlock() > > can be a no-op for them. > > Ok, but what about arm64? We use acquire for lock() and release for > unlock(), so in Linus' example: Right, I did forget the arm vs. arm64 split! > write A; > spin_lock() > mb__after_spinlock(); > read B > > Then A could very well be reordered after B if mb__after_spinlock() is a nop. > Making that a full barrier kind of defeats the point of using acquire in the > first place... The trick is that you don't have mb__after_spinlock() unless you need the ordering, which we expect in a small minority of the lock acquisitions. So you would normally get the benefit of acquire/release efficiency. > It's one thing ordering unlock -> lock, but another getting those two to > behave as full barriers for any arbitrary memory accesses. 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. Thanx, Paul -- 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>