On Tue, Jan 26, 2016 at 11:19:27AM +0100, Peter Zijlstra wrote: > On Mon, Jan 25, 2016 at 10:03:22PM -0800, Paul E. McKenney wrote: > > On Mon, Jan 25, 2016 at 04:42:43PM +0000, Will Deacon wrote: > > > On Fri, Jan 15, 2016 at 01:58:53PM -0800, Paul E. McKenney wrote: > > > > On Fri, Jan 15, 2016 at 10:27:14PM +0100, Peter Zijlstra wrote: > > > > > > Yes, that seems a good start. But yesterday you raised the 'fun' point > > > > > of two globally ordered sequences connected by a single local link. > > > > > > > > The conclusion that I am slowly coming to is that litmus tests should > > > > not be thought of as linear chains, but rather as cycles. If you think > > > > of it as a cycle, then it doesn't matter where the local link is, just > > > > how many of them and how they are connected. > > > > > > Do you have some examples of this? I'm struggling to make it work in my > > > mind, or are you talking specifically in the context of the kernel > > > memory model? > > > > Now that you mention it, maybe it would be best to keep the transitive > > and non-transitive separate for the time being anyway. Just because it > > might be possible to deal with does not necessarily mean that we should > > be encouraging it. ;-) > > So isn't smp_mb__after_unlock_lock() exactly such a scenario? And would > not someone trying to implement RCsc locks using locally transitive > RELEASE/ACQUIRE operations need exactly this stuff? > > That is, I am afraid we need to cover the mix of local and global > transitive operations at least in overview. True, but we haven't gotten to locking yet. That said, I would argue that smp_mb__after_unlock_lock() upgrades locks to transitive, and thus would not be an exception to the "no combining transitive and non-transitive steps in cycles" rule. Thanx, Paul
