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.
