On Fri, Aug 26, 2022 at 06:23:24PM +0200, Peter Zijlstra wrote: > On Fri, Aug 26, 2022 at 05:48:12AM -0700, Paul E. McKenney wrote: > > Hello! > > > > I have not yet done more than glance at this one, but figured I should > > send it along sooner rather than later. > > > > "Verifying and Optimizing Compact NUMA-Aware Locks on Weak > > Memory Models", Antonio Paolillo, Hernán Ponce-de-León, Thomas > > Haas, Diogo Behrens, Rafael Chehab, Ming Fu, and Roland Meyer. > > https://arxiv.org/abs/2111.15240 > > > > The claim is that the queued spinlocks implementation with CNA violates > > LKMM but actually works on all architectures having a formal hardware > > memory model. > > > > Thoughts? > > So the paper mentions the following defects: > > - LKMM doesn't carry a release-acquire chain across a relaxed op That's right, although I'm not so sure this should be considered a defect... > - some babbling about a missing propagation -- ISTR Linux if stuffed > full of them, specifically we require stores to auto propagate > without help from barriers Not a missing propagation; a late one. Don't understand what you mean by "auto propagate without help from barriers". > - some handoff that is CNA specific and I've not looked too hard at > presently. > > > I think we should address that first one in LKMM, it seems very weird to > me a RmW would break the chain like that. An explicitly relaxed RMW (atomic_cmpxchg_relaxed(), to be precise). If the authors wanted to keep the release-acquire chain intact, why not use a cmpxchg version that has release semantics instead of going out of their way to use a relaxed version? To put it another way, RMW accesses and release-acquire accesses are unrelated concepts. You can have one without the other (in principle, anyway). So a relaxed RMW is just as capable of breaking a release-acquire chain as any other relaxed operation is. > Is there actual hardware that > doesn't behave? Not as far as I know, although that isn't very far. Certainly an other-multicopy-atomic architecture would make the litmus test succeed. But the LKMM does not require other-multicopy-atomicity. Alan
