On Thu, Mar 04, 2021 at 04:27:53PM -0500, Alan Stern wrote: > On Thu, Mar 04, 2021 at 11:05:15AM -0800, Paul E. McKenney wrote: > > On Thu, Mar 04, 2021 at 10:35:24AM -0500, Alan Stern wrote: > > > On Wed, Mar 03, 2021 at 09:04:07PM -0800, Paul E. McKenney wrote: > > > > On Wed, Mar 03, 2021 at 10:21:01PM -0500, Alan Stern wrote: > > > > > On Wed, Mar 03, 2021 at 02:03:48PM -0800, Paul E. McKenney wrote: > > > > > > On Wed, Mar 03, 2021 at 03:22:46PM -0500, Alan Stern wrote: > > > > > > > > > > > And I cannot immediately think of a situation where > > > > > > > > this approach would break that would not result in a data race being > > > > > > > > flagged. Or is this yet another failure of my imagination? > > > > > > > > > > > > > > By definition, an access to a local variable cannot participate in a > > > > > > > data race because all such accesses are confined to a single thread. > > > > > > > > > > > > True, but its value might have come from a load from a shared variable. > > > > > > > > > > Then that load could have participated in a data race. But the store to > > > > > the local variable cannot. > > > > > > > > Agreed. My thought was that if the ordering from the initial (non-local) > > > > load mattered, then that initial load must have participated in a > > > > data race. Is that true, or am I failing to perceive some corner case? > > > > > > Ordering can matter even when no data race is involved. Just think > > > about how much of the memory model is concerned with ordering of > > > marked accesses, which don't participate in data races unless there is > > > a conflicting plain access somewhere. > > > > Fair point. Should I have instead said "then that initial load must > > have run concurrently with a store to that same variable"? > > I'm losing track of the point you were originally trying to make. > > Does ordering matter when there are no conflicting accesses? Sure. > Consider this: > > A: r1 = READ_ONCE(x); > B: WRITE_ONCE(y, r1); > smp_wmb(); > C: WRITE_ONCE(z, 1); > > Even if there are no other accesses to y at all (let alone any > conflicting ones), the mere existence of B forces A to be ordered before > C, and this is easily detectable by a litmus test. Given that herd7 treats all local variables as registers (including forbidding taking their addresses), and given that we are not thinking of treating local-variable accesses as if they were marked, this is likely all moot. But just in case... I was trying to figure out if there was a litmus test of the following form where it might make a difference if local-variable accesses were treated as if they were marked. So is there something like this: r1 = x; if (r1) WRITE_ONCE(y, 1); where implicitly treating the accesses to r1 as marked would make a difference. I was thinking that any such example would have to result in LKMM flagging the load from x as a data race. However, your example inserting the smp_wmb() does shed some doubt on that theory. This of course is moot unless we come back to treating local-variable accesses as if they were marked. Thanx, Paul
