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. Alan
