----- On Aug 4, 2020, at 12:51 PM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote: > On Tue, Aug 04, 2020 at 10:48:41AM -0400, Mathieu Desnoyers wrote: >> Here is the scenario I have in mind: > >> Userspace variables: >> >> int x = 0, y = 0; >> >> CPU 0 CPU 1 >> Thread A Thread B >> (in thread group A) (in thread group B) >> >> x = 1 >> barrier() >> y = 1 >> exit() >> exit_mm() >> current->mm = NULL; >> r1 = load y >> membarrier() >> skips CPU 0 (no IPI) because its current mm is NULL >> r2 = load x >> BUG_ON(r1 == 1 && r2 == 0) >> > > Ah, yes of course. > > We really should have a bunch of these scenarios in membarrier.c. Good point. > > > > Now, the above cannot happen because we have an unconditional > atomic_dec_and_test() in do_exit() before exit_mm(), but I'm sure > relying on that is a wee bit dodgy. I am not against using this already existing barrier to provide the guarantee we need, but it would have to be documented in the code. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
