On Tue, 15 Jan 2019, Andrea Parri wrote: > Unless I'm mis-reading/-applying this definition, this will flag the > following test (a variation on your "race.litmus") with "data-race": > > C no-race > > {} > > P0(int *x, spinlock_t *s) > { > spin_lock(s); > WRITE_ONCE(*x, 1); /* A */ > spin_unlock(s); /* B */ > } > > P1(int *x, spinlock_t *s) > { > int r1; > > spin_lock(s); /* C */ > r1 = *x; /* D */ > spin_unlock(s); > } > > exists (1:r1=1) > > Broadly speaking, this is due to the fact that the modified "happens- > before" axiom does not forbid the execution with the (MP-) cycle > > A ->po-rel B ->rfe C ->acq-po D ->fre A > > and then to the link "D ->race-from-r A" here defined.
Yes, that cycle certainly should be forbidden. On the other hand, we don't want to insist that C happens before D, given that D may not happen at all. This is a real problem. Can we solve it by adding a modified "happens-before" which says essentially that _if_ D is preserved _then_ C happens before D? But then what about cycles involving more than one possibly preserved access? Or maybe a relation which says that D cannot execute before C (so if D executes at all, it has to come after C)? Now you see why this stuff is so difficult... At the moment, I don't know how to fix this. > (In part., similar considerations hold for the following litmus test: > > C MP1 > > {} > > P0(int *x, int *y) > { > *x = 1; > smp_store_release(y, 1); > } > > P1(int *x, int *y) > { > int r0; > int r1 = -1; > > r0 = smp_load_acquire(y); > if (r0) > r1 = *x; > } > > exists (1:r0=1 /\ 1:r1=0) > > ) > > I wonder whether you actually intended to introduce these "races"...? No, I did not. Alan