LKMM: failing cmpxchg should have no ordering

[JonasOberhauser]

In LKMM, a failing cmpxchg is just a once access with no implied barriers.

Per my understanding, GenMC currently inserts atomic-related mb() around cmpxchg

https://github.com/MPI-SWS/genmc/blob/master/include/lkmm.h#L133

which in LKMM also provide ordering in case the RMW fails:

P0(int *x, int *y, int *z)
{
	WRITE_ONCE(*x, 1);
	smp_store_release(y, 1);
}

P1(int *x, int *y, int *z)
{
	int r = 2;
	if (atomic_cmpxchg(y, 2, 2)) {
		smp_mb__after_atomic();
		r = READ_ONCE(*x);
	}
}

exists (1:r=0) // impossible

[JonasOberhauser]

https://github.com/MPI-SWS/genmc/blob/master/include/lkmm.h#L140C1-L144C1

the same issue appears here, these failed cmpxchg should have relaxed failure mode