C C-relseq.litmus { } P0(int *x, int *y) { WRITE_ONCE(*x, 1); smp_store_release(y, 1); WRITE_ONCE(*y, 2); } P1(int *y) { r1 = xchg_relaxed(y, 3); } P2(int *y, int *z) { r2 = smp_load_acquire(y); WRITE_ONCE(*z, 1); } P3(int *x, int *z) { r3 = READ_ONCE(*z); smp_rmb(); r4 = READ_ONCE(*x); } exists (1:r1=2 /\ 2:r2=3 /\ 3:r3=1 /\ 3:r4=0)