C MP+locked+rmb {} P0(int* y,int* x,spinlock_t* lo) { spin_lock(lo); WRITE_ONCE(*x,1); spin_unlock(lo); spin_lock(lo); WRITE_ONCE(*y,1); spin_unlock(lo); } P1(int* y,int* x) { int r0 = READ_ONCE(*y); smp_rmb(); int r1 = READ_ONCE(*x); } Observed 1:r1=0; 1:r0=1;