Question about ISA2+pooncelock+pooncelock+pombonce litmus

[Alan Huang]

Hi,

I’m playing with the LKMM, then I saw the ISA2+pooncelock+pooncelock+pombonce.

The original litmus is as follows:
------------------------------------------------------
P0(int *x, int *y, spinlock_t *mylock)
{
        spin_lock(mylock);
        WRITE_ONCE(*x, 1);
        WRITE_ONCE(*y, 1);
        spin_unlock(mylock);
}

P1(int *y, int *z, spinlock_t *mylock)
{
        int r0;

        spin_lock(mylock);
        r0 = READ_ONCE(*y);
        WRITE_ONCE(*z, 1);
        spin_unlock(mylock);
}

P2(int *x, int *z)
{
        int r1;
        int r2;

        r2 = READ_ONCE(*z);
        smp_mb();
        r1 = READ_ONCE(*x);
}

exists (1:r0=1 /\ 2:r2=1 /\ 2:r1=0)
------------------------------------------------------
Of course, the result is Never. 

But when I delete P0’s spin_lock and P1’s spin_unlock:
-------------------------------------------------------
P0(int *x, int *y, spinlock_t *mylock)
{
        WRITE_ONCE(*x, 1);
        WRITE_ONCE(*y, 1);
        spin_unlock(mylock);
}

P1(int *y, int *z, spinlock_t *mylock)
{
        int r0;

        spin_lock(mylock);
        r0 = READ_ONCE(*y);
        WRITE_ONCE(*z, 1);
}

P2(int *x, int *z)
{
        int r1;
        int r2;

        r2 = READ_ONCE(*z);
        smp_mb();
        r1 = READ_ONCE(*x);
}

exists (1:r0=1 /\ 2:r2=1 /\ 2:r1=0)
------------------------------------------------------
Then herd told me the result is Sometimes.

Is this expected?