linux-kernel - Question about ISA2+pooncelock+pooncelock+pombonce litmus

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Date: Mon, 18 Mar 2024 01:47:43 +0800
From: Alan Huang <mmpgouride@...il.com>
To: linux-kernel@...r.kernel.org,
 linux-arch@...r.kernel.org,
 rcu@...r.kernel.org
Subject: Question about ISA2+pooncelock+pooncelock+pombonce litmus

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?