On Mon, Mar 18, 2024 at 01:47:43AM +0800, Alan Huang wrote:
> 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.
You mean like this?
Test ISA2+pooncelock+pooncelock+pombonce Allowed
States 8
1:r0=0; 2:r1=0; 2:r2=0;
1:r0=0; 2:r1=0; 2:r2=1;
1:r0=0; 2:r1=1; 2:r2=0;
1:r0=0; 2:r1=1; 2:r2=1;
1:r0=1; 2:r1=0; 2:r2=0;
1:r0=1; 2:r1=0; 2:r2=1;
1:r0=1; 2:r1=1; 2:r2=0;
1:r0=1; 2:r1=1; 2:r2=1;
Ok
Witnesses
Positive: 1 Negative: 7
Flag unmatched-unlock
Condition exists (1:r0=1 /\ 2:r2=1 /\ 2:r1=0)
Observation ISA2+pooncelock+pooncelock+pombonce Sometimes 1 7
Time ISA2+pooncelock+pooncelock+pombonce 0.01
Hash=f55b8515e48310f812aa676084f2cc88
> Is this expected?
There are no locks held initially, so why can't the following
sequence of events unfold:
o P1() acquires the lock.
o P0() does WRITE_ONCE(*y, 1). (Yes, out of order)
o P1() does READ_ONCE(*y), and gets 1.
o P1() does WRITE_ONCE(*z, 1).
o P2() does READ_ONCE(*z) and gets 1.
o P2() does smp_mb(), but there is nothing to order with.
o P2() does READ_ONCE(*x) and gets 0.
o P0() does WRITE_ONCE(*x, 1), but too late to affect P2().
o P0() releases the lock that is does not hold, which is why you see
the "Flag unmatched-unlock" in the output. LKMM is complaining
that the litmus test is not legitimate, and rightly so!
Or am I missing your point?
Thanx, Paul
