2024-03-17 17:48:08

by Alan Huang

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?

2024-03-17 23:27:14

by Paul E. McKenney

Subject: Re: Question about ISA2+pooncelock+pooncelock+pombonce litmus

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

2024-03-18 11:28:42

by Alan Huang

Subject: Re: Question about ISA2+pooncelock+pooncelock+pombonce litmus

> 2024年3月18日 07:02，Paul E. McKenney <[email protected]> wrote：
>
> 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!


Oh! I missed that line, Thank you for pointing this out! :)

>
> Or am I missing your point?
>
> Thanx, Paul