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

[Alan Huang]

> 2024年3月18日 07:02，Paul E. McKenney <paulmck@xxxxxxxxxx> 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