On 04/11/2018 11:22 AM, Catalin Marinas wrote:
> Hi Waiman,
>
> On Mon, Apr 09, 2018 at 02:08:52PM -0400, Waiman Long wrote:
>> @@ -311,13 +320,19 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, 
>> u32 val)
>>              return;
>>  
>>      /*
>> -     * wait for in-progress pending->locked hand-overs
>> +     * wait for in-progress pending->locked hand-overs with a
>> +     * limited number of spins.
>>       *
>>       * 0,1,0 -> 0,0,1
>>       */
>>      if (val == _Q_PENDING_VAL) {
>> -            while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL)
>> +            int cnt = _Q_PENDING_LOOP;
>> +
>> +            while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL) {
>> +                    if (!--cnt)
>> +                            goto queue;
>>                      cpu_relax();
>> +            }
>>      }
> In my model, the pathological case is not this loop but the following
> one (trylock || pending):
>
> P0:                                   P1:
> queued_spin_lock() fails              queued_spin_lock() succeeds
> queued_spin_lock_slowpath()
>       val == _Q_LOCKED_VAL
>       new = _Q_LOCKED_VAL |
>               _Q_PENDING_VAL
>                                       queued_spin_unlock()
>                                               lock->val == 0
>       cmpxchg(&lock->val, val, new)
>               fails
>       val = old (0)
>       repeat for (;;) loop:
>       new = _Q_LOCKED_VAL
>                                       queued_spin_lock() succeeds
>                                               lock->val == _Q_LOCKED_VAL
>       cmpxchg(&lock->val, val, new)
>               fails
>       val = old (_Q_LOCKED_VAL)
>       repeat for (;;) loop:
>
>       ... and we are back to the P0 state above when it first entered
>       the loop, hence no progress. P1 never enters slowpath.

I don't see any problem in removing this second loop. Thanks for running
tool to check for problem.

-Longman

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