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SubjectRe: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt
On 10/13/2015 03:56 PM, Peter Zijlstra wrote:
> On Tue, Sep 22, 2015 at 04:50:43PM -0400, Waiman Long wrote:
>
>> +gotlock:
>> /*
>> + * We now have the lock. We need to either clear the tail code or
>> + * notify the next one in queue as the new queue head.
>> */
>> + old = atomic_read(&lock->val);
>> + while ((old& _Q_TAIL_MASK) == tail) {
>> + int val;
>> + int new = old& ~_Q_TAIL_MASK;
>> +
>> + /*
>> + * We are the only one in the queue, so clear the tail code
>> + * and return.
>> + */
>> + val = atomic_cmpxchg(&lock->val, old, new);
>> + if (old == val)
>> + goto done;
>> + old = val;
>> + }
>> +
> This i need to think about a wee bit; its almost the same...
>
>
> So the below is exactly duplicated from the normal slowpath, so why
> don't you keep that there?
>
> It would get you something like:
>
> if (pv_wait_head_or_steal(..))
> goto stolen;
>
>
> stolen:
>> + /*
>> + * contended path; wait for next, release.
>> + */
>> + while (!(next = READ_ONCE(node->next)))
>> + cpu_relax();
>> +
>> + arch_mcs_spin_unlock_contended(&next->locked);
>> + pv_kick_node(lock, next);
> release:
> ...

Yes, it is largely the same. I thought that you don't like too much
change in the logic flow of the generic qspinlock code. I will make the
change in the next revision.

Cheers,
Longman


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