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:
...
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