Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list is not NULL.

[Davidlohr Bueso]

On Fri, 22 Jan 2016, Waiman Long wrote:

> The patch that I sent out is just a proof of concept to make sure
> that it can fix that particular case. I do plan to refactor it if I
> decide to go ahead with an official one. Unlike the OSQ, there can be
> no more than one waiter spinner as the wakeup function is directed to
> only the first task in the wait list and the spinning won't happen
> until the task is first woken up. In the worst case scenario, there
> are only 2 spinners spinning on the lock and the owner field, one
> from OSQ and one from the wait list. That shouldn't put too much
> cacheline contention traffic to the system.

Similarly, I guess we should also wakeup the next waiter in line after
releasing the wait_lock via wake_q. This would allow the woken waiter a
slightly better chance of finding the wait_lock free when continuing to
take the mutex.

Thanks,
Davidlohr