Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin()

[Jason Low <jason.low2@hp.com>]

On Mon, 2016-02-15 at 18:15 -0800, Jason Low wrote:
> On Fri, 2016-02-12 at 14:14 -0800, Davidlohr Bueso wrote:
> > On Fri, 12 Feb 2016, Peter Zijlstra wrote:
> > 
> > >On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote:
> > >>  static bool mutex_optimistic_spin(struct mutex *lock,
> > >> +				  struct ww_acquire_ctx *ww_ctx,
> > >> +				  const bool use_ww_ctx, int waiter)
> > >>  {
> > >>  	struct task_struct *task = current;
> > >> +	bool acquired = false;
> > >>
> > >> +	if (!waiter) {
> > >> +		if (!mutex_can_spin_on_owner(lock))
> > >> +			goto done;
> > >
> > >Why doesn't the waiter have to check mutex_can_spin_on_owner() ?
> > 
> > afaict because mutex_can_spin_on_owner() fails immediately when the counter
> > is -1, which is a nono for the waiters case.
> 
> mutex_can_spin_on_owner() returns false if the task needs to reschedule
> or if the lock owner is not on_cpu. In either case, the task will end up
> not spinning when it enters the spin loop. So it makes sense if the
> waiter also checks mutex_can_spin_on_owner() so that the optimistic spin
> queue overhead can be avoided in those cases.

Actually, since waiters bypass the optimistic spin queue, that means the
the mutex_can_spin_on_owner() isn't really beneficial. So Waiman is
right in that it's fine to skip this in the waiter case.