Re: sem_lock() vs qspinlocks

[Peter Zijlstra]

On Fri, May 20, 2016 at 08:00:49AM -0700, Davidlohr Bueso wrote:
> On Fri, 20 May 2016, Peter Zijlstra wrote:
> 
> >On Thu, May 19, 2016 at 10:39:26PM -0700, Davidlohr Bueso wrote:
> >> In addition, this makes me wonder if queued_spin_is_locked() should then be:
> >>
> >>-	return atomic_read(&lock->val);
> >>+	return atomic_read(&lock->val) & _Q_LOCKED_MASK;
> >>
> >>And avoid considering pending waiters as locked.
> >
> >Probably
> 
> Similarly, and I know you hate it, but afaict, then semantically
> queued_spin_is_contended() ought to be:
> 
> -       return atomic_read(&lock->val) & ~_Q_LOCKED_MASK;
> +       return atomic_read(&lock->val);

Nah, that would make it return true for (0,0,1), ie. uncontended locked.