Re: sem_lock() vs qspinlocks

[Davidlohr Bueso]

On Fri, 20 May 2016, Peter Zijlstra wrote:

> On Fri, May 20, 2016 at 04:47:43PM -0400, Waiman Long wrote:
>
> > >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);
> > >
>
> > Looking for contended lock, you need to consider the lock waiters also. So
> > looking at the whole word is right.
>
> No, you _only_ need to look at the lock waiters.

Is there anyway to do this in a single atomic_read? My thought is that otherwise
we could further expand the race window of when the lock is and isn't
contended (as returned to by the user). Ie avoiding crap like:

atomic_read(&lock->val) && atomic_read(&lock->val) != _Q_LOCKED_VAL

In any case, falsely returning for the 'locked, uncontended' case, vs completely
ignoring waiters is probably the lesser evil :).

Thanks,
Davidlohr