Re: sem_lock() vs qspinlocks

[Waiman Long]

On 05/20/2016 08:59 PM, Davidlohr Bueso wrote:
> 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

The existing code is doing that, but I would argue that including the 
locked, but uncontended case isn't a bad idea.

Cheers,
Longman