Re: sem_lock() vs qspinlocks

From: Peter Zijlstra
Date: Fri May 20 2016 - 11:05:26 EST


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.