On Fri, May 20, 2016 at 05:05:05PM +0200, Peter Zijlstra wrote: > 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.
FWIW, the only usage of spin_is_contended() should be for lock breaking, see spin_needbreak(). This also means that #define spin_is_contended(l) (false) is a valid implementation, where the only down-side is worse latency. This is done (together with GENERIC_LOCKBREAK), to allow trivial test-and-set spinlock implementations; as these cannot tell if the lock is contended.