On Thu, Apr 05, 2018 at 05:16:16PM -0400, Waiman Long wrote: > On 04/05/2018 12:58 PM, Will Deacon wrote: > > /* > > - * we're pending, wait for the owner to go away. > > - * > > - * *,1,1 -> *,1,0 > > - * > > - * this wait loop must be a load-acquire such that we match the > > - * store-release that clears the locked bit and create lock > > - * sequentiality; this is because not all clear_pending_set_locked() > > - * implementations imply full barriers. > > - */ > > - smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_MASK)); > > - > > - /* > > - * take ownership and clear the pending bit. > > - * > > - * *,1,0 -> *,0,1 > > + * If pending was clear but there are waiters in the queue, then > > + * we need to undo our setting of pending before we queue ourselves. > > */ > > - clear_pending_set_locked(lock); > > - return; > > + if (!(val & _Q_PENDING_MASK)) > > + atomic_andnot(_Q_PENDING_VAL, &lock->val); > Can we add a clear_pending() helper that will just clear the byte if > _Q_PENDING_BITS == 8? That will eliminate one atomic instruction from > the failure path.
Good idea! Will