On Fri, 2014-08-08 at 13:21 -0700, Davidlohr Bueso wrote: > On Fri, 2014-08-08 at 12:50 -0700, Jason Low wrote: > > > __visible __used noinline > > > @@ -730,6 +744,23 @@ __mutex_unlock_common_slowpath(struct mutex *lock, > > > int nested) > > > if (__mutex_slowpath_needs_to_unlock()) > > > atomic_set(&lock->count, 1); > > > > > > +/* > > > + * Skipping the mutex_has_owner() check when DEBUG, allows us to > > > + * avoid taking the wait_lock in order to do not call mutex_release() > > > + * and debug_mutex_unlock() when !DEBUG. This can otherwise result in > > > + * deadlocks when another task enters the lock's slowpath in > > > mutex_lock(). > > > + */ > > > +#ifndef CONFIG_DEBUG_MUTEXES > > > + /* > > > + * Abort the wakeup operation if there is an another mutex owner, as the > > > + * lock was stolen. mutex_unlock() should have cleared the owner field > > > + * before calling this function. If that field is now set, another task > > > + * must have acquired the mutex. > > > + */ > > > + if (mutex_has_owner(lock)) > > > + return; > > > > Would we need the mutex lock count to eventually get set to a negative > > value if there are waiters? An optimistic spinner can get the lock and > > set lock->count to 0. Then the lock count might remain 0 since a waiter > > might not get waken up here to try-lock and set lock->count to -1 if it > > goes back to sleep in the lock path. > > This is a good point, but I think we are safe because we do not rely on > strict dependence between the mutex counter and the wait list. So to see > if there are waiters to wakeup, we do a !list_empty() check, but to > determine the lock state, we rely on the counter.
Right, though if an optimistic spinner gets the lock, it would set lock->count to 0. After it is done with its critical region and calls mutex_unlock(), it would skip the slowpath and not wake up the next thread either, because it sees that the lock->count is 0. In that case, there might be a situation where the following mutex_unlock() call would skip waking up the waiter as there's no call to slowpath. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
