On Wednesday, 17 April 2024 06:37:03 CDT Peter Zijlstra wrote: > On Mon, Apr 15, 2024 at 08:08:12PM -0500, Elizabeth Figura wrote: > > + if (atomic_read(&sem->all_hint) > 0) { > > + spin_lock(&dev->wait_all_lock); > > + spin_lock_nest_lock(&sem->lock, &dev->wait_all_lock); > > > > + prev_count = sem->u.sem.count; > > + ret = post_sem_state(sem, args); > > + if (!ret) { > > + try_wake_all_obj(dev, sem); > > + try_wake_any_sem(sem); > > + } > > > > + spin_unlock(&sem->lock); > > + spin_unlock(&dev->wait_all_lock); > > + } else { > > + spin_lock(&sem->lock); > > + > > + prev_count = sem->u.sem.count; > > + ret = post_sem_state(sem, args); > > + if (!ret) > > + try_wake_any_sem(sem); > > + > > + spin_unlock(&sem->lock); > > + } > > > > if (!ret && put_user(prev_count, user_args)) > > ret = -EFAULT; > > vs. > > > + /* queue ourselves */ > > + > > + spin_lock(&dev->wait_all_lock); > > + > > + for (i = 0; i < args.count; i++) { > > + struct ntsync_q_entry *entry = &q->entries[i]; > > + struct ntsync_obj *obj = entry->obj; > > + > > + atomic_inc(&obj->all_hint); > > + > > + /* > > + * obj->all_waiters is protected by dev->wait_all_lock rather > > + * than obj->lock, so there is no need to acquire obj->lock > > + * here. > > + */ > > + list_add_tail(&entry->node, &obj->all_waiters); > > + } > > This looks racy, consider: > > atomic_read(all_hints) /* 0 */ > > spin_lock(wait_all_lock) > atomic_inc(all_hint) /* 1 */ > list_add_tail() > > spin_lock(sem->lock) > /* try_wake_all_obj() missing */ > > > > > I've not yet thought about if this is harmful or not, but if not, it > definitely needs a comment. > > Anyway, I need a break, maybe more this evening. Ach. I wrote this with the idea that the race isn't meaningful, but looking at it again you're right—there is a harmful race here. I think it should be fixable by moving the atomic_read inside the lock, though.