Re: [PATCH v4 02/27] ntsync: Introduce NTSYNC_IOC_WAIT_ALL.

Peter Zijlstra
2024-04-17T11:37:15Z

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.