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

[Peter Zijlstra]

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.