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

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.