[net/sched] Question: Locks for clearing ERR_PTR() value from idrinfo->action_idr ?

[Tetsuo Handa]

Hello.

syzbot is reporting hung task problems involving rtnl_muxex. A debug printk()
patch added to linux-next-20240611 suggested that many of them are caused by
an infinite busy loop inside tcf_idr_check_alloc().

----------
again:
		rcu_read_lock();
		p = idr_find(&idrinfo->action_idr, *index);

		if (IS_ERR(p)) {
			/* This means that another process allocated
			 * index but did not assign the pointer yet.
			 */
			rcu_read_unlock();
			goto again;
		}
----------

Since there is no sleep (e.g. cond_resched()/schedule_timeout_uninterruptible(1))
before "goto again;", once idr_find() returns an IS_ERR() value, all of that CPU's
computation resource is wasted forever with rtnl_mutex held (and anybody else who
tries to hold rtnl_mutex at rtnl_lock() is reported as hung task, resulting in
various hung task reports waiting for rtnl_mutex at rtnl_lock()).

Therefore, I tried to add a sleep before "goto again;", but I can't know whether
a sleep added to linux-next-20240612 solves the hung task problem because syzbot
currently cannot test linux-next kernels due to some different problem.

Therefore, I'm posting a question here before syzbot can resume testing of
linux-next kernels. As far as I can see, the ERR_PTR(-EBUSY) assigned at

	mutex_lock(&idrinfo->lock);
	ret = idr_alloc_u32(&idrinfo->action_idr, ERR_PTR(-EBUSY), index, max,
			    GFP_KERNEL);
	mutex_unlock(&idrinfo->lock);

in tcf_idr_check_alloc() is cleared by either

	mutex_lock(&idrinfo->lock);
	/* Remove ERR_PTR(-EBUSY) allocated by tcf_idr_check_alloc */
	WARN_ON(!IS_ERR(idr_remove(&idrinfo->action_idr, index)));
	mutex_unlock(&idrinfo->lock);

in tcf_idr_cleanup() or

	mutex_lock(&idrinfo->lock);
	/* Replace ERR_PTR(-EBUSY) allocated by tcf_idr_check_alloc */
	idr_replace(&idrinfo->action_idr, a, a->tcfa_index);
	mutex_unlock(&idrinfo->lock);

in tcf_idr_insert_many().

But is there a possibility that rtnl_mutex is released between
tcf_idr_check_alloc() and tcf_idr_{cleanup,insert_many}() ? If yes,
adding a sleep before "goto again;" won't be sufficient. But if no,
how can

	/* This means that another process allocated
	 * index but did not assign the pointer yet.
	 */

happen (because both setting ERR_PTR(-EBUSY) and replacing with an !IS_ERR()
value are done without temporarily releasing rtnl_mutex) ?

Is there a possibility that tcf_idr_check_alloc() is called without holding
rtnl_mutex? If yes, adding a sleep before "goto again;" would help. But if no,
is this a sign that some path forgot to call tcf_idr_{cleanup,insert_many}() ?