net/sched: cls_api: Fix lockup on flushing explicitly created chain

Mingshuai Ren reports:

When a new chain is added by using tc, one soft lockup alarm will be
 generated after delete the prio 0 filter of the chain. To reproduce
 the problem, perform the following steps:
(1) tc qdisc add dev eth0 root handle 1: htb default 1
(2) tc chain add dev eth0
(3) tc filter del dev eth0 chain 0 parent 1: prio 0
(4) tc filter add dev eth0 chain 0 parent 1:

Fix the issue by accounting for additional reference to chains that are
explicitly created by RTM_NEWCHAIN message as opposed to implicitly by
RTM_NEWTFILTER message.

Fixes: 726d061 ("net: sched: prevent insertion of new classifiers during chain flush")
Reported-by: Mingshuai Ren <renmingshuai@huawei.com>
Closes: https://lore.kernel.org/lkml/87legswvi3.fsf@nvidia.com/T/
Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Link: https://lore.kernel.org/r/20230612093426.2867183-1-vladbu@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: w1ldptr

net/sched/cls_api.c

@@ -657,8 +657,8 @@ static void __tcf_chain_put(struct tcf_chain *chain, bool by_act,
 {
 	struct tcf_block *block = chain->block;
 	const struct tcf_proto_ops *tmplt_ops;
+	unsigned int refcnt, non_act_refcnt;
 	bool free_block = false;
-	unsigned int refcnt;
 	void *tmplt_priv;
 
 	mutex_lock(&block->lock);
@@ -678,13 +678,15 @@ static void __tcf_chain_put(struct tcf_chain *chain, bool by_act,
 	 * save these to temporary variables.
 	 */
 	refcnt = --chain->refcnt;
+	non_act_refcnt = refcnt - chain->action_refcnt;
 	tmplt_ops = chain->tmplt_ops;
 	tmplt_priv = chain->tmplt_priv;
 
-	/* The last dropped non-action reference will trigger notification. */
-	if (refcnt - chain->action_refcnt == 0 && !by_act) {
-		tc_chain_notify_delete(tmplt_ops, tmplt_priv, chain->index,
-				       block, NULL, 0, 0, false);
+	if (non_act_refcnt == chain->explicitly_created && !by_act) {
+		if (non_act_refcnt == 0)
+			tc_chain_notify_delete(tmplt_ops, tmplt_priv,
+					       chain->index, block, NULL, 0, 0,
+					       false);
 		/* Last reference to chain, no need to lock. */
 		chain->flushing = false;
 	}