sctp: add a refcnt in sctp_stream_priorities to avoid a nested loop

With this refcnt added in sctp_stream_priorities, we don't need to
traverse all streams to check if the prio is used by other streams
when freeing one stream's prio in sctp_sched_prio_free_sid(). This
can avoid a nested loop (up to 65535 * 65535), which may cause a
stuck as Ying reported:

watchdog: BUG: soft lockup - CPU#23 stuck for 26s! [ksoftirqd/23:136]
Call Trace:
<TASK>
sctp_sched_prio_free_sid+0xab/0x100 [sctp]
sctp_stream_free_ext+0x64/0xa0 [sctp]
sctp_stream_free+0x31/0x50 [sctp]
sctp_association_free+0xa5/0x200 [sctp]

Note that it doesn't need to use refcount_t type for this counter,
as its accessing is always protected under the sock lock.

v1->v2:
- add a check in sctp_sched_prio_set to avoid the possible prio_head
refcnt overflow.

Fixes: 9ed7bfc79542 ("sctp: fix memory leak in sctp_stream_outq_migrate()")
Reported-by: Ying Xu <[e-mail]>
Acked-by: Marcelo Ricardo Leitner <[e-mail]>
Signed-off-by: Xin Long <[e-mail]>
Link: https://lore.kernel.org/r/82[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/sctp/structs.h

···
 	/* The next stream in line */
 	struct sctp_stream_out_ext *next;
 	__u16 prio;
+	__u16 users;
 };
 
 struct sctp_stream_out_ext {

net/sctp/stream_sched_prio.c

···
 
 static void sctp_sched_prio_unsched_all(struct sctp_stream *stream);
 
+static struct sctp_stream_priorities *sctp_sched_prio_head_get(struct sctp_stream_priorities *p)
+{
+	p->users++;
+	return p;
+}
+
+static void sctp_sched_prio_head_put(struct sctp_stream_priorities *p)
+{
+	if (p && --p->users == 0)
+		kfree(p);
+}
+
 static struct sctp_stream_priorities *sctp_sched_prio_new_head(
 			struct sctp_stream *stream, int prio, gfp_t gfp)
 {
···
 	INIT_LIST_HEAD(&p->active);
 	p->next = NULL;
 	p->prio = prio;
+	p->users = 1;
 
 	return p;
 }
···
 	 */
 	list_for_each_entry(p, &stream->prio_list, prio_sched) {
 		if (p->prio == prio)
-			return p;
+			return sctp_sched_prio_head_get(p);
 		if (p->prio > prio)
 			break;
 	}
···
 			 */
 			break;
 		if (p->prio == prio)
-			return p;
+			return sctp_sched_prio_head_get(p);
 	}
 
 	/* If not even there, allocate a new one. */
···
 	struct sctp_stream_out_ext *soute = sout->ext;
 	struct sctp_stream_priorities *prio_head, *old;
 	bool reschedule = false;
-	int i;
+
+	old = soute->prio_head;
+	if (old && old->prio == prio)
+		return 0;
 
 	prio_head = sctp_sched_prio_get_head(stream, prio, gfp);
 	if (!prio_head)
 		return -ENOMEM;
 
 	reschedule = sctp_sched_prio_unsched(soute);
-	old = soute->prio_head;
 	soute->prio_head = prio_head;
 	if (reschedule)
 		sctp_sched_prio_sched(stream, soute);
 
-	if (!old)
-		/* Happens when we set the priority for the first time */
-		return 0;
-
-	for (i = 0; i < stream->outcnt; i++) {
-		soute = SCTP_SO(stream, i)->ext;
-		if (soute && soute->prio_head == old)
-			/* It's still in use, nothing else to do here. */
-			return 0;
-	}
-
-	/* No hits, we are good to free it. */
-	kfree(old);
-
+	sctp_sched_prio_head_put(old);
 	return 0;
 }
 
···
 
 static void sctp_sched_prio_free_sid(struct sctp_stream *stream, __u16 sid)
 {
-	struct sctp_stream_priorities *prio = SCTP_SO(stream, sid)->ext->prio_head;
-	int i;
-
-	if (!prio)
-		return;
-
+	sctp_sched_prio_head_put(SCTP_SO(stream, sid)->ext->prio_head);
 	SCTP_SO(stream, sid)->ext->prio_head = NULL;
-	for (i = 0; i < stream->outcnt; i++) {
-		if (SCTP_SO(stream, i)->ext &&
-		    SCTP_SO(stream, i)->ext->prio_head == prio)
-			return;
-	}
-
-	kfree(prio);
 }
 
 static void sctp_sched_prio_enqueue(struct sctp_outq *q,