net/sched: sch_hfsc: Don't make class passive twice

[ Upstream commit 90b662ea25 ]

update_vf() is called from two places for the same class during a single
dequeue when the class's child qdisc (e.g. codel/fq_codel) drops its last
packets while dequeuing:

1. The child calls qdisc_tree_reduce_backlog(), which, now that the child
   is empty, invokes hfsc_qlen_notify() -> update_vf(cl, 0, 0) and turns
   the class passive (cl_nactive is decremented up the hierarchy).

2. hfsc_dequeue() then calls update_vf(cl, qdisc_pkt_len(skb), cur_time)
   to charge the dequeued bytes.

On the second call the class is already passive, but its child qdisc is
still empty, so update_vf() arms go_passive again:

      if (cl->qdisc->q.qlen == 0 && cl->cl_flags & HFSC_FSC)
              go_passive = 1;

The leaf is then skipped by the cl_nactive == 0 check inside the loop,
which does not clear go_passive, so the stale go_passive propagates to the
parent and decrements its cl_nactive a second time. A parent that still
has other active children is driven to cl_nactive == 0 and removed from
the vttree, even though those siblings are still backlogged. They are
never dequeued again and the qdisc stalls.

Fix this by only arming go_passive when the class is actually active, so an
already-passive class no longer triggers a second passive transition. The
byte accounting (cl->cl_total += len) still runs for every ancestor, so
dequeued bytes continue to be counted exactly once.

Fixes: 51eb3b6554 ("sch_hfsc: make hfsc_qlen_notify() idempotent")
Reported-by: Anirudh Gupta <anirudhrudr@gmail.com>
Closes: https://lore.kernel.org/netdev/CAN2cbVe79oj0O9==m4+4x3v+O+qzRagA=2=wkrp9i9=CqYvyZA@mail.gmail.com/
Tested-by: Anirudh Gupta <anirudhrudr@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/20260610132824.3027549-1-victor@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

net/sched/sch_hfsc.c

@@ -753,7 +753,7 @@ update_vf(struct hfsc_class *cl, unsigned int len, u64 cur_time)
 	u64 f; /* , myf_bound, delta; */
 	int go_passive = 0;
 
-	if (cl->qdisc->q.qlen == 0 && cl->cl_flags & HFSC_FSC)
+	if (cl->qdisc->q.qlen == 0 && cl->cl_flags & HFSC_FSC && cl->cl_nactive)
 		go_passive = 1;
 
 	for (; cl->cl_parent != NULL; cl = cl->cl_parent) {