sch_hfsc: make hfsc_qlen_notify() idempotent

hfsc_qlen_notify() is not idempotent either and not friendly
to its callers, like fq_codel_dequeue(). Let's make it idempotent
to ease qdisc_tree_reduce_backlog() callers' life:

1. update_vf() decreases cl->cl_nactive, so we can check whether it is
non-zero before calling it.

2. eltree_remove() always removes RB node cl->el_node, but we can use
   RB_EMPTY_NODE() + RB_CLEAR_NODE() to make it safe.

Reported-by: Gerrard Tai <gerrard.tai@starlabs.sg>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250403211033.166059-4-xiyou.wangcong@gmail.com
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/net/sched/sch_hfsc.c b/net/sched/sch_hfsc.c
index c287bf8423b47b7ca022fc2e6ca19b77f3ec13a0..ce5045eea065689e4aa54ad1bf9dfa08bcef46a9 100644 (file)
--- a/net/sched/sch_hfsc.c
+++ b/net/sched/sch_hfsc.c
@@ -203,7 +203,10 @@ eltree_insert(struct hfsc_class *cl)
 static inline void
 eltree_remove(struct hfsc_class *cl)
 {
-       rb_erase(&cl->el_node, &cl->sched->eligible);
+       if (!RB_EMPTY_NODE(&cl->el_node)) {
+               rb_erase(&cl->el_node, &cl->sched->eligible);
+               RB_CLEAR_NODE(&cl->el_node);
+       }
 }
 
 static inline void
@@ -1220,7 +1223,8 @@ hfsc_qlen_notify(struct Qdisc *sch, unsigned long arg)
        /* vttree is now handled in update_vf() so that update_vf(cl, 0, 0)
         * needs to be called explicitly to remove a class from vttree.
         */
-       update_vf(cl, 0, 0);
+       if (cl->cl_nactive)
+               update_vf(cl, 0, 0);
        if (cl->cl_flags & HFSC_RSC)
                eltree_remove(cl);
 }