Re: [PATCH] net_sched: fix order of queue length updates in qdisc_replace()

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I've seen this as loops in hfsc rb-trees and hungs in hfsc_dequeue() -> rb_first().
Simple debug catched insering already inserted rb-tree nodes and similar mess.

Most likely this could explain warning in hfsc_dequeue()
https://bugzilla.kernel.org/show_bug.cgi?id=109581

On 19.08.2017 15:37, Konstantin Khlebnikov wrote:
This important to call qdisc_tree_reduce_backlog() after changing queue
length. Parent qdisc should deactivate class in ->qlen_notify() called from
qdisc_tree_reduce_backlog() but this happens only if qdisc->q.qlen in zero.

Missed class deactivations leads to crashes/warnings at picking packets
from empty qdisc and corrupting state at reactivating this class in future.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Fixes: 86a7996cc8a0 ("net_sched: introduce qdisc_replace() helper")hfsc_dequeue
Cc: Stable <stable@xxxxxxxxxxxxxxx>
---
  include/net/sch_generic.h |    5 ++++-
  1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 1c123e2b2415..67f815e5d525 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -806,8 +806,11 @@ static inline struct Qdisc *qdisc_replace(struct Qdisc *sch, struct Qdisc *new,
  	old = *pold;
  	*pold = new;
  	if (old != NULL) {
-		qdisc_tree_reduce_backlog(old, old->q.qlen, old->qstats.backlog);
+		unsigned int qlen = old->q.qlen;
+		unsigned int backlog = old->qstats.backlog;
+
  		qdisc_reset(old);
+		qdisc_tree_reduce_backlog(old, qlen, backlog);
  	}
  	sch_tree_unlock(sch);



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