linux-kernel - [PATCH 3.16 126/294] net_sched: fix order of queue length updates in qdisc

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Date:   Mon, 06 Nov 2017 23:03:02 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "David S. Miller" <davem@...emloft.net>,
        "Cong Wang" <xiyou.wangcong@...il.com>,
        "Konstantin Khlebnikov" <khlebnikov@...dex-team.ru>
Subject: [PATCH 3.16 126/294] net_sched: fix order of queue length updates
 in qdisc_replace()

3.16.50-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>

commit 68a66d149a8c78ec6720f268597302883e48e9fa upstream.

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@...dex-team.ru>
Fixes: 86a7996cc8a0 ("net_sched: introduce qdisc_replace() helper")
Acked-by: Cong Wang <xiyou.wangcong@...il.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 include/net/sch_generic.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -618,8 +618,11 @@ static inline struct Qdisc *qdisc_replac
 	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);