[PATCH 5.2 12/94] taprio: Fix kernel panic in taprio_destroy

[Greg Kroah-Hartman]

From: Vladimir Oltean <olte...@gmail.com>

taprio_init may fail earlier than this line:

        list_add(&q->taprio_list, &taprio_list);

i.e. due to the net device not being multi queue.

Attempting to remove q from the global taprio_list when it is not part
of it will result in a kernel panic.

Fix it by matching list_add and list_del better to one another in the
order of operations. This way we can keep the deletion unconditional
and with lower complexity - O(1).

Cc: Leandro Dorileo <leandro.maciel.dori...@intel.com>
Fixes: 7b9eba7ba0c1 ("net/sched: taprio: fix picos_per_byte miscalculation")
Signed-off-by: Vladimir Oltean <olte...@gmail.com>
Acked-by: Vinicius Costa Gomes <vinicius.go...@intel.com>
Signed-off-by: David S. Miller <da...@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 net/sched/sch_taprio.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -903,6 +903,10 @@ static int taprio_init(struct Qdisc *sch
         */
        q->clockid = -1;
 
+       spin_lock(&taprio_list_lock);
+       list_add(&q->taprio_list, &taprio_list);
+       spin_unlock(&taprio_list_lock);
+
        if (sch->parent != TC_H_ROOT)
                return -EOPNOTSUPP;
 
@@ -920,10 +924,6 @@ static int taprio_init(struct Qdisc *sch
        if (!opt)
                return -EINVAL;
 
-       spin_lock(&taprio_list_lock);
-       list_add(&q->taprio_list, &taprio_list);
-       spin_unlock(&taprio_list_lock);
-
        for (i = 0; i < dev->num_tx_queues; i++) {
                struct netdev_queue *dev_queue;
                struct Qdisc *qdisc;