pkt_sched: Use qdisc_lock() on already sampled root qdisc.

Based upon a bug report by Jeff Kirsher.

Don't use qdisc_root_lock() in these cases as the root
qdisc could have been changed, and we'd thus lock the
wrong object.

Tested by Emil S Tantilov who confirms that this seems
to fix the problem.

Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 2 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 69320a56a084..da7acacf02b5 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1796,7 +1796,7 @@ gso:
         skb->tc_verd = SET_TC_AT(skb->tc_verd,AT_EGRESS);
 #endif
         if (q->enqueue) {
-                spinlock_t *root_lock = qdisc_root_lock(q);
+                spinlock_t *root_lock = qdisc_lock(q);
 
                 spin_lock(root_lock);
 
@@ -1995,7 +1995,7 @@ static void net_tx_action(struct softirq_action *h)
                         smp_mb__before_clear_bit();
                         clear_bit(__QDISC_STATE_SCHED, &q->state);
 
-                        root_lock = qdisc_root_lock(q);
+                        root_lock = qdisc_lock(q);
                         if (spin_trylock(root_lock)) {
                                 qdisc_run(q);
                                 spin_unlock(root_lock);