[PATCH] net: core: fix unbalanced qdisc_run_begin/qdisc_run_end

3598 static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
3599                                  struct net_device *dev,
3600                                  struct netdev_queue *txq)
3601 {
...
3650         } else if ((q->flags & TCQ_F_CAN_BYPASS) && !qdisc_qlen(q) &&
3651                    qdisc_run_begin(q)) {

---> Those multiple *and conditions* in this if statement are not
      necessarily executed sequentially. If the qdisc_run_begin(q)
      statement is executed first and the other conditions are not
      satisfied, qdisc_run_end will have no chance to be executed,
      and the lowest bit of q->running will always be 1.
      This may lead to a softlockup:
      https://bugzilla.kernel.org/show_bug.cgi?id=205427
...
3657
3658                 qdisc_bstats_update(q, skb);
...
3661                         if (unlikely(contended)) {
3662                                 spin_unlock(&q->busylock);
3663                                 contended = false;
3664                         }
3665                         __qdisc_run(q);
3666                 }
3667
3668                 qdisc_run_end(q);
3669                 rc = NET_XMIT_SUCCESS;
3670         }
...

We ensure the correct execution order by explicitly
specifying those dependencies.
Fixes: edb09eb17ed8 ("net: sched: do not acquire qdisc spinlock in qdisc/class stats dump")
Signed-off-by: Wen Yang <redacted>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Cong Wang <redacted>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Kevin Athey <redacted>
Cc: Xiaotian Pei <redacted>
Cc: netdev@vger.kernel.org
Cc: bpf@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
  net/core/dev.c | 63 ++++++++++++++++++++++++++++++----------------------------
  1 file changed, 33 insertions(+), 30 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 20c7a67..d2690ee 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -3602,27 +3602,28 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
  	spinlock_t *root_lock = qdisc_lock(q);
  	struct sk_buff *to_free = NULL;
  	bool contended;
-	int rc;
+	int rc = NET_XMIT_SUCCESS;
  
  	qdisc_calculate_pkt_len(skb, q);
  
  	if (q->flags & TCQ_F_NOLOCK) {
-		if ((q->flags & TCQ_F_CAN_BYPASS) && q->empty &&
-		    qdisc_run_begin(q)) {
-			if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED,
-					      &q->state))) {
-				__qdisc_drop(skb, &to_free);
-				rc = NET_XMIT_DROP;
-				goto end_run;
-			}
-			qdisc_bstats_cpu_update(q, skb);
+		if ((q->flags & TCQ_F_CAN_BYPASS) && q->empty) {
+			if (qdisc_run_begin(q)) {
+				if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED,
+						      &q->state))) {
+					__qdisc_drop(skb, &to_free);
+					rc = NET_XMIT_DROP;
+					goto end_run;
+				}
+				qdisc_bstats_cpu_update(q, skb);
  
-			rc = NET_XMIT_SUCCESS;
-			if (sch_direct_xmit(skb, q, dev, txq, NULL, true))
-				__qdisc_run(q);
+				if (sch_direct_xmit(skb, q, dev, txq, NULL,
+						    true))
+					__qdisc_run(q);
  
  end_run:
-			qdisc_run_end(q);
+				qdisc_run_end(q);
+			}
  		} else {
  			rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
  			qdisc_run(q);

@@ -3647,26 +3648,28 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
  	if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
  		__qdisc_drop(skb, &to_free);
  		rc = NET_XMIT_DROP;
-	} else if ((q->flags & TCQ_F_CAN_BYPASS) && !qdisc_qlen(q) &&
-		   qdisc_run_begin(q)) {
-		/*
-		 * This is a work-conserving queue; there are no old skbs
-		 * waiting to be sent out; and the qdisc is not running -
-		 * xmit the skb directly.
-		 */
+	} else if ((q->flags & TCQ_F_CAN_BYPASS) && !qdisc_qlen(q)) {
+		if (qdisc_run_begin(q)) {
+			/* This is a work-conserving queue;
+			 * there are no old skbs waiting to be sent out;
+			 * and the qdisc is not running -
+			 * xmit the skb directly.
+			 */
  
-		qdisc_bstats_update(q, skb);
+			qdisc_bstats_update(q, skb);
  
-		if (sch_direct_xmit(skb, q, dev, txq, root_lock, true)) {
-			if (unlikely(contended)) {
-				spin_unlock(&q->busylock);
-				contended = false;
+			if (sch_direct_xmit(skb, q, dev, txq, root_lock,
+					    true)) {
+				if (unlikely(contended)) {
+					spin_unlock(&q->busylock);
+					contended = false;
+				}
+				__qdisc_run(q);
  			}
-			__qdisc_run(q);
-		}
  
-		qdisc_run_end(q);
-		rc = NET_XMIT_SUCCESS;
+			qdisc_run_end(q);
+			rc = NET_XMIT_SUCCESS;
+		}
  	} else {
  		rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
  		if (qdisc_run_begin(q)) {