Re: [syzbot] [net?] possible deadlock in sch_direct_xmit

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Subject: [net?] possible deadlock in sch_direct_xmit
Author: eadavis@xxxxxx

please test deadlock in sch_direct_xmit

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 4195a4bc26ca..9e418f94757d 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -337,13 +337,16 @@ bool sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
 #endif
 
 	if (likely(skb)) {
-		HARD_TX_LOCK(dev, txq, smp_processor_id());
-		if (!netif_xmit_frozen_or_stopped(txq))
-			skb = dev_hard_start_xmit(skb, dev, txq, &ret);
-		else
-			qdisc_maybe_clear_missed(q, txq);
+		int cpu = smp_processor_id();
+		if (READ_ONCE(txq->xmit_lock_owner) != cpu) {
+			HARD_TX_LOCK(dev, txq, cpu);
+			if (!netif_xmit_frozen_or_stopped(txq))
+				skb = dev_hard_start_xmit(skb, dev, txq, &ret);
+			else
+				qdisc_maybe_clear_missed(q, txq);
 
-		HARD_TX_UNLOCK(dev, txq);
+			HARD_TX_UNLOCK(dev, txq);
+		}
 	} else {
 		if (root_lock)
 			spin_lock(root_lock);