linux-kernel - [PATCH v3 2/2] af_packet: fix soft lockup issue caused by tpacket

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Message-ID: <20250709095653.62469-3-luyun_611@163.com>
Date: Wed,  9 Jul 2025 17:56:53 +0800
From: Yun Lu <luyun_611@....com>
To: willemdebruijn.kernel@...il.com,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com
Cc: netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v3 2/2] af_packet: fix soft lockup issue caused by tpacket_snd()

From: Yun Lu <luyun@...inos.cn>

When MSG_DONTWAIT is not set, the tpacket_snd operation will wait for
pending_refcnt to decrement to zero before returning. The pending_refcnt
is decremented by 1 when the skb->destructor function is called,
indicating that the skb has been successfully sent and needs to be
destroyed.

If an error occurs during this process, the tpacket_snd() function will
exit and return error, but pending_refcnt may not yet have decremented to
zero. Assuming the next send operation is executed immediately, but there
are no available frames to be sent in tx_ring (i.e., packet_current_frame
returns NULL), and skb is also NULL, the function will not execute
wait_for_completion_interruptible_timeout() to yield the CPU. Instead, it
will enter a do-while loop, waiting for pending_refcnt to be zero. Even
if the previous skb has completed transmission, the skb->destructor
function can only be invoked in the ksoftirqd thread (assuming NAPI
threading is enabled). When both the ksoftirqd thread and the tpacket_snd
operation happen to run on the same CPU, and the CPU trapped in the
do-while loop without yielding, the ksoftirqd thread will not get
scheduled to run. As a result, pending_refcnt will never be reduced to
zero, and the do-while loop cannot exit, eventually leading to a CPU soft
lockup issue.

In fact, as long as pending_refcnt is not zero, even if skb is NULL,
wait_for_completion_interruptible_timeout() should be executed to yield
the CPU, allowing the ksoftirqd thread to be scheduled. Therefore, move
the penging_refcnt check to the start of the do-while loop, and reuse ph
to continue for the next iteration.

Fixes: 89ed5b519004 ("af_packet: Block execution of tasks waiting for transmit to complete in AF_PACKET")
Cc: stable@...nel.org
Suggested-by: LongJun Tang <tanglongjun@...inos.cn>
Signed-off-by: Yun Lu <luyun@...inos.cn>

---
Changes in v3:
- Simplify the code and reuse ph to continue. Thanks: Eric Dumazet.
- Link to v2: https://lore.kernel.org/all/20250708020642.27838-1-luyun_611@163.com/

Changes in v2:
- Add a Fixes tag.
- Link to v1: https://lore.kernel.org/all/20250707081629.10344-1-luyun_611@163.com/
---
 net/packet/af_packet.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 7089b8c2a655..89a5d2a3a720 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2846,11 +2846,21 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 		ph = packet_current_frame(po, &po->tx_ring,
 					  TP_STATUS_SEND_REQUEST);
 		if (unlikely(ph == NULL)) {
-			if (need_wait && skb) {
+			/* Note: packet_read_pending() might be slow if we
+			 * have to call it as it's per_cpu variable, but in
+			 * fast-path we don't have to call it, only when ph
+			 * is NULL, we need to check pending_refcnt.
+			 */
+			if (need_wait && packet_read_pending(&po->tx_ring)) {
 				timeo = wait_for_completion_interruptible_timeout(&po->skb_completion, timeo);
 				if (timeo <= 0) {
 					err = !timeo ? -ETIMEDOUT : -ERESTARTSYS;
 					goto out_put;
+				} else {
+					/* Just reuse ph to continue for the next iteration, and
+					 * ph will be reassigned at the start of the next iteration.
+					 */
+					ph = (void *)1;
 				}
 			}
 			/* check for additional frames */
@@ -2943,14 +2953,7 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 		}
 		packet_increment_head(&po->tx_ring);
 		len_sum += tp_len;
-	} while (likely((ph != NULL) ||
-		/* Note: packet_read_pending() might be slow if we have
-		 * to call it as it's per_cpu variable, but in fast-path
-		 * we already short-circuit the loop with the first
-		 * condition, and luckily don't have to go that path
-		 * anyway.
-		 */
-		 (need_wait && packet_read_pending(&po->tx_ring))));
+	} while (likely(ph != NULL))

 	err = len_sum;
 	goto out_put;
-- 
2.43.0