From: Yun Lu 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@kernel.org Suggested-by: LongJun Tang Signed-off-by: Yun Lu --- 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