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(unknown []) by gzga-smtp-mtada-g1-1 (Coremail) with SMTP id _____wAXowFflW9oGKJ1Dw--.26998S4; Thu, 10 Jul 2025 18:26:41 +0800 (CST) From: Yun Lu To: willemdebruijn.kernel@gmail.com, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v4 2/3] af_packet: fix soft lockup issue caused by tpacket_snd() Date: Thu, 10 Jul 2025 18:26:38 +0800 Message-ID: <20250710102639.280932-3-luyun_611@163.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20250710102639.280932-1-luyun_611@163.com> References: <20250710102639.280932-1-luyun_611@163.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-CM-TRANSID: _____wAXowFflW9oGKJ1Dw--.26998S4 X-Coremail-Antispam: 1Uf129KBjvJXoWxZF1xXFy7Zr4rKw17Aw1rtFb_yoW5urWxpa yYg347t3WDGr1Iqr18Ga1kJr12vw4rJFsrGrWkJ34SywnIyF9ayrWIkrWj9FyUZFWDta4a vF4qvr1UAa4DAaDanT9S1TB71UUUUU7qnTZGkaVYY2UrUUUUjbIjqfuFe4nvWSU5nxnvy2 9KBjDUYxBIdaVFxhVjvjDU0xZFpf9x07jV1vsUUUUU= X-CM-SenderInfo: pox130jbwriqqrwthudrp/1tbiQxqGzmhvkAiYvQAAsr Content-Type: text/plain; charset="utf-8" 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, skb is true for all but the first iterations of that loop, and as long as pending_refcnt is not zero, even if incremented by a previous call, wait_for_completion_interruptible_timeout() should be executed to yield the CPU, allowing the ksoftirqd thread to be scheduled. Therefore, the execution condition of this function should be modified to check if pending_refcnt is not zero, instead of check skb. As a result, packet_read_pending() may be called twice in the loop. This will be optimized in the following patch. Fixes: 89ed5b519004 ("af_packet: Block execution of tasks waiting for trans= mit to complete in AF_PACKET") Cc: stable@kernel.org Suggested-by: LongJun Tang Signed-off-by: Yun Lu --- Changes in v4: - Split to the fix alone. Thanks: Willem de Bruijn. - Link to v3: https://lore.kernel.org/all/20250709095653.62469-3-luyun_611@= 163.com/ 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 | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c index 7089b8c2a655..581a96ec8e1a 100644 --- a/net/packet/af_packet.c +++ b/net/packet/af_packet.c @@ -2846,7 +2846,7 @@ static int tpacket_snd(struct packet_sock *po, struct= msghdr *msg) ph =3D packet_current_frame(po, &po->tx_ring, TP_STATUS_SEND_REQUEST); if (unlikely(ph =3D=3D NULL)) { - if (need_wait && skb) { + if (need_wait && packet_read_pending(&po->tx_ring)) { timeo =3D wait_for_completion_interruptible_timeout(&po->skb_completio= n, timeo); if (timeo <=3D 0) { err =3D !timeo ? -ETIMEDOUT : -ERESTARTSYS; --=20 2.43.0