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[36.236.241.206]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-83515b864c7sm3356061b3a.59.2026.05.01.08.03.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 01 May 2026 08:03:13 -0700 (PDT) From: Dao Zhong Ma To: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, antonio@openvpn.net Cc: sd@queasysnail.net, andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, Dao Zhong Ma Subject: [PATCH 1/1] ovpn: tcp - defer TX from softirq to workqueue Date: Fri, 1 May 2026 22:54:25 +0800 Message-ID: <20260501145425.757147-2-cz1346219@gmail.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260501145425.757147-1-cz1346219@gmail.com> References: <20260501145425.757147-1-cz1346219@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit ovpn_tcp_send_skb() holds sk->sk_lock.slock while performing the full TCP send in softirq context. This can hold the spinlock for a long time (large skb), blocking lock_sock() users. This can starve the RCU GP kthread and trigger RCU stalls warnings and hung tasks. Defer the TCP send operation to process context: - In interrupt context, only enqueue the skb under the spinlock schedule tcp_tx_work. - In process context, dequeue and flush the send queue under lock_sock() This reduces the softirq critical section to a short duration, allowing lock_sock() users to make progress and preventing RCU stalls. Signed-off-by: Dao Zhong Ma --- drivers/net/ovpn/tcp.c | 80 +++++++++++++++++++++++++++++++----------- 1 file changed, 59 insertions(+), 21 deletions(-) diff --git a/drivers/net/ovpn/tcp.c b/drivers/net/ovpn/tcp.c index 65054cc84be5..d75ad0c22a30 100644 --- a/drivers/net/ovpn/tcp.c +++ b/drivers/net/ovpn/tcp.c @@ -6,6 +6,7 @@ * Author: Antonio Quartulli */ +#include #include #include #include @@ -312,6 +313,40 @@ static void ovpn_tcp_send_sock(struct ovpn_peer *peer, struct sock *sk) peer->tcp.tx_in_progress = false; } +/* Caller must hold sk->sk_lock.slock. */ +static bool ovpn_tcp_queue_skb(struct ovpn_peer *peer, struct sk_buff *skb) +{ + if (skb_queue_len(&peer->tcp.out_queue) >= + READ_ONCE(net_hotdata.max_backlog)) { + dev_dstats_tx_dropped(peer->ovpn->dev); + kfree_skb(skb); + return false; + } + + __skb_queue_tail(&peer->tcp.out_queue, skb); + return true; +} + +/* Caller must hold sk->sk_lock.slock and own the socket. */ +static void ovpn_tcp_tx_flush(struct ovpn_peer *peer, struct sock *sk) +{ + struct sk_buff *skb; + + if (peer->tcp.out_msg.skb) + ovpn_tcp_send_sock(peer, sk); + + while (!peer->tcp.out_msg.skb) { + skb = __skb_dequeue(&peer->tcp.out_queue); + if (!skb) + break; + + peer->tcp.out_msg.skb = skb; + peer->tcp.out_msg.len = skb->len; + peer->tcp.out_msg.offset = 0; + ovpn_tcp_send_sock(peer, sk); + } +} + void ovpn_tcp_tx_work(struct work_struct *work) { struct ovpn_socket *sock; @@ -320,7 +355,7 @@ void ovpn_tcp_tx_work(struct work_struct *work) lock_sock(sock->sk); if (sock->peer) - ovpn_tcp_send_sock(sock->peer, sock->sk); + ovpn_tcp_tx_flush(sock->peer, sock->sk); release_sock(sock->sk); } @@ -345,32 +380,38 @@ static void ovpn_tcp_send_sock_skb(struct ovpn_peer *peer, struct sock *sk, void ovpn_tcp_send_skb(struct ovpn_peer *peer, struct sock *sk, struct sk_buff *skb) { + struct ovpn_socket *sock; u16 len = skb->len; + bool queued; *(__be16 *)__skb_push(skb, sizeof(u16)) = htons(len); - spin_lock_nested(&sk->sk_lock.slock, OVPN_TCP_DEPTH_NESTING); - if (sock_owned_by_user(sk)) { - if (skb_queue_len(&peer->tcp.out_queue) >= - READ_ONCE(net_hotdata.max_backlog)) { - dev_dstats_tx_dropped(peer->ovpn->dev); - kfree_skb(skb); - goto unlock; - } - __skb_queue_tail(&peer->tcp.out_queue, skb); - } else { - ovpn_tcp_send_sock_skb(peer, sk, skb); + if (unlikely(in_interrupt())) { + spin_lock_nested(&sk->sk_lock.slock, OVPN_TCP_DEPTH_NESTING); + queued = ovpn_tcp_queue_skb(peer, skb); + spin_unlock(&sk->sk_lock.slock); + if (!queued) + return; + + rcu_read_lock(); + sock = rcu_dereference_sk_user_data(sk); + if (sock) + schedule_work(&sock->tcp_tx_work); + rcu_read_unlock(); + return; } -unlock: - spin_unlock(&sk->sk_lock.slock); + + lock_sock_nested(sk, OVPN_TCP_DEPTH_NESTING); + queued = ovpn_tcp_queue_skb(peer, skb); + if (queued) + ovpn_tcp_tx_flush(peer, sk); + release_sock(sk); } static void ovpn_tcp_release(struct sock *sk) { - struct sk_buff_head queue; struct ovpn_socket *sock; struct ovpn_peer *peer; - struct sk_buff *skb; rcu_read_lock(); sock = rcu_dereference_sk_user_data(sk); @@ -390,11 +431,7 @@ static void ovpn_tcp_release(struct sock *sk) } rcu_read_unlock(); - __skb_queue_head_init(&queue); - skb_queue_splice_init(&peer->tcp.out_queue, &queue); - - while ((skb = __skb_dequeue(&queue))) - ovpn_tcp_send_sock_skb(peer, sk, skb); + ovpn_tcp_tx_flush(peer, sk); peer->tcp.sk_cb.prot->release_cb(sk); ovpn_peer_put(peer); @@ -653,3 +690,4 @@ void __init ovpn_tcp_init(void) &inet6_stream_ops); #endif } + -- 2.54.0