[PATCH v2] ksmbd: server: avoid busy polling in accept loop

[Qingfang Deng <dqfext@gmail.com>]

The ksmbd listener thread was using busy waiting on a listening socket by
calling kernel_accept() with SOCK_NONBLOCK and retrying every 100ms on
-EAGAIN. Since this thread is dedicated to accepting new connections,
there is no need for non-blocking mode.

Switch to a blocking accept() call instead, allowing the thread to sleep
until a new connection arrives. This avoids unnecessary wakeups and CPU
usage. During teardown, call shutdown() on the listening socket so that
accept() returns -EINVAL and the thread exits cleanly.

The socket release mutex is redundant because kthread_stop() blocks until
the listener thread returns, guaranteeing safe teardown ordering.

Also remove sk_rcvtimeo and sk_sndtimeo assignments, which only caused
accept() to return -EAGAIN prematurely.

Fixes: 0626e6641f6b ("cifsd: add server handler for central processing and tranport layers")
Signed-off-by: Qingfang Deng <dqfext@gmail.com>
---
v1 -> v2: https://lore.kernel.org/linux-cifs/20251030064736.24061-1-dqfext@gmail.com
 Do not remove TCP_NODELAY, as accepted sockets inherits from it.
 Fix accept() blocking forever on older kernel versions.
 Remove a redundant mutex

 fs/smb/server/transport_tcp.c | 28 ++++++----------------------
 1 file changed, 6 insertions(+), 22 deletions(-)

diff --git a/fs/smb/server/transport_tcp.c b/fs/smb/server/transport_tcp.c
index 6e03e93321b8..79319815b2fc 100644
--- a/fs/smb/server/transport_tcp.c
+++ b/fs/smb/server/transport_tcp.c
@@ -22,7 +22,6 @@ struct interface {
 	struct socket		*ksmbd_socket;
 	struct list_head	entry;
 	char			*name;
-	struct mutex		sock_release_lock;
 	int			state;
 };
 
@@ -236,20 +235,14 @@ static int ksmbd_kthread_fn(void *p)
 	unsigned int max_ip_conns;
 
 	while (!kthread_should_stop()) {
-		mutex_lock(&iface->sock_release_lock);
 		if (!iface->ksmbd_socket) {
-			mutex_unlock(&iface->sock_release_lock);
 			break;
 		}
-		ret = kernel_accept(iface->ksmbd_socket, &client_sk,
-				    SOCK_NONBLOCK);
-		mutex_unlock(&iface->sock_release_lock);
-		if (ret) {
-			if (ret == -EAGAIN)
-				/* check for new connections every 100 msecs */
-				schedule_timeout_interruptible(HZ / 10);
+		ret = kernel_accept(iface->ksmbd_socket, &client_sk, 0);
+		if (ret == -EINVAL)
+			break;
+		if (ret)
 			continue;
-		}
 
 		if (!server_conf.max_ip_connections)
 			goto skip_max_ip_conns_limit;
@@ -458,10 +451,6 @@ static void tcp_destroy_socket(struct socket *ksmbd_socket)
 	if (!ksmbd_socket)
 		return;
 
-	/* set zero to timeout */
-	ksmbd_tcp_rcv_timeout(ksmbd_socket, 0);
-	ksmbd_tcp_snd_timeout(ksmbd_socket, 0);
-
 	ret = kernel_sock_shutdown(ksmbd_socket, SHUT_RDWR);
 	if (ret)
 		pr_err("Failed to shutdown socket: %d\n", ret);
@@ -532,9 +521,6 @@ static int create_socket(struct interface *iface)
 		goto out_error;
 	}
 
-	ksmbd_socket->sk->sk_rcvtimeo = KSMBD_TCP_RECV_TIMEOUT;
-	ksmbd_socket->sk->sk_sndtimeo = KSMBD_TCP_SEND_TIMEOUT;
-
 	ret = kernel_listen(ksmbd_socket, KSMBD_SOCKET_BACKLOG);
 	if (ret) {
 		pr_err("Port listen() error: %d\n", ret);
@@ -604,12 +590,11 @@ static int ksmbd_netdev_event(struct notifier_block *nb, unsigned long event,
 		if (iface && iface->state == IFACE_STATE_CONFIGURED) {
 			ksmbd_debug(CONN, "netdev-down event: netdev(%s) is going down\n",
 					iface->name);
+			kernel_sock_shutdown(iface->ksmbd_socket, SHUT_RDWR);
 			tcp_stop_kthread(iface->ksmbd_kthread);
 			iface->ksmbd_kthread = NULL;
-			mutex_lock(&iface->sock_release_lock);
-			tcp_destroy_socket(iface->ksmbd_socket);
+			sock_release(iface->ksmbd_socket);
 			iface->ksmbd_socket = NULL;
-			mutex_unlock(&iface->sock_release_lock);
 
 			iface->state = IFACE_STATE_DOWN;
 			break;
@@ -672,7 +657,6 @@ static struct interface *alloc_iface(char *ifname)
 	iface->name = ifname;
 	iface->state = IFACE_STATE_DOWN;
 	list_add(&iface->entry, &iface_list);
-	mutex_init(&iface->sock_release_lock);
 	return iface;
 }
 
-- 
2.43.0