staging: usbip: fix the usage of kthread_stop()

stub_shutdown_connection() and vhci_shutdown_connection() use
task_is_dead() before kthread_stop(). This buys nothing and wrong.
kthread_stop() is fine even if this thread is dead. However, if it
is dead nothing protects this task_struct, we shouldn't touch this
memory.

Change the code to do the necessary get_task_struct/put_task_struct.

This patch assumes that

	- xxx_shutdown_connection() is always called, so we can't
	  leak the task_struct.

	- kthread_stop_put() can't be called twice on the same task.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Tobias Klauser <tklauser@distanz.ch>
Cc: Matt Mooney <mfm@muteddisk.com>,
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/usbip/stub_dev.c

@@ -113,8 +113,8 @@ static ssize_t store_sockfd(struct device *dev, struct device_attribute *attr,
 
 		spin_unlock(&sdev->ud.lock);
 
-		sdev->ud.tcp_rx = kthread_run(stub_rx_loop, &sdev->ud, "stub_rx");
-		sdev->ud.tcp_tx = kthread_run(stub_tx_loop, &sdev->ud, "stub_tx");
+		sdev->ud.tcp_rx = kthread_get_run(stub_rx_loop, &sdev->ud, "stub_rx");
+		sdev->ud.tcp_tx = kthread_get_run(stub_tx_loop, &sdev->ud, "stub_tx");
 
 		spin_lock(&sdev->ud.lock);
 		sdev->ud.status = SDEV_ST_USED;
@@ -187,10 +187,10 @@ static void stub_shutdown_connection(struct usbip_device *ud)
 	}
 
 	/* 1. stop threads */
-	if (ud->tcp_rx && !task_is_dead(ud->tcp_rx))
-		kthread_stop(ud->tcp_rx);
-	if (ud->tcp_tx && !task_is_dead(ud->tcp_tx))
-		kthread_stop(ud->tcp_tx);
+	if (ud->tcp_rx)
+		kthread_stop_put(ud->tcp_rx);
+	if (ud->tcp_tx)
+		kthread_stop_put(ud->tcp_tx);
 
 	/*
 	 * 2. close the socket

drivers/staging/usbip/usbip_common.h

@@ -292,6 +292,23 @@ struct usbip_device {
 	} eh_ops;
 };
 
+#define kthread_get_run(threadfn, data, namefmt, ...)			   \
+({									   \
+	struct task_struct *__k						   \
+		= kthread_create(threadfn, data, namefmt, ## __VA_ARGS__); \
+	if (!IS_ERR(__k)) {						   \
+		get_task_struct(__k);					   \
+		wake_up_process(__k);					   \
+	}								   \
+	__k;								   \
+})
+
+#define kthread_stop_put(k)		\
+	do {				\
+		kthread_stop(k);	\
+		put_task_struct(k);	\
+	} while (0)
+
 /* usbip_common.c */
 void usbip_dump_urb(struct urb *purb);
 void usbip_dump_header(struct usbip_header *pdu);

drivers/staging/usbip/vhci_hcd.c

@@ -821,10 +821,10 @@ static void vhci_shutdown_connection(struct usbip_device *ud)
 	}
 
 	/* kill threads related to this sdev, if v.c. exists */
-	if (vdev->ud.tcp_rx && !task_is_dead(vdev->ud.tcp_rx))
-		kthread_stop(vdev->ud.tcp_rx);
-	if (vdev->ud.tcp_tx && !task_is_dead(vdev->ud.tcp_tx))
-		kthread_stop(vdev->ud.tcp_tx);
+	if (vdev->ud.tcp_rx)
+		kthread_stop_put(vdev->ud.tcp_rx);
+	if (vdev->ud.tcp_tx)
+		kthread_stop_put(vdev->ud.tcp_tx);
 
 	pr_info("stop threads\n");
 

drivers/staging/usbip/vhci_sysfs.c

@@ -222,8 +222,8 @@ static ssize_t store_attach(struct device *dev, struct device_attribute *attr,
 	spin_unlock(&the_controller->lock);
 	/* end the lock */
 
-	vdev->ud.tcp_rx = kthread_run(vhci_rx_loop, &vdev->ud, "vhci_rx");
-	vdev->ud.tcp_tx = kthread_run(vhci_tx_loop, &vdev->ud, "vhci_tx");
+	vdev->ud.tcp_rx = kthread_get_run(vhci_rx_loop, &vdev->ud, "vhci_rx");
+	vdev->ud.tcp_tx = kthread_get_run(vhci_tx_loop, &vdev->ud, "vhci_tx");
 
 	rh_port_connect(rhport, speed);