drm/udl: Fix inconsistent urbs.count value during udl_free_urb_list()

In the current design, udl_get_urb() may be called asynchronously
during the driver freeing its URL list via udl_free_urb_list().
The problem is that the sync is determined by comparing the urbs.count
and urbs.available fields, while we clear urbs.count field only once
after udl_free_urb_list() finishes, i.e. during udl_free_urb_list(),
the state becomes inconsistent.

For fixing this inconsistency and also for hardening the locking
scheme, this patch does a slight refactoring of the code around
udl_get_urb() and udl_free_urb_list().  Now urbs.count is updated in
the same spinlock at extracting a URB from the list in
udl_free_url_list().
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220908095115.23396-11-tiwai@suse.de

drivers/gpu/drm/udl/udl_drv.h

@@ -74,13 +74,7 @@ static inline struct usb_device *udl_to_usb_device(struct udl_device *udl)
 int udl_modeset_init(struct drm_device *dev);
 struct drm_connector *udl_connector_init(struct drm_device *dev);
 
-struct urb *udl_get_urb_timeout(struct drm_device *dev, long timeout);
-
-#define GET_URB_TIMEOUT	HZ
-static inline struct urb *udl_get_urb(struct drm_device *dev)
-{
-	return udl_get_urb_timeout(dev, GET_URB_TIMEOUT);
-}
+struct urb *udl_get_urb(struct drm_device *dev);
 
 int udl_submit_urb(struct drm_device *dev, struct urb *urb, size_t len);
 int udl_sync_pending_urbs(struct drm_device *dev);

drivers/gpu/drm/udl/udl_main.c

@@ -23,6 +23,8 @@
 #define WRITES_IN_FLIGHT (20)
 #define MAX_VENDOR_DESCRIPTOR_SIZE 256
 
+static struct urb *udl_get_urb_locked(struct udl_device *udl, long timeout);
+
 static int udl_parse_vendor_descriptor(struct udl_device *udl)
 {
 	struct usb_device *udev = udl_to_usb_device(udl);
@@ -146,15 +148,17 @@ void udl_urb_completion(struct urb *urb)
 static void udl_free_urb_list(struct drm_device *dev)
 {
 	struct udl_device *udl = to_udl(dev);
-	int count = udl->urbs.count;
 	struct urb_node *unode;
 	struct urb *urb;
 
 	DRM_DEBUG("Waiting for completes and freeing all render urbs\n");
 
 	/* keep waiting and freeing, until we've got 'em all */
-	while (count--) {
-		urb = udl_get_urb_timeout(dev, MAX_SCHEDULE_TIMEOUT);
+	while (udl->urbs.count) {
+		spin_lock_irq(&udl->urbs.lock);
+		urb = udl_get_urb_locked(udl, MAX_SCHEDULE_TIMEOUT);
+		udl->urbs.count--;
+		spin_unlock_irq(&udl->urbs.lock);
 		if (WARN_ON(!urb))
 			break;
 		unode = urb->context;
@@ -164,7 +168,8 @@ static void udl_free_urb_list(struct drm_device *dev)
 		usb_free_urb(urb);
 		kfree(unode);
 	}
-	udl->urbs.count = 0;
+
+	wake_up_all(&udl->urbs.sleep);
 }
 
 static int udl_alloc_urb_list(struct drm_device *dev, int count, size_t size)
@@ -228,33 +233,44 @@ static int udl_alloc_urb_list(struct drm_device *dev, int count, size_t size)
 	return udl->urbs.count;
 }
 
-struct urb *udl_get_urb_timeout(struct drm_device *dev, long timeout)
+static struct urb *udl_get_urb_locked(struct udl_device *udl, long timeout)
 {
-	struct udl_device *udl = to_udl(dev);
-	struct urb_node *unode = NULL;
+	struct urb_node *unode;
 
-	if (!udl->urbs.count)
-		return NULL;
+	assert_spin_locked(&udl->urbs.lock);
 
 	/* Wait for an in-flight buffer to complete and get re-queued */
-	spin_lock_irq(&udl->urbs.lock);
 	if (!wait_event_lock_irq_timeout(udl->urbs.sleep,
+					 !udl->urbs.count ||
 					 !list_empty(&udl->urbs.list),
 					 udl->urbs.lock, timeout)) {
 		DRM_INFO("wait for urb interrupted: available: %d\n",
 			 udl->urbs.available);
-		goto unlock;
+		return NULL;
 	}
 
+	if (!udl->urbs.count)
+		return NULL;
+
 	unode = list_first_entry(&udl->urbs.list, struct urb_node, entry);
 	list_del_init(&unode->entry);
 	udl->urbs.available--;
 
-unlock:
-	spin_unlock_irq(&udl->urbs.lock);
 	return unode ? unode->urb : NULL;
 }
 
+#define GET_URB_TIMEOUT	HZ
+struct urb *udl_get_urb(struct drm_device *dev)
+{
+	struct udl_device *udl = to_udl(dev);
+	struct urb *urb;
+
+	spin_lock_irq(&udl->urbs.lock);
+	urb = udl_get_urb_locked(udl, GET_URB_TIMEOUT);
+	spin_unlock_irq(&udl->urbs.lock);
+	return urb;
+}
+
 int udl_submit_urb(struct drm_device *dev, struct urb *urb, size_t len)
 {
 	struct udl_device *udl = to_udl(dev);