drm/nouveau/core: Allow asymmetric nouveau_event_get/_put

Most nouveau event handlers have storage in 'static' containers
(structures with lifetimes nearly equivalent to the drm_device),
but are dangerously reused via nouveau_event_get/_put. For
example, if nouveau_event_get is called more than once for a
given handler, the event handler list will be corrupted.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/core/core/event.c

@@ -27,11 +27,13 @@ static void
 nouveau_event_put_locked(struct nouveau_event *event, int index,
 			 struct nouveau_eventh *handler)
 {
+	if (__test_and_clear_bit(NVKM_EVENT_ENABLE, &handler->flags)) {
 		if (!--event->index[index].refs) {
 			if (event->disable)
 				event->disable(event, index);
 		}
 		list_del(&handler->head);
+	}
 }
 
 void
@@ -58,11 +60,13 @@ nouveau_event_get(struct nouveau_event *event, int index,
 		return;
 
 	spin_lock_irqsave(&event->lock, flags);
+	if (!__test_and_set_bit(NVKM_EVENT_ENABLE, &handler->flags)) {
 		list_add(&handler->head, &event->index[index].list);
 		if (!event->index[index].refs++) {
 			if (event->enable)
 				event->enable(event, index);
 		}
+	}
 	spin_unlock_irqrestore(&event->lock, flags);
 }
 

drivers/gpu/drm/nouveau/core/include/core/event.h

@@ -5,8 +5,12 @@
 #define NVKM_EVENT_DROP 0
 #define NVKM_EVENT_KEEP 1
 
+/* nouveau_eventh.flags bit #s */
+#define NVKM_EVENT_ENABLE 0
+
 struct nouveau_eventh {
 	struct list_head head;
+	unsigned long flags;
 	void *priv;
 	int (*func)(struct nouveau_eventh *, int index);
 };

drivers/gpu/drm/nouveau/nouveau_drm.c

@@ -95,7 +95,6 @@ nouveau_drm_vblank_enable(struct drm_device *dev, int head)
 
 	if (WARN_ON_ONCE(head >= ARRAY_SIZE(drm->vblank)))
 		return -EIO;
-	WARN_ON_ONCE(drm->vblank[head].func);
 	drm->vblank[head].func = nouveau_drm_vblank_handler;
 	nouveau_event_get(pdisp->vblank, head, &drm->vblank[head]);
 	return 0;
@@ -106,11 +105,8 @@ nouveau_drm_vblank_disable(struct drm_device *dev, int head)
 {
 	struct nouveau_drm *drm = nouveau_drm(dev);
 	struct nouveau_disp *pdisp = nouveau_disp(drm->device);
-	if (drm->vblank[head].func)
+
 	nouveau_event_put(pdisp->vblank, head, &drm->vblank[head]);
-	else
-		WARN_ON_ONCE(1);
-	drm->vblank[head].func = NULL;
 }
 
 static u64