drm/i915: Ensure requests stick around during waits

Added reference counting of the request structure around __wait_seqno() calls.
This is a precursor to updating the wait code itself to take the request rather
than a seqno. At that point, it would be a Bad Idea for a request object to be
retired and freed while the wait code is still using it.

v3:

Note that even though the mutex lock is held during a call to i915_wait_seqno(),
it is still necessary to explicitly bump the reference count. It appears that
the shrinker can asynchronously retire items even though the mutex is locked.

For: VIZ-4377
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Thomas Daniel <Thomas.Daniel@intel.com>
[danvet: Remove wrongly squashed hunk which breaks the build.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -1417,10 +1417,12 @@ i915_gem_object_wait_rendering__nonblocking(struct drm_i915_gem_object *obj,
 		return ret;
 
 	reset_counter = atomic_read(&dev_priv->gpu_error.reset_counter);
+	i915_gem_request_reference(req);
 	mutex_unlock(&dev->struct_mutex);
 	ret = __i915_wait_seqno(ring, seqno, reset_counter, true, NULL,
 				file_priv);
 	mutex_lock(&dev->struct_mutex);
+	i915_gem_request_unreference(req);
 	if (ret)
 		return ret;
 
@@ -2920,6 +2922,7 @@ i915_gem_wait_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct drm_i915_gem_wait *args = data;
 	struct drm_i915_gem_object *obj;
+	struct drm_i915_gem_request *req;
 	struct intel_engine_cs *ring = NULL;
 	unsigned reset_counter;
 	u32 seqno = 0;
@@ -2946,7 +2949,8 @@ i915_gem_wait_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
 	if (!obj->active || !obj->last_read_req)
 		goto out;
 
-	seqno = i915_gem_request_get_seqno(obj->last_read_req);
+	req = obj->last_read_req;
+	seqno = i915_gem_request_get_seqno(req);
 	WARN_ON(seqno == 0);
 	ring = obj->ring;
 
@@ -2960,10 +2964,15 @@ i915_gem_wait_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
 
 	drm_gem_object_unreference(&obj->base);
 	reset_counter = atomic_read(&dev_priv->gpu_error.reset_counter);
+	i915_gem_request_reference(req);
 	mutex_unlock(&dev->struct_mutex);
 
-	return __i915_wait_seqno(ring, seqno, reset_counter, true,
-				 &args->timeout_ns, file->driver_priv);
+	ret = __i915_wait_seqno(ring, seqno, reset_counter, true, &args->timeout_ns,
+			   file->driver_priv);
+	mutex_lock(&dev->struct_mutex);
+	i915_gem_request_unreference(req);
+	mutex_unlock(&dev->struct_mutex);
+	return ret;
 
 out:
 	drm_gem_object_unreference(&obj->base);
@@ -4118,6 +4127,8 @@ i915_gem_ring_throttle(struct drm_device *dev, struct drm_file *file)
 		target = request;
 	}
 	reset_counter = atomic_read(&dev_priv->gpu_error.reset_counter);
+	if (target)
+		i915_gem_request_reference(target);
 	spin_unlock(&file_priv->mm.lock);
 
 	if (target == NULL)
@@ -4129,6 +4140,10 @@ i915_gem_ring_throttle(struct drm_device *dev, struct drm_file *file)
 	if (ret == 0)
 		queue_delayed_work(dev_priv->wq, &dev_priv->mm.retire_work, 0);
 
+	mutex_lock(&dev->struct_mutex);
+	i915_gem_request_unreference(target);
+	mutex_unlock(&dev->struct_mutex);
+
 	return ret;
 }