drm/i915: Be defensive when starting vma activity

Before we acquire the vma for GPU activity, ensure that the underlying
object is not already in the process of being freed.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190820100531.8430-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_vma.c

@@ -87,8 +87,7 @@ static inline struct i915_vma *active_to_vma(struct i915_active *ref)
 
 static int __i915_vma_active(struct i915_active *ref)
 {
-	i915_vma_get(active_to_vma(ref));
-	return 0;
+	return i915_vma_tryget(active_to_vma(ref)) ? 0 : -ENOENT;
 }
 
 static void __i915_vma_retire(struct i915_active *ref)

drivers/gpu/drm/i915/i915_vma.h

@@ -231,6 +231,14 @@ static inline struct i915_vma *i915_vma_get(struct i915_vma *vma)
 	return vma;
 }
 
+static inline struct i915_vma *i915_vma_tryget(struct i915_vma *vma)
+{
+	if (likely(kref_get_unless_zero(&vma->obj->base.refcount)))
+		return vma;
+
+	return NULL;
+}
+
 static inline void i915_vma_put(struct i915_vma *vma)
 {
 	i915_gem_object_put(vma->obj);