drm/i915: Remove the vma refcount

Now that i915_vma_parked() is taking the object lock on vma destruction,
and the only user of the vma refcount, i915_gem_object_unbind()
also takes the object lock, remove the vma refcount.

v3: Documentation update.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220304082641.308069-3-thomas.hellstrom@linux.intel.com

drivers/gpu/drm/i915/i915_gem.c

@@ -151,14 +151,25 @@ try_again:
 			break;
 		}
 
+		/*
+		 * Requiring the vm destructor to take the object lock
+		 * before destroying a vma would help us eliminate the
+		 * i915_vm_tryget() here, AND thus also the barrier stuff
+		 * at the end. That's an easy fix, but sleeping locks in
+		 * a kthread should generally be avoided.
+		 */
 		ret = -EAGAIN;
 		if (!i915_vm_tryget(vma->vm))
 			break;
 
-		/* Prevent vma being freed by i915_vma_parked as we unbind */
-		vma = __i915_vma_get(vma);
 		spin_unlock(&obj->vma.lock);
 
+		/*
+		 * Since i915_vma_parked() takes the object lock
+		 * before vma destruction, it won't race us here,
+		 * and destroy the vma from under us.
+		 */
+
 		if (vma) {
 			bool vm_trylock = !!(flags & I915_GEM_OBJECT_UNBIND_VM_TRYLOCK);
 			ret = -EBUSY;
@@ -180,8 +191,6 @@ try_again:
 					ret = i915_vma_unbind(vma);
 				}
 			}
-
-			__i915_vma_put(vma);
 		}
 
 		i915_vm_put(vma->vm);

drivers/gpu/drm/i915/i915_vma.c

@@ -122,7 +122,6 @@ vma_create(struct drm_i915_gem_object *obj,
 	if (vma == NULL)
 		return ERR_PTR(-ENOMEM);
 
-	kref_init(&vma->ref);
 	vma->ops = &vm->vma_ops;
 	vma->obj = obj;
 	vma->size = obj->base.size;
@@ -1628,15 +1627,6 @@ void i915_vma_reopen(struct i915_vma *vma)
 		__i915_vma_remove_closed(vma);
 }
 
-void i915_vma_release(struct kref *ref)
-{
-	struct i915_vma *vma = container_of(ref, typeof(*vma), ref);
-
-	i915_active_fini(&vma->active);
-	GEM_WARN_ON(vma->resource);
-	i915_vma_free(vma);
-}
-
 static void force_unbind(struct i915_vma *vma)
 {
 	if (!drm_mm_node_allocated(&vma->node))
@@ -1665,7 +1655,9 @@ static void release_references(struct i915_vma *vma, bool vm_ddestroy)
 	if (vm_ddestroy)
 		i915_vm_resv_put(vma->vm);
 
-	__i915_vma_put(vma);
+	i915_active_fini(&vma->active);
+	GEM_WARN_ON(vma->resource);
+	i915_vma_free(vma);
 }
 
 /**
@@ -1693,9 +1685,6 @@ static void release_references(struct i915_vma *vma, bool vm_ddestroy)
  * - vm->mutex
  * - obj->vma.lock
  * - gt->closed_lock
- *
- * A vma user can also temporarily keep the vma alive while holding a vma
- * reference.
  */
 void i915_vma_destroy_locked(struct i915_vma *vma)
 {

drivers/gpu/drm/i915/i915_vma.h

@@ -222,20 +222,6 @@ void i915_vma_unlink_ctx(struct i915_vma *vma);
 void i915_vma_close(struct i915_vma *vma);
 void i915_vma_reopen(struct i915_vma *vma);
 
-static inline struct i915_vma *__i915_vma_get(struct i915_vma *vma)
-{
-	if (kref_get_unless_zero(&vma->ref))
-		return vma;
-
-	return NULL;
-}
-
-void i915_vma_release(struct kref *ref);
-static inline void __i915_vma_put(struct i915_vma *vma)
-{
-	kref_put(&vma->ref, i915_vma_release);
-}
-
 void i915_vma_destroy_locked(struct i915_vma *vma);
 void i915_vma_destroy(struct i915_vma *vma);
 

drivers/gpu/drm/i915/i915_vma_types.h

@@ -211,7 +211,6 @@ struct i915_vma {
 	 * handles (but same file) for execbuf, i.e. the number of aliases
 	 * that exist in the ctx->handle_vmas LUT for this vma.
 	 */
-	struct kref ref;
 	atomic_t open_count;
 	atomic_t flags;
 	/**