drm/ttm: Optimize vm locking using kref_get_unless_zero v3

Removes the need for a write lock each time we call ttm_bo_unref().

v2: Remove an unused variable.
v3: Really remove the unused variable.
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ttm/ttm_bo.c

@@ -696,6 +696,7 @@ static void ttm_bo_release(struct kref *kref)
 	struct ttm_bo_device *bdev = bo->bdev;
 	struct ttm_mem_type_manager *man = &bdev->man[bo->mem.mem_type];
 
+	write_lock(&bdev->vm_lock);
 	if (likely(bo->vm_node != NULL)) {
 		rb_erase(&bo->vm_rb, &bdev->addr_space_rb);
 		drm_mm_put_block(bo->vm_node);
@@ -707,18 +708,14 @@ static void ttm_bo_release(struct kref *kref)
 	ttm_mem_io_unlock(man);
 	ttm_bo_cleanup_refs_or_queue(bo);
 	kref_put(&bo->list_kref, ttm_bo_release_list);
-	write_lock(&bdev->vm_lock);
 }
 
 void ttm_bo_unref(struct ttm_buffer_object **p_bo)
 {
 	struct ttm_buffer_object *bo = *p_bo;
-	struct ttm_bo_device *bdev = bo->bdev;
 
 	*p_bo = NULL;
-	write_lock(&bdev->vm_lock);
 	kref_put(&bo->kref, ttm_bo_release);
-	write_unlock(&bdev->vm_lock);
 }
 EXPORT_SYMBOL(ttm_bo_unref);
 

drivers/gpu/drm/ttm/ttm_bo_vm.c

@@ -259,8 +259,8 @@ int ttm_bo_mmap(struct file *filp, struct vm_area_struct *vma,
 	read_lock(&bdev->vm_lock);
 	bo = ttm_bo_vm_lookup_rb(bdev, vma->vm_pgoff,
 				 (vma->vm_end - vma->vm_start) >> PAGE_SHIFT);
-	if (likely(bo != NULL))
-		ttm_bo_reference(bo);
+	if (likely(bo != NULL) && !kref_get_unless_zero(&bo->kref))
+		bo = NULL;
 	read_unlock(&bdev->vm_lock);
 
 	if (unlikely(bo == NULL)) {