[PATCH v3] drm: modify drm_global_item_ref to avoid two times of writing ref->object

[Christian König]

Am 08.09.2016 um 09:35 schrieb Chris Wilson:
> On Thu, Sep 08, 2016 at 03:22:48PM +0800, Huang Rui wrote:
>> On Thu, Sep 08, 2016 at 02:36:06PM +0800, Chris Wilson wrote:
>>> On Wed, Sep 07, 2016 at 10:07:57PM -0400, Huang Rui wrote:
>>>> In previous drm_global_item_ref, there are two times of writing
>>>> ref->object if item->refcount is 0. So this patch does a minor update
>>>> to put alloc and init ref firstly, and then to modify the item of glob
>>>> array. Use "else" to avoid two times of writing ref->object. It can
>>>> make the code logic more clearly.
>>>>
>>>> Signed-off-by: Huang Rui <ray.huang at amd.com>
>>>> ---
>>>>
>>>> Changes from V2 -> V3:
>>>> - Use duplicate mutex release to avoid "goto" in non-error patch.
>>>> - Rename error label
>>>>
>>>> ---
>>>>   drivers/gpu/drm/drm_global.c | 24 ++++++++++++++----------
>>>>   1 file changed, 14 insertions(+), 10 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_global.c b/drivers/gpu/drm/drm_global.c
>>>> index 3d2e91c..b404287 100644
>>>> --- a/drivers/gpu/drm/drm_global.c
>>>> +++ b/drivers/gpu/drm/drm_global.c
>>>> @@ -65,30 +65,34 @@ void drm_global_release(void)
>>>>   
>>>>   int drm_global_item_ref(struct drm_global_reference *ref)
>>>>   {
>>>> -	int ret;
>>>> +	int ret = 0;
>>>>   	struct drm_global_item *item = &glob[ref->global_type];
>>>>   
>>>>   	mutex_lock(&item->mutex);
>>>>   	if (item->refcount == 0) {
>>>> -		item->object = kzalloc(ref->size, GFP_KERNEL);
>>>> -		if (unlikely(item->object == NULL)) {
>>>> +		ref->object = kzalloc(ref->size, GFP_KERNEL);
>>> So the item refcount is 0, we operate on ref, whereas previous we
>>> inspected item and operated on item. Not an improvement.
>> Hmm, when item refcount is 0, we operate on ref to create object and init
>> it for item, so although item->object and ref->object here should point the
>> same thing, but we should alloc and init ref firstly and pass the
>> ref->object address to item (item actually points a static area). This make
>> the code logic more clearly and readable. So I updated it to "ref" here.
>> :-)
> The object is owned by item. ref is just a pointer to it.

Yeah, so what? We initialized ref->item when the refcount was 0 before 
as well.

Why not create the reference first and initialize the item later on?

Regards,
Christian.

> -Chris
>