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

[Sean Paul <seanpaul@xxxxxxxxxxxx>]

On Mon, Sep 5, 2016 at 3:00 AM, Huang Rui <ray.huang@xxxxxxx> 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@xxxxxxx>
> ---
>  drivers/gpu/drm/drm_global.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_global.c b/drivers/gpu/drm/drm_global.c
> index 3d2e91c..3abe738 100644
> --- a/drivers/gpu/drm/drm_global.c
> +++ b/drivers/gpu/drm/drm_global.c
> @@ -70,25 +70,25 @@ int drm_global_item_ref(struct drm_global_reference *ref)
>
>         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);
> +               if (unlikely(ref->object == NULL)) {
>                         ret = -ENOMEM;
>                         goto out_err;
>                 }
> -
> -               ref->object = item->object;
>                 ret = ref->init(ref);
>                 if (unlikely(ret != 0))
>                         goto out_err;
>
> -       }
> +               item->object = ref->object;
> +       } else
> +               ref->object = item->object;
> +

You should add the braces to all blocks if they're present in any block.

>         ++item->refcount;
> -       ref->object = item->object;
>         mutex_unlock(&item->mutex);
>         return 0;
>  out_err:
>         mutex_unlock(&item->mutex);
> -       item->object = NULL;
> +       ref->object = NULL;

It seems like you're leaking memory here. Do you need a
kfree(ref->object) before this?

I think perhaps you can simplify (or maybe streamline is a better
word) things better by unifying the unlock and return. Something like:


diff --git a/drivers/gpu/drm/drm_global.c b/drivers/gpu/drm/drm_global.c
index 3d2e91c..5b255e4 100644
--- a/drivers/gpu/drm/drm_global.c
+++ b/drivers/gpu/drm/drm_global.c
@@ -65,30 +65,31 @@ 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);
+               if (unlikely(ref->object == NULL)) {
                        ret = -ENOMEM;
-                       goto out_err;
+                       goto out;
                }
-
-               ref->object = item->object;
                ret = ref->init(ref);
                if (unlikely(ret != 0))
                        goto out_err;
-
+       } else {
+               ref->object = item->object;
        }
-       ++item->refcount;
-       ref->object = item->object;
-       mutex_unlock(&item->mutex);
-       return 0;
+
+       item->refcount++;
+       goto out;
+
 out_err:
+       kfree(ref->object);
+       ref->object = NULL;
+out:
        mutex_unlock(&item->mutex);
-       item->object = NULL;
        return ret;
 }
 EXPORT_SYMBOL(drm_global_item_ref);


>         return ret;
>  }
>  EXPORT_SYMBOL(drm_global_item_ref);
> --
> 2.7.4
>
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