Re: [PATCH] drm/gma500: fix double freeing

[Patrik Jakobsson]

On Fri, Oct 2, 2015 at 5:56 PM, Sudip Mukherjee
<sudipm.mukherjee@xxxxxxxxx> wrote:
> On Thu, Oct 01, 2015 at 07:07:33PM +0200, Patrik Jakobsson wrote:
>> On Wed, Sep 30, 2015 at 8:12 AM, Sudip Mukherjee
>> <sudipm.mukherjee@xxxxxxxxx> wrote:
>> > On Tue, Sep 29, 2015 at 03:20:35PM +0200, Patrik Jakobsson wrote:
>> >> On Thu, Sep 24, 2015 at 5:57 PM, Sudip Mukherjee
>> >> <sudipm.mukherjee@xxxxxxxxx> wrote:
>> >> > On Wed, Sep 09, 2015 at 06:20:40PM +0530, Sudip Mukherjee wrote:
>> >> >> If backing->stolen is true then we were freeing backing by calling
>> >> >> psb_gtt_free_range() but we called it again after unlocking the mutex.
>> >> >> Lets make it NULL after freeing in psb_gtt_free_range() and check for
>> >> >> NULL before calling the function for the second time.
>> >> >>
>> >> >> Signed-off-by: Sudip Mukherjee <sudip@xxxxxxxxxxxxxxx>
>> >> >> ---
>> >> > Hi Patrik,
>> >> > A gentle ping.
>> >> >
>> >> > regards
>> >> > sudip
>> >>
>> >> Hi, sorry for the late reply.
>> >>
>> >> Why are we freeing the range twice in the first case?
>> > I think,
>> > if backing->stolen is true then backing is released using
>> > psb_gtt_free_range() but if backing->stolen is false then the gem object
>> > is freed but the backing is not yet freed. To free that backing
>> > psb_gtt_free_range() has been called second time. My patch tried to fix
>> > the possibility of backing->stolen being true and backing being freed 2
>> > times.
>> >
>> > regards
>> > sudip
>>
>> There are some special handling of the stolen framebuffer that I don't
>> remember entirely but the basic concept is that we free the backing
>> when we drop the last reference on a gem object. That will trigger a
>> psb_gtt_free_range(). So in this case it looks to me that the extra
>> free is not needed at all. That's my quick reasoning, feel free to
>> prove me wrong :)
>
> In this case we are allocating backing using psbfb_alloc() and so
> backing->stolen is always true. So we can remove the backing->stolen
> condition. And if drm_fb_helper_alloc_fbi() fails then we
> are jumping to out_err1. So the fitst free will not be needed.

Sounds good, could you also rename the labels to what they're doing
now. I'm thinking out_release and out_unlock or something you feel is
suitable.

Thanks
Patrik

> diff --git a/drivers/gpu/drm/gma500/framebuffer.c b/drivers/gpu/drm/gma500/framebuffer.c
> index 2eaf1b3..932f07b 100644
> --- a/drivers/gpu/drm/gma500/framebuffer.c
> +++ b/drivers/gpu/drm/gma500/framebuffer.c
> @@ -466,11 +466,6 @@ static int psbfb_create(struct psb_fbdev *fbdev,
>         mutex_unlock(&dev->struct_mutex);
>         return 0;
>  out_unref:
> -       if (backing->stolen)
> -               psb_gtt_free_range(dev, backing);
> -       else
> -               drm_gem_object_unreference(&backing->gem);
> -
>         drm_fb_helper_release_fbi(&fbdev->psb_fb_helper);
>  out_err1:
>         mutex_unlock(&dev->struct_mutex);
>
>
> If it is ok, I can submit the v2.
>
> regards
> sudip
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