linux-kernel - Re: [PATCH] drm/gma500: fix double freeing

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Message-ID: <CAMeQTsYhfXrbkxqpoCkYPL_scfCqrqhiJ-9x=46+YB+aY0reaw@mail.gmail.com>
Date:	Thu, 1 Oct 2015 19:07:33 +0200
From:	Patrik Jakobsson <patrik.r.jakobsson@...il.com>
To:	Sudip Mukherjee <sudipm.mukherjee@...il.com>
Cc:	David Airlie <airlied@...ux.ie>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH] drm/gma500: fix double freeing

On Wed, Sep 30, 2015 at 8:12 AM, Sudip Mukherjee
<sudipm.mukherjee@...il.com> 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@...il.com> 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@...torindia.org>
>> >> ---
>> > 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 :)

Thanks
Patrik
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