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

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Date:	Fri, 2 Oct 2015 21:26:41 +0530
From:	Sudip Mukherjee <sudipm.mukherjee@...il.com>
To:	Patrik Jakobsson <patrik.r.jakobsson@...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 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@...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 :)

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.

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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