Re: Linux 2.6.38-rc6

[Paul Mundt <lethal@linux-sh.org>]

On Fri, Feb 25, 2011 at 11:49:21AM -0300, Herton Ronaldo Krzesinski wrote:
> On Fri, Feb 25, 2011 at 03:56:20AM +0200, Anca Emanuel wrote:
> > On Fri, Feb 25, 2011 at 3:47 AM, Anca Emanuel <anca.emanuel@gmail.com> wrote:
> > > On Fri, Feb 25, 2011 at 3:14 AM, Dave Airlie <airlied@redhat.com> wrote:
> > >> On Thu, 2011-02-24 at 16:54 -0800, Linus Torvalds wrote:
> > >>> On Thu, Feb 24, 2011 at 4:48 PM, Anca Emanuel <anca.emanuel@gmail.com> wrote:
> > >>> >
> > >>> > diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
> > >>> > index e2bf953..e8f8925 100644
> > >>> > --- a/drivers/video/fbmem.c
> > >>> > +++ b/drivers/video/fbmem.c
> > >>> > @@ -1511,6 +1511,7 @@ void remove_conflicting_framebuffers(struct
> > >>> > apertures_struct *a,
> > >>> > ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? "%s vs %s - removing generic driver\n",
> > >>> > ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? name, registered_fb[i]->fix.id);
> > >>> > ? ? ? ? ? ? ? ? ? ? ? ?unregister_framebuffer(registered_fb[i]);
> > >>> > + ? ? ? ? ? ? ? ? ? ? ? registered_fb[i] = NULL;
> > >>> >
> > >>> > Tested the patch, and now I get this:
> > >>> > dmesg: http://pastebin.com/ieMNrA7C
> > >>> >
> > >>> > [ ? 12.252328] BUG: unable to handle kernel NULL pointer dereference
> > >>> > at 00000000000003b8
> > >>> > [ ? 12.252342] IP: [<ffffffff81311178>] fb_mmap+0x58/0x1d0
> > >>>
> > >>> Ok, goodie.
> > >>>
> > >>> Or not so goodie, but it does make it clear that yeah, the fb code
> > >>> seems to be using stale pointers from that registered_fb[] array, and
> > >>> the whole unregistration process is just racing with people using it.
> > >>>
> > >>> Herton had that much bigger patch, can you test it?
> > >>
> > >> I think Andy's patch worked, not sure why it fell between the cracks,
> > >> either didn't appear on lkml or in my inbox at all.
> > >>
> > >> if we can get Herton to repost it properly + a tested by I'm happy for
> > >> it to go in.
> > >>
> > >> Dave.
> > >>
> > >>
> > >
> > > Tested Andy's patch and it works !
> > > http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-natty.git;a=commit;hÅa742b5f78e161d6a13853a7e3e6e1dfa429e69
> > >
> > > Tested-by: Anca Emanuel <anca.emanuel@gmail.com>
> > >
> > 
> > link to patch: http://is.gd/otIfGc
> 
> Adding Andy on CC (btw he is away for today, may get some time to answer).
> 
> Andy, can you repost the patch?
> 
This is the first I've seen the patch as well, but fortunately patchwork
caught it on the Cc.

There's also an outstanding patch for fixing an AB-BA deadlock between
the fb_info lock and the console lock which this will clash with. I'm
happy to rework that patch on top of Andy's patch for Anca and/or Herton
to test, though.

I'll need to do some more testing locally as well..