[PATCH] fbcon: fix race condition between console lock and cursor timer

[Josh Boyer]

On Tue, Aug 21, 2012 at 2:40 AM, Dave Airlie <airlied at redhat.com> wrote:
> So we've had a fair few reports of fbcon handover breakage between
> efi/vesafb and i915 surface recently, so I dedicated a couple of
> days to finding the problem.
>
> Essentially the last thing we saw was the conflicting framebuffer
> message and that was all.
>
> So after much tracing with direct netconsole writes (printks
> under console_lock not so useful), I think I found the race.
>
> Thread A (driver load)    Thread B (timer thread)
>   unbind_con_driver ->              |
>   bind_con_driver ->                |
>   vc->vc_sw->con_deinit ->          |
>   fbcon_deinit ->                   |
>   console_lock()                    |
>       |                             |
>       |                       fbcon_flashcursor timer fires
>       |                       console_lock() <- blocked for A
>       |
>       |
> fbcon_del_cursor_timer ->
>   del_timer_sync
>   (BOOM)
>
> Of course because all of this is under the console lock,
> we never see anything, also since we also just unbound the active
> console guess what we never see anything.
>
> Hopefully this fixes the problem for anyone seeing vesafb->kms
> driver handoff.
>
> Signed-off-by: David Airlie <airlied at redhat.com>
> ---
>  drivers/video/console/fbcon.c |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/video/console/fbcon.c b/drivers/video/console/fbcon.c
> index 2e471c2..f8a79fc 100644
> --- a/drivers/video/console/fbcon.c
> +++ b/drivers/video/console/fbcon.c
> @@ -372,8 +372,12 @@ static void fb_flashcursor(struct work_struct *work)
>         struct vc_data *vc = NULL;
>         int c;
>         int mode;
> +       int ret;
> +
> +       ret = console_trylock();
> +       if (ret == 0)
> +               return;
>
> -       console_lock();
>         if (ops && ops->currcon != -1)
>                 vc = vc_cons[ops->currcon].d;
>

I have a Dell XPS 8300 machine with a Radeon card in it that started
showing this problem yesterday with 3.6-rc2 kernels.  I tested this
patch on top of v3.6-rc2-206-g10c63c9 this morning and the problem
seems to have been cleared up for me.  That includes making sure the
grub2 file has the gfxterm set, etc.

I know we've been seeing this quite a bit more on Fedora 17, so we'll
want to have some people test a 3.5 build with it but things are
looking better.

josh

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