Regression in panic

[Mandeep Singh Baines]

On Mon, Jun 20, 2011 at 4:03 PM, David Rientjes <rientjes at google.com> wrote:
> On Mon, 20 Jun 2011, Mandeep Singh Baines wrote:
>
>> Hi Dave,
>>
>> I think this change is causing a regression I'm seeing in panic.
>> Before this change, I'd get a
>> reboot on panic (we've configured as such).
>>
>> With this change, my machine gets wedged if the machine is running in
>> X when the panic occurs.
>>
>> I traced the code flow to this:
>>
>> bust_spinlocks(0);
>>  ->unblank_screen();
>>    ->do_unblank_screen(0);
>>      ->vc->vc_sw->con_blank(vc, 0, 0);
>>        ->fbcon_blank(vc, 0, 0);
>>          ->update_screen(vc);
>>            ->redraw_screen(vc, 0);
>>              ->vc->vc_sw->con_switch(vc);
>>                ->fbcon_switch(vc);
>>                  ->ops->update_start(info);
>>                    ->bit_update_start(info);
>>                     ->fb_pan_display(info, &ops->var);
>>                       ->info->fbops->fb_pan_display(var, info);
>>                         ->drm_fb_helper_pan_display(var, info);
>>                           ->mutex_lock(&dev->mode_config.mutex); *this blocks*
>>
>> With this change, there is now a lot going on in the panic path. Stuff
>> that I'm not sure is safe when panicking. In addition to the
>> mutex_lock, there is also a del_timer_sync()
>> now happening in the context of panic().
>>
>> I see this bug with a 2.6.38 kernel but did a quick scan of a newer
>> kernels and did not see anything that changed in this path so I
>> suspect its still there.
>>
>> Reverting this change fixes the regression.
>>
>
> Chris Fowler reports something similar when running 2.6.38 by inducing a
> kernel panic via the oom killer -- see
> http://marc.info/?l=linux-kernel&m=130805985022791.  I've added him to the
> cc so he can participate in the thread and cherry-pick any fixes (last
> status update was that he was going to be trying 2.6.38.8).
>

One potential fix might be to convert the mutex_lock to a try if
oops_in_progress but
I suspect oops_in_progress checks may be needed in a bunch of other places in
the screen_unblank code path.

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