linux-kernel - Re: Regression in panic

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Message-ID: <alpine.DEB.2.00.1106201601220.15259@chino.kir.corp.google.com>
Date:	Mon, 20 Jun 2011 16:03:37 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Mandeep Singh Baines <msb@...omium.org>
cc:	Dave Airlie <airlied@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Olof Johansson <olofj@...omium.org>,
	linux-fbdev@...r.kernel.org, dri-devel@...ts.sf.net,
	linux-kernel@...r.kernel.org, Chris Fowler <cfowler@...dc.com>
Subject: Re: Regression in panic

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).
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