Re: [patch] fbcon: Fix workqueue shutdown

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 3 Dec 2008 16:23:12 -0800 Geoff Levand <geoffrey.levand@am.sony.com> wrote:

> Add a call to cancel_work_sync() in fbcon_exit() to cancel
> any pending work in the fbcon workqueue.  Also, change the
> assignment of the workqueue function info->queue.func from
> NULL to fb_flashcursor(),

OK.

> and add a corresponding check in
> fb_flashcursor() to exit immediately if the variable
> fbcon_has_exited is set.

Why was this second part added?

> The current implementation of fbcon_exit() sets the fbcon
> workqueue function info->queue.func to NULL, but does not
> assure that there is no work pending when it does so.  On
> occasion, depending on system timing, there will still be
> pending work in the queue when fbcon_exit() is called.
> This results in a null pointer deference when
> run_workqueue() tries to call the queue's work function.
> 
> Fixes errors on shutdown similar to these:
> 
>   Console: switching to colour dummy device 80x25
>   Unable to handle kernel paging request for data at address 0x00000000
> 
> ...
>
> --- a/drivers/video/console/fbcon.c
> +++ b/drivers/video/console/fbcon.c
> @@ -394,13 +394,19 @@ static void fbcon_update_softback(struct
>  
>  static void fb_flashcursor(struct work_struct *work)
>  {
> -	struct fb_info *info = container_of(work, struct fb_info, queue);
> -	struct fbcon_ops *ops = info->fbcon_par;
> +	struct fb_info *info;
> +	struct fbcon_ops *ops;
>  	struct display *p;
>  	struct vc_data *vc = NULL;
>  	int c;
>  	int mode;
>  
> +	if (fbcon_has_exited)
> +		return;

This whole fbcon_has_exited thing has big red "wrong" lights flashing
all over it.

If drivers get all their refcounting, locking and synchronisation
correct, such things are unneeded.


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