linux-kernel - Re: [Linux-fbdev-devel] [2.6.29-rc2] fb_mmap: circular locking dependency on hibernation

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Date:	Sat, 31 Jan 2009 18:44:45 +0100 (CET)
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Andrea Righi <righi.andrea@...il.com>
cc:	Andrey Borzenkov <arvidjaar@...l.ru>,
	Linux Frame Buffer Device Development 
	<linux-fbdev-devel@...ts.sourceforge.net>,
	"Antonino A. Daplas" <adaplas@...il.com>,
	linux-pm@...ts.linux-foundation.org,
	Linux Kernel Development <linux-kernel@...r.kernel.org>
Subject: Re: [Linux-fbdev-devel] [2.6.29-rc2] fb_mmap: circular locking
 dependency on hibernation

On Sat, 31 Jan 2009, Andrea Righi wrote:
> On 2009-01-30 05:15, Andrey Borzenkov wrote:
> > On 29 of January 2009 12:10:11 Geert Uytterhoeven wrote:
> >> On Tue, 27 Jan 2009, Andrey Borzenkov wrote:
> fbcon: avoid circular locking dependency between fb_info->lock and mm->mmap_sem
> 
> In fbcon notifier the handler for FB_EVENT_SET_CONSOLE_MAP doesn't need
> to hold fb_info->lock.
> 
> Simply unlock it before calling set_con2fb_map(), that could try to
> acquire mm->mmap_sem to avoid a circular locking dependency with
> fb_mmap() (that acquires mm->mmap_sem -> fb_info-lock).

However, set_con2fb_map() accesses the array of fb_info pointers
registered_fb[], which seems to be a bit unsafe now the BKL is no longer held.

> Reported-by: Andrey Borzenkov <arvidjaar@...l.ru>
> Signed-off-by: Andrea Righi <righi.andrea@...il.com>
> ---
>  drivers/video/console/fbcon.c |    6 ++++++
>  1 files changed, 6 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/video/console/fbcon.c b/drivers/video/console/fbcon.c
> index 1657b96..81fc2f7 100644
> --- a/drivers/video/console/fbcon.c
> +++ b/drivers/video/console/fbcon.c
> @@ -3169,9 +3169,15 @@ static int fbcon_event_notify(struct notifier_block *self,
>  		ret = fbcon_fb_unregistered(info);
>  		break;
>  	case FB_EVENT_SET_CONSOLE_MAP:
> +		/*
> +		 * XXX: info is not used here, we can unlock it to avoid
> +		 * circular locking with &current->mm->mmap_sem in fb_mmap().
> +		 */
> +		unlock_fb_info(info);
>  		con2fb = event->data;
>  		ret = set_con2fb_map(con2fb->console - 1,
>  				     con2fb->framebuffer, 1);
> +		lock_fb_info(info);
>  		break;
>  	case FB_EVENT_GET_CONSOLE_MAP:
>  		con2fb = event->data;
> 

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds
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