linux-kernel - Re: Deadlock in fb and tty

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Message-ID: <5238A8E6.4000506@hurleysoftware.com>
Date:	Tue, 17 Sep 2013 15:09:26 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	johnflux <johnflux@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Deadlock in fb and tty

On 09/12/2013 09:22 AM, johnflux wrote:
> The following seems to be better:
>
>  From d93c1e9761ff66365d658da7d8d0d33823aa946f Mon Sep 17 00:00:00 2001
> From: John Tapsell<johnflux@...il.com>
> Date: Thu, 12 Sep 2013 09:16:12 +0100
> Subject: [PATCH] Fix deadlock between fb_info and console.  Do not lock
>   fb_info when calling sending the FB_EVENT_CONBLANK
>
> In fbmem.c, the semantics are that we acquire the lock_fb_info first,
> and then console_lock.  However when fbcon.c fbcon_generic_blank() is
> called, the console lock could already be held.  Locking fb_info can
> thus cause a deadlock.
>
> fbmem.c sends the FB_EVENT_BLANK without locking lock_fb_info first, so
> this change introduces similar behaviour.
> ---
>   drivers/video/console/fbcon.c | 4 ----
>   1 file changed, 4 deletions(-)
>
> diff --git a/drivers/video/console/fbcon.c b/drivers/video/console/fbcon.c
> index 6b4fb5c..8546441 100644
> --- a/drivers/video/console/fbcon.c
> +++ b/drivers/video/console/fbcon.c
> @@ -2333,13 +2333,9 @@ static void fbcon_generic_blank(struct vc_data *vc,
> struct fb_info *info,
>                  vc->vc_video_erase_char = oldc;
>          }
>
> -
> -       if (!lock_fb_info(info))
> -               return;
>          event.info = info;
>          event.data = &blank;
>          fb_notifier_call_chain(FB_EVENT_CONBLANK, &event);
> -       unlock_fb_info(info);
>   }
>
>   static int fbcon_blank(struct vc_data *vc, int blank, int mode_switch)
> -- 1.8.1.2

> How can I get this reviewed/acked please?

The get_maintainer.pl script can help to discover to whom to
address patches. For example,

peter@...r:~/src/kernels/next$ ./scripts/get_maintainer.pl -f drivers/video/console/fbcon.c
Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com> (maintainer:FRAMEBUFFER LAYER)
Tomi Valkeinen <tomi.valkeinen@...com> (maintainer:FRAMEBUFFER LAYER)
Dave Airlie <airlied@...hat.com> (commit_signer:5/11=45%)
Greg Kroah-Hartman <gregkh@...uxfoundation.org> (commit_signer:4/11=36%)
Wang YanQing <udknight@...il.com> (commit_signer:4/11=36%)
Andrew Morton <akpm@...ux-foundation.org> (commit_signer:3/11=27%)
Kamal Mostafa <kamal@...nce.com> (commit_signer:1/11=9%)
linux-fbdev@...r.kernel.org (open list:FRAMEBUFFER LAYER)
linux-kernel@...r.kernel.org (open list)

In this case, you'll want to send this patch addressed to the two maintainers
and cc linux-fbdev and linux-kernel. You can cc me if you want.

This is covered in Documentation/SubmittingPatches, which also covers the
required subject line format <hint: too long, needs subsystem identifier>.

Also, if you have the lockdep report of the deadlock, it's customary to
include it in the commit message.

Regards,
Peter Hurley

PS - Don't put stuff after the '--' tag at the end of the patch. Many
mailers treat that as don't care. I missed your question first time around
because Thunderbird grays that text :(

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