linux-kernel - Re: [PATCH] fbmem: fix race condition between register_framebuffer() and fb

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Message-ID: <20110525211253.4985ee79@neptune.home>
Date:	Wed, 25 May 2011 21:12:53 +0200
From:	Bruno Prémont <bonbons@...ux-vserver.org>
To:	Fabio Erculiani <lxnay@...ayon.org>
Cc:	linux-fbdev@...r.kernel.org, lethal@...ux-sh.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fbmem: fix race condition between
 register_framebuffer() and fb_open()

On Wed, 25 May 2011 Fabio Erculiani <lxnay@...ayon.org> wrote:
> On Wed, May 25, 2011 at 8:57 PM, Bruno Prémont wrote:
> > On Wed, 25 May 2011 Fabio Erculiani <lxnay@...ayon.org> wrote:
> >> On Wed, May 25, 2011 at 8:46 PM, Bruno Prémont wrote:
> >> > On Wed, 25 May 2011 Fabio Erculiani <lxnay@...ayon.org> wrote:
> >> >> I'm not a fbdev expert. So I leave the real fix to real men ( ;-) ).
> >> >> It is causing deadlock during boot, so I would consider it quite critical.
> >> >> Users using any fb driver will get into troubles.
> >> >> The workaround is to boot with vga=normal.
> >> >
> >> > What is your system doing during boot? I've never seen it here but maybe
> >> > my boot sequence is too simple.
> >>
> >> I'm using vesafb and vga=791. It is quite simple to reproduce.
> >> Also see: http://bugs.gentoo.org/show_bug.cgi?id=368109
> >
> > Looks like gentoo kernel, might be splash is related to the hang
> 
> Then, if you say so, it must be the fbsplash patch for sure, I keep
> forgetting of that :-/

I've had a look at the bug report which points at fbcon_decore
patch.

Looking into that patch confirms my impression:
  fbcon_decor calls a userspace helper at the time fbcon takes over
  console and that userspace helper then tries to open fb device
  with the aim of calling some IOCTLs.

Probably changing fbcon_decor to just call the userspace helper in
non-blocking mode (or having userspace helper "fork and detach") would
avoid the deadlock as well.
Though fbcon_decor seems to rely on helper's return code...

What is the matching piece on userspace side so I can look at it as well?

Bruno
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