linux-kernel - Re: [BUG] drivers/video/sis: deadlock introduced by "fbdev: add mutex for fb

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Message-Id: <20090705181808.93be24a9.krzysztof.h1@poczta.fm>
Date:	Sun, 5 Jul 2009 18:18:08 +0200
From:	Krzysztof Helt <krzysztof.h1@...zta.fm>
To:	Paul Mundt <lethal@...ux-sh.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Wu Zhangjin <wuzhangjin@...il.com>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mips@...ux-mips.org, Krzysztof Helt <krzysztof.h1@...pl>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ralf Baechle <ralf@...ux-mips.org>, ???? <yanh@...ote.com>,
	zhangfx <zhangfx@...ote.com>
Subject: Re: [BUG] drivers/video/sis: deadlock introduced by
 "fbdev: add mutex for fb_mmap locking"

On Mon, 6 Jul 2009 00:25:57 +0900
Paul Mundt <lethal@...ux-sh.org> wrote:

> On Sun, Jul 05, 2009 at 08:19:40AM -0700, Linus Torvalds wrote:
> > 
> > 
> > On Mon, 6 Jul 2009, Paul Mundt wrote:
> > > >
> > > > Why not "lock" as well?
> > > 
> > > I had that initially, but matroxfb will break if we do that, and
> > > presently nothing cares about trying to take ->lock that early on.
> > 
> > I really would rather have consistency than some odd rules like that.
> > 
> > In particular - if matroxfb is different and needs its own lock 
> > initialization because it doesn't use the common allocation routine, then 
> > please make _that_ consistent too. Rather than have it special-case just 
> > one lock that it needs to initialize separately, make it clear that since 
> > it does its own allocations it needs to initialize _everything_ 
> > separately.
> > 
> Ok, here is an updated version with an updated matroxfb and the sm501fb
> change reverted.
> 
> Signed-off-by: Paul Mundt <lethal@...ux-sh.org>
> 
> ---
> 

This is incorrect way to fix this as some drivers do not use the framebuffer_alloc() 
at all. They use global (for a file) fb_info structure. I have done some cleanups to
the fbdev layer before the 2.6.31 and there should no drivers which uses kmalloc or
kzalloc to allocate the fb_info (your patch would break these drivers too).

A root of the whole mm_lock issue is that the fb_mmap() BKL protected two fb_info
fields which were never protected when set. I changed this by add the mm_lock 
around these fields but only in drivers which modified this fields AFTER call
to the register_framebuffer(). Some drivers set these fields using the same
function before and after the register_framebuffer(). I strongly believe that
setting these fields before the register_framebuffer() is wrong or redundant for
these drivers. See my fix for the sisfb driver below. 

I have tested the patch below. Wu Zhangjin, can you also confirm that this 
works for you (without your patch)?

I will look into the matroxfb and sm501fb drivers now. The same problem is
already fixed for the mx3fb driver and the patch is sent to Andrew Morton.

Regards,
Krzysztof

From: Krzysztof Helt <krzysztof.h1@...pl>

Remove redundant call to the sisfb_get_fix() before sis frambuffer is registered.

This fixes a problem with uninitialized the fb_info->mm_lock mutex.

Signed-off-by: Krzysztof Helt <krzysztof.h1@...pl>
---

diff -urp linux-ref/drivers/video/sis/sis_main.c linux-next/drivers/video/sis/sis_main.c
--- linux-ref/drivers/video/sis/sis_main.c	2009-07-01 18:07:05.000000000 +0200
+++ linux-next/drivers/video/sis/sis_main.c	2009-07-05 17:20:33.000000000 +0200
@@ -6367,7 +6367,6 @@ error_3:	vfree(ivideo->bios_abase);
 		sis_fb_info->fix = ivideo->sisfb_fix;
 		sis_fb_info->screen_base = ivideo->video_vbase + ivideo->video_offset;
 		sis_fb_info->fbops = &sisfb_ops;
-		sisfb_get_fix(&sis_fb_info->fix, -1, sis_fb_info);
 		sis_fb_info->pseudo_palette = ivideo->pseudo_palette;

 		fb_alloc_cmap(&sis_fb_info->cmap, 256 , 0);

----------------------------------------------------------------------
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