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

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Date:	Sun, 5 Jul 2009 23:52:03 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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 Sun, Jul 05, 2009 at 07:19:33AM -0700, Linus Torvalds wrote:
> 
> 
> On Sun, 5 Jul 2009, Wu Zhangjin wrote:
> > 
> > then it works! so, I guess there is a deadlock introduced by the above
> > commit.
> 
> Hmm. Perhaps more likely, the 'mm_lock' mutex hasn't even been initialized 
> yet.  We appear to have had that problem with matroxfb and sm501fb, and it 
> may be more common than that. See commit f50bf2b2.
> 
> That said, I do agree that the mm_lock seems to be causing more problems 
> than it actually fixes, and maybe we should revert it. Krzysztof?
> 
Looking at this a bit closer, just moving the mutex initialization in to
framebuffer_alloc() should basically fix most of these, at least it
certainly does for sm501fb and for this sis case as well. So, here's a
patch to do that.

As an aside note, matroxfb is the odd one out, as it doesn't use
framebuffer_alloc() directly for whatever reason. This actually raises an
additional issue, in that framebuffer_alloc() is already where other
mutexes are initialized, which will simply never happen on matroxfb
(suggesting that at least the FB_BACKLIGHT and matroxfb combination will
blow up, although perhaps that's not a valid combination).

Signed-off-by: Paul Mundt <lethal@...ux-sh.org>

---

 drivers/video/fbmem.c   |    1 -
 drivers/video/fbsysfs.c |    2 ++
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
index 53ea056..27a5271 100644
--- a/drivers/video/fbmem.c
+++ b/drivers/video/fbmem.c
@@ -1514,7 +1514,6 @@ register_framebuffer(struct fb_info *fb_info)
 			break;
 	fb_info->node = i;
 	mutex_init(&fb_info->lock);
-	mutex_init(&fb_info->mm_lock);
 
 	fb_info->dev = device_create(fb_class, fb_info->device,
 				     MKDEV(FB_MAJOR, i), NULL, "fb%d", i);
diff --git a/drivers/video/fbsysfs.c b/drivers/video/fbsysfs.c
index d4a2c11..dd413ad 100644
--- a/drivers/video/fbsysfs.c
+++ b/drivers/video/fbsysfs.c
@@ -62,6 +62,8 @@ struct fb_info *framebuffer_alloc(size_t size, struct device *dev)
 	mutex_init(&info->bl_curve_mutex);
 #endif
 
+	mutex_init(&info->mm_lock);
+
 	return info;
 #undef PADDING
 #undef BYTES_PER_LONG
--
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