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Date:	Sat, 18 Jun 2011 11:19:34 +0200
From:	Bruno Prémont <bonbons@...ux-vserver.org>
To:	Guennadi Liakhovetski <g.liakhovetski@....de>
Cc:	Florian Tobias Schandinat <FlorianSchandinat@....de>,
	lethal@...ux-sh.org, linux-fbdev@...r.kernel.org,
	francis.moro@...il.com, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Herton Ronaldo Krzesinski <herton@...driva.com.br>,
	stable@...nel.org
Subject: Re: [PATCH] fb: avoid possible deadlock caused by fb_set_suspend

Guennadi, could you have a look at (completely untested) patch which avoids
possible deadlock due to inverted lock taking order on hotplug as well
as "readding" lock_fb_info() for fb_set_suspend() call after Herton's
patch to fb_set_suspend().

Thanks,
Bruno


On Sat, 18 June 2011 Bruno Prémont <bonbons@...ux-vserver.org> wrote:
> On Fri, 17 June 2011 Florian Tobias Schandinat <FlorianSchandinat@....de> wrote:
> > From: Herton Ronaldo Krzesinski <herton@...driva.com.br>
> > 
> > A lock ordering issue can cause deadlocks: in framebuffer/console code,
> > all needed struct fb_info locks are taken before acquire_console_sem(),
> > in places which need to take console semaphore.
> > 
> > But fb_set_suspend is always called with console semaphore held, and
> > inside it we call lock_fb_info which gets the fb_info lock, inverse
> > locking order of what the rest of the code does. This causes a real
> > deadlock issue, when we write to state fb sysfs attribute (which calls
> > fb_set_suspend) while a framebuffer is being unregistered by
> > remove_conflicting_framebuffers, as can be shown by following show
> > blocked state trace on a test program which loads i915 and runs another
> > forked processes writing to state attribute:
> > 
> > Test process with semaphore held and trying to get fb_info lock:
> 
> ...
> 
> > fb-test2 which reproduces above is available on kernel.org bug #26232.
> > To solve this issue, avoid calling lock_fb_info inside fb_set_suspend,
> > and move it out to where needed (callers of fb_set_suspend must call
> > lock_fb_info before if needed). So far, the only place which needs to
> > call lock_fb_info is store_fbstate, all other places which calls
> > fb_set_suspend are suspend/resume hooks that should not need the lock as
> > they should be run only when processes are already frozen in
> > suspend/resume.
> 
> From a quick look through FB drivers in 2.6.39 I've found one that would need
> more work:
> - drivers/video/sh_mobile_hdmi.c: sh_hdmi_edid_work_fn()
>   Should get changed to
>   a) right locking order in case (hdmi->hp_state == HDMI_HOTPLUG_CONNECTED)
>   b) lock fb_info in the other case
>   For this one fb_set_suspend() does get call in a hotplug worker,
>   thus independently on suspend/resume process.
> 
> The rest does match the suspend/resume hook pattern mentioned.
> 
> Bruno
> 
> 
> > References: https://bugzilla.kernel.org/show_bug.cgi?id=26232
> > Signed-off-by: Herton Ronaldo Krzesinski <herton@...driva.com.br>
> > Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@....de>
> > Cc: stable@...nel.org
> > ---
> >  drivers/video/fbmem.c   |    3 ---
> >  drivers/video/fbsysfs.c |    3 +++
> >  2 files changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
> > index 5aac00e..ad93629 100644
> > --- a/drivers/video/fbmem.c
> > +++ b/drivers/video/fbmem.c
> > @@ -1738,8 +1738,6 @@ void fb_set_suspend(struct fb_info *info, int state)
> >  {
> >  	struct fb_event event;
> >  
> > -	if (!lock_fb_info(info))
> > -		return;
> >  	event.info = info;
> >  	if (state) {
> >  		fb_notifier_call_chain(FB_EVENT_SUSPEND, &event);
> > @@ -1748,7 +1746,6 @@ void fb_set_suspend(struct fb_info *info, int state)
> >  		info->state = FBINFO_STATE_RUNNING;
> >  		fb_notifier_call_chain(FB_EVENT_RESUME, &event);
> >  	}
> > -	unlock_fb_info(info);
> >  }
> >  
> >  /**
> > diff --git a/drivers/video/fbsysfs.c b/drivers/video/fbsysfs.c
> > index 04251ce..67afa9c 100644
> > --- a/drivers/video/fbsysfs.c
> > +++ b/drivers/video/fbsysfs.c
> > @@ -399,9 +399,12 @@ static ssize_t store_fbstate(struct device *device,
> >  
> >  	state = simple_strtoul(buf, &last, 0);
> >  
> > +	if (!lock_fb_info(fb_info))
> > +		return -ENODEV;
> >  	console_lock();
> >  	fb_set_suspend(fb_info, (int)state);
> >  	console_unlock();
> > +	unlock_fb_info(fb_info);
> >  
> >  	return count;
> >  }


diff --git a/drivers/video/sh_mobile_hdmi.c b/drivers/video/sh_mobile_hdmi.c
index 2b9e56a..b1a13ab 100644
--- a/drivers/video/sh_mobile_hdmi.c
+++ b/drivers/video/sh_mobile_hdmi.c
@@ -1151,27 +1151,27 @@ static void sh_hdmi_edid_work_fn(struct work_struct *work)
 
 		ch = info->par;
 
-		console_lock();
+		if (lock_fb_info(info)) {
+			console_lock();
 
-		/* HDMI plug in */
-		if (!sh_hdmi_must_reconfigure(hdmi) &&
-		    info->state == FBINFO_STATE_RUNNING) {
-			/*
-			 * First activation with the default monitor - just turn
-			 * on, if we run a resume here, the logo disappears
-			 */
-			if (lock_fb_info(info)) {
+			/* HDMI plug in */
+			if (!sh_hdmi_must_reconfigure(hdmi) &&
+			    info->state == FBINFO_STATE_RUNNING) {
+				/*
+				 * First activation with the default monitor - just turn
+				 * on, if we run a resume here, the logo disappears
+				 */
 				info->var.width = hdmi->var.width;
 				info->var.height = hdmi->var.height;
 				sh_hdmi_display_on(hdmi, info);
-				unlock_fb_info(info);
+			} else {
+				/* New monitor or have to wake up */
+				fb_set_suspend(info, 0);
 			}
-		} else {
-			/* New monitor or have to wake up */
-			fb_set_suspend(info, 0);
-		}
 
-		console_unlock();
+			console_unlock();
+			unlock_fb_info(info);
+		}
 	} else {
 		ret = 0;
 		if (!hdmi->info)
@@ -1181,12 +1181,15 @@ static void sh_hdmi_edid_work_fn(struct work_struct *work)
 		fb_destroy_modedb(hdmi->monspec.modedb);
 		hdmi->monspec.modedb = NULL;
 
-		console_lock();
+		if (lock_fb_info(info)) {
+			console_lock();
 
-		/* HDMI disconnect */
-		fb_set_suspend(hdmi->info, 1);
+			/* HDMI disconnect */
+			fb_set_suspend(hdmi->info, 1);
 
-		console_unlock();
+			console_unlock();
+			unlock_fb_info(info);
+		}
 		pm_runtime_put(hdmi->dev);
 	}
 
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