Re: [Linux-fbdev-devel] [2.6.29-rc2] fb_mmap: circular locking dependency on hibernation

[Andrea Righi <righi.andrea@gmail.com>]

On 2009-02-02 18:36, Andrey Borzenkov wrote:
> I hope you do not think I'm doing this on purpose? :)

LOL!

Don't worry, we'll be able to fix it before linux 3.0.0 (maybe).

> 
> [  241.668005] =======================================================
> [  241.668044] [ INFO: possible circular locking dependency detected ]
> [  241.668068] 2.6.29-rc3-1avb #17
> [  241.668080] -------------------------------------------------------
> [  241.668100] s2disk/4219 is trying to acquire lock:
> [  241.668118]  (&fb_info->lock){--..}, at: [<c02235e7>] 
> fb_mmap+0x97/0x170
> [  241.668188]
> [  241.668190] but task is already holding lock:
> [  241.668207]  (&mm->mmap_sem){----}, at: [<c010639e>] 
> sys_mmap2+0x8e/0xc0
> [  241.668256]
> [  241.668258] which lock already depends on the new lock.
> [  241.668262]
> [  241.668282]
> [  241.668284] the existing dependency chain (in reverse order) is:
> [  241.668305]
> [  241.668307] -> #2 (&mm->mmap_sem){----}:
> [  241.668331]        [<c014520f>] __lock_acquire+0x129f/0x1930
> [  241.668362]        [<c01458fc>] lock_acquire+0x5c/0x80
> [  241.668382]        [<c0182117>] might_fault+0x77/0xa0
> [  241.668413]        [<c0209916>] copy_to_user+0x36/0x120
> [  241.668439]        [<c01a60f7>] filldir+0x97/0xe0
> [  241.668482]        [<c01de339>] sysfs_readdir+0x129/0x220
> [  241.668508]        [<c01a62a6>] vfs_readdir+0x86/0xa0
> [  241.668529]        [<c01a63e8>] sys_getdents+0x68/0xc0
> [  241.668549]        [<c010344a>] syscall_call+0x7/0xb
> [  241.668570]        [<ffffffff>] 0xffffffff
> [  241.668650]
> [  241.668653] -> #1 (sysfs_mutex){--..}:
> [  241.668676]        [<c014520f>] __lock_acquire+0x129f/0x1930
> [  241.668698]        [<c01458fc>] lock_acquire+0x5c/0x80
> [  241.668718]        [<c030820a>] mutex_lock_nested+0xba/0x2f0
> [  241.668749]        [<c01de6cc>] sysfs_addrm_start+0x2c/0xc0
> [  241.668771]        [<c01dec80>] create_dir+0x40/0x90
> [  241.668792]        [<c01decfb>] sysfs_create_dir+0x2b/0x50
> [  241.668812]        [<c02036bc>] kobject_add_internal+0xbc/0x1b0
> [  241.668853]        [<c0203881>] kobject_add_varg+0x31/0x50
> [  241.668874]        [<c02038fc>] kobject_add+0x2c/0x60
> [  241.668895]        [<c0273718>] device_add+0xa8/0x550
> [  241.668938]        [<c0273bd2>] device_register+0x12/0x20
> [  241.668960]        [<c0273c8b>] device_create_vargs+0xab/0xc0
> [  241.668982]        [<c0273cc8>] device_create+0x28/0x30
> [  241.669003]        [<c026cf3d>] register_con_driver+0xed/0x130
> [  241.669027]        [<c026e45b>] take_over_console+0x1b/0x50
> [  241.669049]        [<c02304dd>] fbcon_takeover+0x5d/0xb0
> [  241.669081]        [<c0231457>] fbcon_event_notify+0xb27/0xc20
> [  241.669104]        [<c030c683>] notifier_call_chain+0x53/0xa0
> [  241.669138]        [<c0138ab4>] 
> __blocking_notifier_call_chain+0x44/0x60
> [  241.669218]        [<c0138aea>] 
> blocking_notifier_call_chain+0x1a/0x20
> [  241.669246]        [<c0222f81>] fb_notifier_call_chain+0x11/0x20
> [  241.669276]        [<c0223f08>] register_framebuffer+0x168/0x220
> [  241.669305]        [<c0414ba2>] vesafb_probe+0x542/0x783
> [  241.669347]        [<c027687f>] platform_drv_probe+0xf/0x20
> [  241.669378]        [<c0275a97>] driver_probe_device+0x87/0x1b0
> [  241.669404]        [<c0275c58>] __device_attach+0x8/0x10
> [  241.669429]        [<c02750db>] bus_for_each_drv+0x5b/0x80
> [  241.669460]        [<c0275d06>] device_attach+0x76/0x80
> [  241.669484]        [<c0274ef7>] bus_attach_device+0x47/0x70
> [  241.669511]        [<c0273993>] device_add+0x323/0x550
> [  241.669536]        [<c0277255>] platform_device_add+0x175/0x1c0
> [  241.669567]        [<c0414e7d>] vesafb_init+0x9a/0x1ec
> [  241.669592]        [<c010111a>] do_one_initcall+0x2a/0x160
> [  241.669619]        [<c04004d5>] kernel_init+0x83/0xd5
> [  241.669661]        [<c0103a77>] kernel_thread_helper+0x7/0x10
> [  241.669688]        [<ffffffff>] 0xffffffff
> [  241.669737]
> [  241.669739] -> #0 (&fb_info->lock){--..}:
> [  241.669770]        [<c014537c>] __lock_acquire+0x140c/0x1930
> [  241.669797]        [<c01458fc>] lock_acquire+0x5c/0x80
> [  241.669821]        [<c030820a>] mutex_lock_nested+0xba/0x2f0
> [  241.669847]        [<c02235e7>] fb_mmap+0x97/0x170
> [  241.669872]        [<c0188c76>] mmap_region+0x1d6/0x530
> [  241.669908]        [<c01891a3>] do_mmap_pgoff+0x1d3/0x2f0
> [  241.669934]        [<c01063bd>] sys_mmap2+0xad/0xc0
> [  241.669959]        [<c0103371>] sysenter_do_call+0x12/0x31
> [  241.669984]        [<ffffffff>] 0xffffffff
> [  241.670009]
> [  241.670012] other info that might help us debug this:
> [  241.670015]
> [  241.670049] 1 lock held by s2disk/4219:
> [  241.670065]  #0:  (&mm->mmap_sem){----}, at: [<c010639e>] 
> sys_mmap2+0x8e/0xc0
> [  241.670092]
> [  241.670092] stack backtrace:
> [  241.670092] Pid: 4219, comm: s2disk Not tainted 2.6.29-rc3-1avb #17
> [  241.670092] Call Trace:
> [  241.670092]  [<c0306e38>] ? printk+0x18/0x20
> [  241.670092]  [<c0143b1f>] print_circular_bug_tail+0xcf/0xe0
> [  241.670092]  [<c014537c>] __lock_acquire+0x140c/0x1930
> [  241.670092]  [<c01487f0>] ? futex_wait+0x170/0x450
> [  241.670092]  [<c011826c>] ? try_to_wake_up+0x10c/0x120
> [  241.670092]  [<c0309d15>] ? _spin_unlock_irqrestore+0x35/0x60
> [  241.670092]  [<c0141105>] ? lock_release_holdtime+0x35/0x210
> [  241.670092]  [<c01458fc>] lock_acquire+0x5c/0x80
> [  241.670092]  [<c02235e7>] ? fb_mmap+0x97/0x170
> [  241.670092]  [<c030820a>] mutex_lock_nested+0xba/0x2f0
> [  241.670092]  [<c02235e7>] ? fb_mmap+0x97/0x170
> [  241.670092]  [<c02235e7>] ? fb_mmap+0x97/0x170
> [  241.670092]  [<c019542d>] ? kmem_cache_alloc+0xad/0x100
> [  241.670092]  [<c02235e7>] fb_mmap+0x97/0x170
> [  241.670092]  [<c0188c76>] mmap_region+0x1d6/0x530
> [  241.670092]  [<c0186cfd>] ? arch_get_unmapped_area_topdown+0x8d/0x170
> [  241.670092]  [<c01891a3>] do_mmap_pgoff+0x1d3/0x2f0
> [  241.670092]  [<c01063bd>] sys_mmap2+0xad/0xc0
> [  241.670092]  [<c0103371>] sysenter_do_call+0x12/0x31
> [  244.217754] Syncing filesystems ... done.
> [  244.228175] Freezing user space processes ... (elapsed 0.00 seconds) 
> done.

OK, another patch. This one is a cumulative patch against the latest
Linus' git.

Thanks again!
-Andrea

---
fbmem: fix circular locking dependency between fb_info->lock and mm->mmap_sem

Avoid calling copy_from/to_user() with fb_info->lock mutex held in the
framebuffer's ioctl().

fb_mmap() is called under mm->mmap_sem (A) held, that also acquires
fb_info->lock (B); fb_ioctl() takes fb_info->lock (B) and does
copy_from/to_user() that might acquire mm->mmap_sem (A), causing a
deadlock.

Also fix other potential circular locking dependencies (always between
fb_info->lock and mm->mmap_sem) in fbcon that occur calling the blocking
fb_notifier_call_chain() with fb_info->lock held.

NOTE: it doesn't push down the fb_info->lock in each own driver's
specific fb_ioctl() implementation, so there are still some potential
deadlocks elsewhere.

CC: Andrey Borzenkov <arvidjaar@mail.ru>
Signed-off-by: Andrea Righi <righi.andrea@gmail.com>
---
 drivers/video/backlight/backlight.c |    3 +
 drivers/video/backlight/lcd.c       |    3 +
 drivers/video/console/fbcon.c       |   73 +++++++++++++++++---
 drivers/video/fbcmap.c              |   20 ++++--
 drivers/video/fbmem.c               |  128 +++++++++++++++++------------------
 include/linux/fb.h                  |   15 ++++
 6 files changed, 160 insertions(+), 82 deletions(-)

diff --git a/drivers/video/backlight/backlight.c b/drivers/video/backlight/backlight.c
index 157057c..dd37cbc 100644
--- a/drivers/video/backlight/backlight.c
+++ b/drivers/video/backlight/backlight.c
@@ -35,6 +35,8 @@ static int fb_notifier_callback(struct notifier_block *self,
 		return 0;
 
 	bd = container_of(self, struct backlight_device, fb_notif);
+	if (!lock_fb_info(evdata->info))
+		return -ENODEV;
 	mutex_lock(&bd->ops_lock);
 	if (bd->ops)
 		if (!bd->ops->check_fb ||
@@ -47,6 +49,7 @@ static int fb_notifier_callback(struct notifier_block *self,
 			backlight_update_status(bd);
 		}
 	mutex_unlock(&bd->ops_lock);
+	unlock_fb_info(evdata->info);
 	return 0;
 }
 
diff --git a/drivers/video/backlight/lcd.c b/drivers/video/backlight/lcd.c
index b644947..0bb13df 100644
--- a/drivers/video/backlight/lcd.c
+++ b/drivers/video/backlight/lcd.c
@@ -40,6 +40,8 @@ static int fb_notifier_callback(struct notifier_block *self,
 	if (!ld->ops)
 		return 0;
 
+	if (!lock_fb_info(evdata->info))
+		return -ENODEV;
 	mutex_lock(&ld->ops_lock);
 	if (!ld->ops->check_fb || ld->ops->check_fb(ld, evdata->info)) {
 		if (event == FB_EVENT_BLANK) {
@@ -51,6 +53,7 @@ static int fb_notifier_callback(struct notifier_block *self,
 		}
 	}
 	mutex_unlock(&ld->ops_lock);
+	unlock_fb_info(evdata->info);
 	return 0;
 }
 
diff --git a/drivers/video/console/fbcon.c b/drivers/video/console/fbcon.c
index 1657b96..2cd500a 100644
--- a/drivers/video/console/fbcon.c
+++ b/drivers/video/console/fbcon.c
@@ -2954,8 +2954,11 @@ static int fbcon_fb_unbind(int idx)
 
 static int fbcon_fb_unregistered(struct fb_info *info)
 {
-	int i, idx = info->node;
+	int i, idx;
 
+	if (!lock_fb_info(info))
+		return -ENODEV;
+	idx = info->node;
 	for (i = first_fb_vc; i <= last_fb_vc; i++) {
 		if (con2fb_map[i] == idx)
 			con2fb_map[i] = -1;
@@ -2979,13 +2982,14 @@ static int fbcon_fb_unregistered(struct fb_info *info)
 		}
 	}
 
-	if (!num_registered_fb)
-		unregister_con_driver(&fb_con);
-
-
 	if (primary_device == idx)
 		primary_device = -1;
 
+	unlock_fb_info(info);
+
+	if (!num_registered_fb)
+		unregister_con_driver(&fb_con);
+
 	return 0;
 }
 
@@ -3021,9 +3025,13 @@ static inline void fbcon_select_primary(struct fb_info *info)
 
 static int fbcon_fb_registered(struct fb_info *info)
 {
-	int ret = 0, i, idx = info->node;
+	int ret = 0, i, idx;
 
+	if (!lock_fb_info(info))
+		return -ENODEV;
+	idx = info->node;
 	fbcon_select_primary(info);
+	unlock_fb_info(info);
 
 	if (info_idx == -1) {
 		for (i = first_fb_vc; i <= last_fb_vc; i++) {
@@ -3124,7 +3132,7 @@ static void fbcon_get_requirement(struct fb_info *info,
 	}
 }
 
-static int fbcon_event_notify(struct notifier_block *self, 
+static int fbcon_event_notify(struct notifier_block *self,
 			      unsigned long action, void *data)
 {
 	struct fb_event *event = data;
@@ -3132,7 +3140,7 @@ static int fbcon_event_notify(struct notifier_block *self,
 	struct fb_videomode *mode;
 	struct fb_con2fbmap *con2fb;
 	struct fb_blit_caps *caps;
-	int ret = 0;
+	int idx, ret = 0;
 
 	/*
 	 * ignore all events except driver registration and deregistration
@@ -3144,23 +3152,54 @@ static int fbcon_event_notify(struct notifier_block *self,
 
 	switch(action) {
 	case FB_EVENT_SUSPEND:
+		if (!lock_fb_info(info)) {
+			ret = -ENODEV;
+			goto done;
+		}
 		fbcon_suspended(info);
+		unlock_fb_info(info);
 		break;
 	case FB_EVENT_RESUME:
+		if (!lock_fb_info(info)) {
+			ret = -ENODEV;
+			goto done;
+		}
 		fbcon_resumed(info);
+		unlock_fb_info(info);
 		break;
 	case FB_EVENT_MODE_CHANGE:
+		if (!lock_fb_info(info)) {
+			ret = -ENODEV;
+			goto done;
+		}
 		fbcon_modechanged(info);
+		unlock_fb_info(info);
 		break;
 	case FB_EVENT_MODE_CHANGE_ALL:
+		if (!lock_fb_info(info)) {
+			ret = -ENODEV;
+			goto done;
+		}
 		fbcon_set_all_vcs(info);
+		unlock_fb_info(info);
 		break;
 	case FB_EVENT_MODE_DELETE:
 		mode = event->data;
+		if (!lock_fb_info(info)) {
+			ret = -ENODEV;
+			goto done;
+		}
 		ret = fbcon_mode_deleted(info, mode);
+		unlock_fb_info(info);
 		break;
 	case FB_EVENT_FB_UNBIND:
-		ret = fbcon_fb_unbind(info->node);
+		if (!lock_fb_info(info)) {
+			ret = -ENODEV;
+			goto done;
+		}
+		idx = info->node;
+		unlock_fb_info(info);
+		ret = fbcon_fb_unbind(idx);
 		break;
 	case FB_EVENT_FB_REGISTERED:
 		ret = fbcon_fb_registered(info);
@@ -3178,17 +3217,31 @@ static int fbcon_event_notify(struct notifier_block *self,
 		con2fb->framebuffer = con2fb_map[con2fb->console - 1];
 		break;
 	case FB_EVENT_BLANK:
+		if (!lock_fb_info(info)) {
+			ret = -ENODEV;
+			goto done;
+		}
 		fbcon_fb_blanked(info, *(int *)event->data);
+		unlock_fb_info(info);
 		break;
 	case FB_EVENT_NEW_MODELIST:
+		if (!lock_fb_info(info)) {
+			ret = -ENODEV;
+			goto done;
+		}
 		fbcon_new_modelist(info);
+		unlock_fb_info(info);
 		break;
 	case FB_EVENT_GET_REQ:
 		caps = event->data;
+		if (!lock_fb_info(info)) {
+			ret = -ENODEV;
+			goto done;
+		}
 		fbcon_get_requirement(info, caps);
+		unlock_fb_info(info);
 		break;
 	}
-
 done:
 	return ret;
 }
diff --git a/drivers/video/fbcmap.c b/drivers/video/fbcmap.c
index 91b78e6..f53b9f1 100644
--- a/drivers/video/fbcmap.c
+++ b/drivers/video/fbcmap.c
@@ -250,10 +250,6 @@ int fb_set_user_cmap(struct fb_cmap_user *cmap, struct fb_info *info)
 	int rc, size = cmap->len * sizeof(u16);
 	struct fb_cmap umap;
 
-	if (cmap->start < 0 || (!info->fbops->fb_setcolreg &&
-			        !info->fbops->fb_setcmap))
-		return -EINVAL;
-
 	memset(&umap, 0, sizeof(struct fb_cmap));
 	rc = fb_alloc_cmap(&umap, cmap->len, cmap->transp != NULL);
 	if (rc)
@@ -262,11 +258,23 @@ int fb_set_user_cmap(struct fb_cmap_user *cmap, struct fb_info *info)
 	    copy_from_user(umap.green, cmap->green, size) ||
 	    copy_from_user(umap.blue, cmap->blue, size) ||
 	    (cmap->transp && copy_from_user(umap.transp, cmap->transp, size))) {
-		fb_dealloc_cmap(&umap);
-		return -EFAULT;
+		rc = -EFAULT;
+		goto out;
 	}
 	umap.start = cmap->start;
+	if (!lock_fb_info(info)) {
+		rc = -ENODEV;
+		goto out;
+	}
+	if (cmap->start < 0 || (!info->fbops->fb_setcolreg &&
+				!info->fbops->fb_setcmap)) {
+		rc = -EINVAL;
+		goto out1;
+	}
 	rc = fb_set_cmap(&umap, info);
+out1:
+	unlock_fb_info(info);
+out:
 	fb_dealloc_cmap(&umap);
 	return rc;
 }
diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
index 756efeb..5cdee72 100644
--- a/drivers/video/fbmem.c
+++ b/drivers/video/fbmem.c
@@ -1013,132 +1013,132 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
 	struct fb_var_screeninfo var;
 	struct fb_fix_screeninfo fix;
 	struct fb_con2fbmap con2fb;
+	struct fb_cmap cmap_from;
 	struct fb_cmap_user cmap;
 	struct fb_event event;
 	void __user *argp = (void __user *)arg;
 	long ret = 0;
 
-	fb = info->fbops;
-	if (!fb)
-		return -ENODEV;
-
 	switch (cmd) {
 	case FBIOGET_VSCREENINFO:
-		ret = copy_to_user(argp, &info->var,
-				    sizeof(var)) ? -EFAULT : 0;
+		if (!lock_fb_info(info))
+			return -ENODEV;
+		var = info->var;
+		unlock_fb_info(info);
+
+		ret = copy_to_user(argp, &var, sizeof(var)) ? -EFAULT : 0;
 		break;
 	case FBIOPUT_VSCREENINFO:
-		if (copy_from_user(&var, argp, sizeof(var))) {
-			ret =  -EFAULT;
-			break;
-		}
+		if (copy_from_user(&var, argp, sizeof(var)))
+			return -EFAULT;
+		if (!lock_fb_info(info))
+			return -ENODEV;
 		acquire_console_sem();
 		info->flags |= FBINFO_MISC_USEREVENT;
 		ret = fb_set_var(info, &var);
 		info->flags &= ~FBINFO_MISC_USEREVENT;
 		release_console_sem();
-		if (ret == 0 && copy_to_user(argp, &var, sizeof(var)))
+		unlock_fb_info(info);
+		if (!ret && copy_to_user(argp, &var, sizeof(var)))
 			ret = -EFAULT;
 		break;
 	case FBIOGET_FSCREENINFO:
-		ret = copy_to_user(argp, &info->fix,
-				    sizeof(fix)) ? -EFAULT : 0;
+		if (!lock_fb_info(info))
+			return -ENODEV;
+		fix = info->fix;
+		unlock_fb_info(info);
+
+		ret = copy_to_user(argp, &fix, sizeof(fix)) ? -EFAULT : 0;
 		break;
 	case FBIOPUTCMAP:
 		if (copy_from_user(&cmap, argp, sizeof(cmap)))
-			ret = -EFAULT;
-		else
-			ret = fb_set_user_cmap(&cmap, info);
+			return -EFAULT;
+		ret = fb_set_user_cmap(&cmap, info);
 		break;
 	case FBIOGETCMAP:
 		if (copy_from_user(&cmap, argp, sizeof(cmap)))
-			ret = -EFAULT;
-		else
-			ret = fb_cmap_to_user(&info->cmap, &cmap);
+			return -EFAULT;
+		if (!lock_fb_info(info))
+			return -ENODEV;
+		cmap_from = info->cmap;
+		unlock_fb_info(info);
+		ret = fb_cmap_to_user(&cmap_from, &cmap);
 		break;
 	case FBIOPAN_DISPLAY:
-		if (copy_from_user(&var, argp, sizeof(var))) {
-			ret = -EFAULT;
-			break;
-		}
+		if (copy_from_user(&var, argp, sizeof(var)))
+			return -EFAULT;
+		if (!lock_fb_info(info))
+			return -ENODEV;
 		acquire_console_sem();
 		ret = fb_pan_display(info, &var);
 		release_console_sem();
+		unlock_fb_info(info);
 		if (ret == 0 && copy_to_user(argp, &var, sizeof(var)))
-			ret = -EFAULT;
+			return -EFAULT;
 		break;
 	case FBIO_CURSOR:
 		ret = -EINVAL;
 		break;
 	case FBIOGET_CON2FBMAP:
 		if (copy_from_user(&con2fb, argp, sizeof(con2fb)))
-			ret = -EFAULT;
-		else if (con2fb.console < 1 || con2fb.console > MAX_NR_CONSOLES)
-			ret = -EINVAL;
-		else {
-			con2fb.framebuffer = -1;
-			event.info = info;
-			event.data = &con2fb;
-			fb_notifier_call_chain(FB_EVENT_GET_CONSOLE_MAP,
-								&event);
-			ret = copy_to_user(argp, &con2fb,
-				    sizeof(con2fb)) ? -EFAULT : 0;
-		}
+			return -EFAULT;
+		if (con2fb.console < 1 || con2fb.console > MAX_NR_CONSOLES)
+			return -EINVAL;
+		con2fb.framebuffer = -1;
+		event.data = &con2fb;
+		event.info = info;
+		fb_notifier_call_chain(FB_EVENT_GET_CONSOLE_MAP, &event);
+
+		ret = copy_to_user(argp, &con2fb, sizeof(con2fb)) ? -EFAULT : 0;
 		break;
 	case FBIOPUT_CON2FBMAP:
-		if (copy_from_user(&con2fb, argp, sizeof(con2fb))) {
-			ret = -EFAULT;
-			break;
-		}
-		if (con2fb.console < 1 || con2fb.console > MAX_NR_CONSOLES) {
-			ret = -EINVAL;
-			break;
-		}
-		if (con2fb.framebuffer < 0 || con2fb.framebuffer >= FB_MAX) {
-			ret = -EINVAL;
-			break;
-		}
+		if (copy_from_user(&con2fb, argp, sizeof(con2fb)))
+			return -EFAULT;
+		if (con2fb.console < 1 || con2fb.console > MAX_NR_CONSOLES)
+			return -EINVAL;
+		if (con2fb.framebuffer < 0 || con2fb.framebuffer >= FB_MAX)
+			return -EINVAL;
 		if (!registered_fb[con2fb.framebuffer])
 			request_module("fb%d", con2fb.framebuffer);
 		if (!registered_fb[con2fb.framebuffer]) {
 			ret = -EINVAL;
 			break;
 		}
-		event.info = info;
 		event.data = &con2fb;
+		event.info = info;
 		ret = fb_notifier_call_chain(FB_EVENT_SET_CONSOLE_MAP,
 					      &event);
 		break;
 	case FBIOBLANK:
+		if (!lock_fb_info(info))
+			return -ENODEV;
 		acquire_console_sem();
 		info->flags |= FBINFO_MISC_USEREVENT;
 		ret = fb_blank(info, arg);
 		info->flags &= ~FBINFO_MISC_USEREVENT;
 		release_console_sem();
-		break;;
+		unlock_fb_info(info);
+		break;
 	default:
-		if (fb->fb_ioctl == NULL)
-			ret = -ENOTTY;
-		else
+		if (!lock_fb_info(info))
+			return -ENODEV;
+		fb = info->fbops;
+		if (fb->fb_ioctl)
 			ret = fb->fb_ioctl(info, cmd, arg);
+		else
+			ret = -ENOTTY;
+		unlock_fb_info(info);
 	}
 	return ret;
 }
 
 static long fb_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
-__acquires(&info->lock)
-__releases(&info->lock)
 {
 	struct inode *inode = file->f_path.dentry->d_inode;
 	int fbidx = iminor(inode);
-	struct fb_info *info;
-	long ret;
+	struct fb_info *info = registered_fb[fbidx];
 
-	info = registered_fb[fbidx];
-	mutex_lock(&info->lock);
-	ret = do_fb_ioctl(info, cmd, arg);
-	mutex_unlock(&info->lock);
-	return ret;
+	return do_fb_ioctl(info, cmd, arg);
 }
 
 #ifdef CONFIG_COMPAT
@@ -1257,8 +1257,6 @@ static int fb_get_fscreeninfo(struct fb_info *info, unsigned int cmd,
 
 static long fb_compat_ioctl(struct file *file, unsigned int cmd,
 			    unsigned long arg)
-__acquires(&info->lock)
-__releases(&info->lock)
 {
 	struct inode *inode = file->f_path.dentry->d_inode;
 	int fbidx = iminor(inode);
@@ -1266,7 +1264,6 @@ __releases(&info->lock)
 	struct fb_ops *fb = info->fbops;
 	long ret = -ENOIOCTLCMD;
 
-	mutex_lock(&info->lock);
 	switch(cmd) {
 	case FBIOGET_VSCREENINFO:
 	case FBIOPUT_VSCREENINFO:
@@ -1292,7 +1289,6 @@ __releases(&info->lock)
 			ret = fb->fb_compat_ioctl(info, cmd, arg);
 		break;
 	}
-	mutex_unlock(&info->lock);
 	return ret;
 }
 #endif
diff --git a/include/linux/fb.h b/include/linux/fb.h
index 818fe21..31527e1 100644
--- a/include/linux/fb.h
+++ b/include/linux/fb.h
@@ -960,6 +960,21 @@ extern struct fb_info *registered_fb[FB_MAX];
 extern int num_registered_fb;
 extern struct class *fb_class;
 
+static inline int lock_fb_info(struct fb_info *info)
+{
+	mutex_lock(&info->lock);
+	if (!info->fbops) {
+		mutex_unlock(&info->lock);
+		return 0;
+	}
+	return 1;
+}
+
+static inline void unlock_fb_info(struct fb_info *info)
+{
+	mutex_unlock(&info->lock);
+}
+
 static inline void __fb_pad_aligned_buffer(u8 *dst, u32 d_pitch,
 					   u8 *src, u32 s_pitch, u32 height)
 {