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SubjectRe: [REGRESSION] commit 66c1ca0: {fbmem: fix fb_info->lock and mm->mmap_sem ...} causes Xfbdev not working
On Sat, Apr 11, 2009 at 11:00:38AM +0200, Stefan Richter wrote:
> Krzysztof Helt wrote:
> > On Sat, 11 Apr 2009 00:05:22 +0200
> > Andrea Righi <righi.andrea@gmail.com> wrote:
> >
> >> mmmh... I may have missed something, but the common fb_mmap() should
> >> acquire mm->mmap_sem and then info->lock, while fb_ioctl() can do that
> >> in reverse order (info->lock first and then mm->mmap_sem) causing the
> >> circular locking dependency. Are you sure that pushing info->lock down
> >> each driver's fb_mmap will fix the problem?
> >
> > Right. The fb_mmap is called with the mmap_sem already held.
> > I will try other possibilities like breaking info->lock() into two
> > mutexex.
>
> I had to deal with interaction of mmap_sem with a driver lock twice yet.
> I solved it cheaply by replacing mutex_lock(driver_mutex) by
> mutex_trylock(driver_mutex).
>
> The obvious drawback is that thic changes kernel behaviour visibly to
> userspace: Under contention, the ioctl/read/write fails with -EAGAIN
> instead of being blocked until the other thread finished its
> ioctl/read/write/mmap on the same fd (if the driver mutex is
> instantiated per fd, like in my case).
>
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=8449fc3ae58bf8ee5acbd2280754cde67b5db128
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=638570b54346f140bc09b986d93e76025d35180f
>
> (In these two drivers, the change of behaviour is irrelevant to real
> userspace clients.)

I've looked at the code again. I think the main problem here (at last
the last one reported) is that fb_notifier_call_chain() is called with
info->lock held, i.e. in do_fb_ioctl() => FBIOPUT_VSCREENINFO =>
fb_set_var() and the some notifier callbacks, like fbcon_event_notify(),
try to re-acquire info->lock again.

Probably we can just remove the lock/unlock_fb_info() in all the fb
notifier callbacks' and be sure to always call fb_notifier_call_chain()
with info->lock held.

Something like this (untested). I'll test it later and re-submit if it
makes sense.

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 | 55 ++---------------------------------
drivers/video/fbmem.c | 19 ++++++++++++
4 files changed, 22 insertions(+), 58 deletions(-)
diff --git a/drivers/video/backlight/backlight.c b/drivers/video/backlight/backlight.c
index dd37cbc..157057c 100644
--- a/drivers/video/backlight/backlight.c
+++ b/drivers/video/backlight/backlight.c
@@ -35,8 +35,6 @@ 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 ||
@@ -49,7 +47,6 @@ 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 0bb13df..b644947 100644
--- a/drivers/video/backlight/lcd.c
+++ b/drivers/video/backlight/lcd.c
@@ -40,8 +40,6 @@ 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) {
@@ -53,7 +51,6 @@ 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 2cd500a..ec7d9ba 100644
--- a/drivers/video/console/fbcon.c
+++ b/drivers/video/console/fbcon.c
@@ -2263,9 +2263,12 @@ static void fbcon_generic_blank(struct vc_data *vc, struct fb_info *info,
}


+ if (!lock_fb_info(info))
+ return -ENODEV;
event.info = info;
event.data = &blank;
fb_notifier_call_chain(FB_EVENT_CONBLANK, &event);
+ unlock_fb_info(info);
}

static int fbcon_blank(struct vc_data *vc, int blank, int mode_switch)
@@ -2956,8 +2959,6 @@ static int fbcon_fb_unregistered(struct fb_info *info)
{
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)
@@ -2985,8 +2986,6 @@ static int fbcon_fb_unregistered(struct fb_info *info)
if (primary_device == idx)
primary_device = -1;

- unlock_fb_info(info);
-
if (!num_registered_fb)
unregister_con_driver(&fb_con);

@@ -3027,11 +3026,8 @@ static int fbcon_fb_registered(struct fb_info *info)
{
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++) {
@@ -3152,53 +3148,23 @@ 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:
- 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:
@@ -3217,29 +3183,14 @@ 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:
diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
index 2ac32e6..d412a1d 100644
--- a/drivers/video/fbmem.c
+++ b/drivers/video/fbmem.c
@@ -1097,8 +1097,11 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
return -EINVAL;
con2fb.framebuffer = -1;
event.data = &con2fb;
+ if (!lock_fb_info(info))
+ return -ENODEV;
event.info = info;
fb_notifier_call_chain(FB_EVENT_GET_CONSOLE_MAP, &event);
+ unlock_fb_info(info);
ret = copy_to_user(argp, &con2fb, sizeof(con2fb)) ? -EFAULT : 0;
break;
case FBIOPUT_CON2FBMAP:
@@ -1115,8 +1118,11 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
break;
}
event.data = &con2fb;
+ if (!lock_fb_info(info))
+ return -ENODEV;
event.info = info;
ret = fb_notifier_call_chain(FB_EVENT_SET_CONSOLE_MAP, &event);
+ unlock_fb_info(info);
break;
case FBIOBLANK:
if (!lock_fb_info(info))
@@ -1521,7 +1527,10 @@ register_framebuffer(struct fb_info *fb_info)
registered_fb[i] = fb_info;

event.info = fb_info;
+ if (!lock_fb_info(fb_info))
+ return -ENODEV;
fb_notifier_call_chain(FB_EVENT_FB_REGISTERED, &event);
+ unlock_fb_info(fb_info);
return 0;
}

@@ -1555,8 +1564,12 @@ unregister_framebuffer(struct fb_info *fb_info)
goto done;
}

+
+ if (!lock_fb_info(fb_info))
+ return -ENODEV;
event.info = fb_info;
ret = fb_notifier_call_chain(FB_EVENT_FB_UNBIND, &event);
+ unlock_fb_info(fb_info);

if (ret) {
ret = -EINVAL;
@@ -1590,6 +1603,8 @@ 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);
@@ -1598,6 +1613,7 @@ 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);
}

/**
@@ -1667,8 +1683,11 @@ int fb_new_modelist(struct fb_info *info)
err = 1;

if (!list_empty(&info->modelist)) {
+ if (!lock_fb_info(info))
+ return -ENODEV;
event.info = info;
err = fb_notifier_call_chain(FB_EVENT_NEW_MODELIST, &event);
+ unlock_fb_info(info);
}

return err;

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