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Subject[RFC PATCH RESEND] fb: reorder the lock sequence to fix a potential lockdep
Following commits:
50e244cc79 fb: rework locking to fix lock ordering on takeover
e93a9a8687 fb: Yet another band-aid for fixing lockdep mess
054430e773 fbcon: fix locking harder

reworked locking to fix related lock ordering on takeover, and introduced console_lock
into fbmem, but it seems that the new lock sequence(fb_info->lock ---> console_lock)
is against with the one in console_callback(console_lock ---> fb_info->lock), and leads to
a potential deadlock as following:
[ 601.079000] ======================================================
[ 601.079000] [ INFO: possible circular locking dependency detected ]
[ 601.079000] 3.11.0 #189 Not tainted
[ 601.079000] -------------------------------------------------------
[ 601.079000] kworker/0:3/619 is trying to acquire lock:
[ 601.079000] (&fb_info->lock){+.+.+.}, at: [<ffffffff81397566>] lock_fb_info+0x26/0x60
[ 601.079000]
but task is already holding lock:
[ 601.079000] (console_lock){+.+.+.}, at: [<ffffffff8141aae3>] console_callback+0x13/0x160
[ 601.079000]
which lock already depends on the new lock.

[ 601.079000]
the existing dependency chain (in reverse order) is:
[ 601.079000]
-> #1 (console_lock){+.+.+.}:
[ 601.079000] [<ffffffff810dc971>] lock_acquire+0xa1/0x140
[ 601.079000] [<ffffffff810c6267>] console_lock+0x77/0x80
[ 601.079000] [<ffffffff81399448>] register_framebuffer+0x1d8/0x320
[ 601.079000] [<ffffffff81cfb4c8>] efifb_probe+0x408/0x48f
[ 601.079000] [<ffffffff8144a963>] platform_drv_probe+0x43/0x80
[ 601.079000] [<ffffffff8144853b>] driver_probe_device+0x8b/0x390
[ 601.079000] [<ffffffff814488eb>] __driver_attach+0xab/0xb0
[ 601.079000] [<ffffffff814463bd>] bus_for_each_dev+0x5d/0xa0
[ 601.079000] [<ffffffff81447e6e>] driver_attach+0x1e/0x20
[ 601.079000] [<ffffffff81447a07>] bus_add_driver+0x117/0x290
[ 601.079000] [<ffffffff81448fea>] driver_register+0x7a/0x170
[ 601.079000] [<ffffffff8144a10a>] __platform_driver_register+0x4a/0x50
[ 601.079000] [<ffffffff8144a12d>] platform_driver_probe+0x1d/0xb0
[ 601.079000] [<ffffffff81cfb0a1>] efifb_init+0x273/0x292
[ 601.079000] [<ffffffff81002132>] do_one_initcall+0x102/0x1c0
[ 601.079000] [<ffffffff81cb80a6>] kernel_init_freeable+0x15d/0x1ef
[ 601.079000] [<ffffffff8166d2de>] kernel_init+0xe/0xf0
[ 601.079000] [<ffffffff816914ec>] ret_from_fork+0x7c/0xb0
[ 601.079000]
-> #0 (&fb_info->lock){+.+.+.}:
[ 601.079000] [<ffffffff810dc1d8>] __lock_acquire+0x1e18/0x1f10
[ 601.079000] [<ffffffff810dc971>] lock_acquire+0xa1/0x140
[ 601.079000] [<ffffffff816835ca>] mutex_lock_nested+0x7a/0x3b0
[ 601.079000] [<ffffffff81397566>] lock_fb_info+0x26/0x60
[ 601.079000] [<ffffffff813a4aeb>] fbcon_blank+0x29b/0x2e0
[ 601.079000] [<ffffffff81418658>] do_blank_screen+0x1d8/0x280
[ 601.079000] [<ffffffff8141ab34>] console_callback+0x64/0x160
[ 601.079000] [<ffffffff8108d855>] process_one_work+0x1f5/0x540
[ 601.079000] [<ffffffff8108e04c>] worker_thread+0x11c/0x370
[ 601.079000] [<ffffffff81095fbd>] kthread+0xed/0x100
[ 601.079000] [<ffffffff816914ec>] ret_from_fork+0x7c/0xb0
[ 601.079000]
other info that might help us debug this:

[ 601.079000] Possible unsafe locking scenario:

[ 601.079000] CPU0 CPU1
[ 601.079000] ---- ----
[ 601.079000] lock(console_lock);
[ 601.079000] lock(&fb_info->lock);
[ 601.079000] lock(console_lock);
[ 601.079000] lock(&fb_info->lock);
[ 601.079000]
*** DEADLOCK ***

so we reorder the lock sequence the same as it in console_callback() to
avoid this issue.
Not very sure this change is suitable, any comments is welcome.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
---
drivers/video/fbmem.c | 50 +++++++++++++++++++++++++++++++-----------------
1 files changed, 32 insertions(+), 18 deletions(-)

diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
index dacaf74..010d191 100644
--- a/drivers/video/fbmem.c
+++ b/drivers/video/fbmem.c
@@ -1108,14 +1108,16 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
case FBIOPUT_VSCREENINFO:
if (copy_from_user(&var, argp, sizeof(var)))
return -EFAULT;
- if (!lock_fb_info(info))
- return -ENODEV;
console_lock();
+ if (!lock_fb_info(info)) {
+ console_unlock();
+ return -ENODEV;
+ }
info->flags |= FBINFO_MISC_USEREVENT;
ret = fb_set_var(info, &var);
info->flags &= ~FBINFO_MISC_USEREVENT;
- console_unlock();
unlock_fb_info(info);
+ console_unlock();
if (!ret && copy_to_user(argp, &var, sizeof(var)))
ret = -EFAULT;
break;
@@ -1144,12 +1146,14 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
case FBIOPAN_DISPLAY:
if (copy_from_user(&var, argp, sizeof(var)))
return -EFAULT;
- if (!lock_fb_info(info))
- return -ENODEV;
console_lock();
+ if (!lock_fb_info(info)) {
+ console_unlock();
+ return -ENODEV;
+ }
ret = fb_pan_display(info, &var);
- console_unlock();
unlock_fb_info(info);
+ console_unlock();
if (ret == 0 && copy_to_user(argp, &var, sizeof(var)))
return -EFAULT;
break;
@@ -1184,23 +1188,27 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
break;
}
event.data = &con2fb;
- if (!lock_fb_info(info))
- return -ENODEV;
console_lock();
+ if (!lock_fb_info(info)) {
+ console_unlock();
+ return -ENODEV;
+ }
event.info = info;
ret = fb_notifier_call_chain(FB_EVENT_SET_CONSOLE_MAP, &event);
- console_unlock();
unlock_fb_info(info);
+ console_unlock();
break;
case FBIOBLANK:
- if (!lock_fb_info(info))
- return -ENODEV;
console_lock();
+ if (!lock_fb_info(info)) {
+ console_unlock();
+ return -ENODEV;
+ }
info->flags |= FBINFO_MISC_USEREVENT;
ret = fb_blank(info, arg);
info->flags &= ~FBINFO_MISC_USEREVENT;
- console_unlock();
unlock_fb_info(info);
+ console_unlock();
break;
default:
if (!lock_fb_info(info))
@@ -1660,12 +1668,15 @@ static int do_register_framebuffer(struct fb_info *fb_info)
registered_fb[i] = fb_info;

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

@@ -1678,13 +1689,16 @@ static int do_unregister_framebuffer(struct fb_info *fb_info)
if (i < 0 || i >= FB_MAX || registered_fb[i] != fb_info)
return -EINVAL;

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

if (ret)
return -EINVAL;
--
1.7.7


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