fb: reorder the lock sequence to fix potential dead lock

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 dead lock 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. And following Tomi's suggestion, fix these similar
issues all in fb subsystem.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

1 files changed, 13 insertions, 6 deletions

diff --git a/drivers/video/fbsysfs.c b/drivers/video/fbsysfs.c
index ef476b02fbe5..53444ac19fe0 100644
--- a/drivers/video/fbsysfs.c
+++ b/drivers/video/fbsysfs.c
@@ -177,9 +177,12 @@ static ssize_t store_modes(struct device *device,
 	if (i * sizeof(struct fb_videomode) != count)
 		return -EINVAL;
 
-	if (!lock_fb_info(fb_info))
-		return -ENODEV;
 	console_lock();
+	if (!lock_fb_info(fb_info)) {
+		console_unlock();
+		return -ENODEV;
+	}
+
 	list_splice(&fb_info->modelist, &old_list);
 	fb_videomode_to_modelist((const struct fb_videomode *)buf, i,
 				 &fb_info->modelist);
@@ -189,8 +192,8 @@ static ssize_t store_modes(struct device *device,
 	} else
 		fb_destroy_modelist(&old_list);
 
-	console_unlock();
 	unlock_fb_info(fb_info);
+	console_unlock();
 
 	return 0;
 }
@@ -404,12 +407,16 @@ static ssize_t store_fbstate(struct device *device,
 
 	state = simple_strtoul(buf, &last, 0);
 
-	if (!lock_fb_info(fb_info))
-		return -ENODEV;
 	console_lock();
+	if (!lock_fb_info(fb_info)) {
+		console_unlock();
+		return -ENODEV;
+	}
+
 	fb_set_suspend(fb_info, (int)state);
-	console_unlock();
+
 	unlock_fb_info(fb_info);
+	console_unlock();
 
 	return count;
 }