watchdog: core: Fix circular locking dependency

lockdep reports the following circular locking dependency.

======================================================
INFO: possible circular locking dependency detected ]
4.6.0-rc3-00191-gfabf418 #162 Not tainted
-------------------------------------------------------
systemd/1 is trying to acquire lock:
((&(&wd_data->work)->work)){+.+...}, at: [<80141650>] flush_work+0x0/0x280

but task is already holding lock:

(&wd_data->lock){+.+...}, at: [<804acfa8>] watchdog_release+0x18/0x190

which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:

-> #1 (&wd_data->lock){+.+...}:
	[<80662310>] mutex_lock_nested+0x64/0x4a8
	[<804aca4c>] watchdog_ping_work+0x18/0x4c
	[<80143128>] process_one_work+0x1ac/0x500
	[<801434b4>] worker_thread+0x38/0x554
	[<80149510>] kthread+0xf4/0x108
	[<80107c10>] ret_from_fork+0x14/0x24

-> #0 ((&(&wd_data->work)->work)){+.+...}:
	[<8017c4e8>] lock_acquire+0x70/0x90
	[<8014169c>] flush_work+0x4c/0x280
	[<801440f8>] __cancel_work_timer+0x9c/0x1e0
	[<804acfcc>] watchdog_release+0x3c/0x190
	[<8022c5e8>] __fput+0x80/0x1c8
	[<80147b28>] task_work_run+0x94/0xc8
	[<8010b998>] do_work_pending+0x8c/0xb4
	[<80107ba8>] slow_work_pending+0xc/0x20

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0                    CPU1
----                    ----
lock(&wd_data->lock);
                        lock((&(&wd_data->work)->work));
                        lock(&wd_data->lock);
lock((&(&wd_data->work)->work));

*** DEADLOCK ***

1 lock held by systemd/1:

stack backtrace:
CPU: 2 PID: 1 Comm: systemd Not tainted 4.6.0-rc3-00191-gfabf418 #162
Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
[<8010f5e4>] (unwind_backtrace) from [<8010c038>] (show_stack+0x10/0x14)
[<8010c038>] (show_stack) from [<8039d7fc>] (dump_stack+0xa8/0xd4)
[<8039d7fc>] (dump_stack) from [<80177ee0>] (print_circular_bug+0x214/0x334)
[<80177ee0>] (print_circular_bug) from [<80179230>] (check_prevs_add+0x4dc/0x8e8)
[<80179230>] (check_prevs_add) from [<8017b3d8>] (__lock_acquire+0xc6c/0x14ec)
[<8017b3d8>] (__lock_acquire) from [<8017c4e8>] (lock_acquire+0x70/0x90)
[<8017c4e8>] (lock_acquire) from [<8014169c>] (flush_work+0x4c/0x280)
[<8014169c>] (flush_work) from [<801440f8>] (__cancel_work_timer+0x9c/0x1e0)
[<801440f8>] (__cancel_work_timer) from [<804acfcc>] (watchdog_release+0x3c/0x190)
[<804acfcc>] (watchdog_release) from [<8022c5e8>] (__fput+0x80/0x1c8)
[<8022c5e8>] (__fput) from [<80147b28>] (task_work_run+0x94/0xc8)
[<80147b28>] (task_work_run) from [<8010b998>] (do_work_pending+0x8c/0xb4)
[<8010b998>] (do_work_pending) from [<80107ba8>] (slow_work_pending+0xc/0x20)

Turns out the call to cancel_delayed_work_sync() in watchdog_release()
is not necessary and can be dropped. If the worker is no longer necessary,
the subsequent call to watchdog_update_worker() will cancel it. If it is
already running, it won't do anything, since the worker function checks
if it needs to ping the watchdog or not.

Reported-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Tested-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Fixes: 11d7aba9ce ("watchdog: imx2: Convert to use infrastructure triggered keepalives")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: stable <stable@vger.kernel.org>

drivers/watchdog/watchdog_dev.c

@@ -736,7 +736,6 @@ static int watchdog_release(struct inode *inode, struct file *file)
 		watchdog_ping(wdd);
 	}
 
-	cancel_delayed_work_sync(&wd_data->work);
 	watchdog_update_worker(wdd);
 
 	/* make sure that /dev/watchdog can be re-opened */