[PATCH 4.6 078/100] watchdog: core: Fix circular locking dependency

Greg Kroah-Hartman
2016-05-30T21:20:29Z

From: Guenter Roeck <linux@roeck-us.net>

commit e1f30282a1d3d0c75d5a08e47c6ac1563065be52 upstream.

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));

---
drivers/watchdog/watchdog_dev.c | 1 -
1 file changed, 1 deletion(-)

--- a/drivers/watchdog/watchdog_dev.c
+++ b/drivers/watchdog/watchdog_dev.c
@@ -736,7 +736,6 @@ static int watchdog_release(struct inode
watchdog_ping(wdd);
}

- cancel_delayed_work_sync(&wd_data->work);
watchdog_update_worker(wdd);

/* make sure that /dev/watchdog can be re-opened */