watchdog: deadlock warning with imx2_wdt driver and systemd

[Clemens Gruber <clemens.gruber@pqgruber.com>]

Hi,

I have an i.MX6Q board with the current mainline tree from Linus and
systemd 229, which also acts as watchdog daemon.
(RuntimeWatchdogSec=60)

Since commit 11d7aba9ceb7 ("watchdog: imx2: Convert to use
infrastructure triggered keepalives") I get a kernel lockdep warning
after systemd launches.

However, according to the backtrace, this happens in watchdog_dev.c
(watchdog_release and watchdog_ping_work) which was not even touched
by said commit.

It also only occurs during the first boot. (I added some initialization
tasks there, e.g. generating SSH keys, expanding filesystems, etc.,
which takes about one minute and must be connected to this somehow,
because on all subsequent boots without that delay, it does not appear
anymore)

Log output:

[    5.426747] ======================================================
[    5.432941] [ INFO: possible circular locking dependency detected ]
[    5.439221] 4.6.0-rc3-00191-gfabf418 #162 Not tainted
[    5.444283] -------------------------------------------------------
[    5.450552] systemd/1 is trying to acquire lock:
[    5.455172]  ((&(&wd_data->work)->work)){+.+...}, at: [<80141650>] flush_work+0x0/0x280
[    5.463271] 
               but task is already holding lock:
[    5.469114]  (&wd_data->lock){+.+...}, at: [<804acfa8>] watchdog_release+0x18/0x190
[    5.476860] 
               which lock already depends on the new lock.

[    5.485050] 
               the existing dependency chain (in reverse order) is:
[    5.492543] 
               -> #1 (&wd_data->lock){+.+...}:
[    5.496967]        [<80662310>] mutex_lock_nested+0x64/0x4a8
[    5.502666]        [<804aca4c>] watchdog_ping_work+0x18/0x4c
[    5.508359]        [<80143128>] process_one_work+0x1ac/0x500
[    5.514044]        [<801434b4>] worker_thread+0x38/0x554
[    5.519389]        [<80149510>] kthread+0xf4/0x108
[    5.524217]        [<80107c10>] ret_from_fork+0x14/0x24
[    5.529482] 
               -> #0 ((&(&wd_data->work)->work)){+.+...}:
[    5.534883]        [<8017c4e8>] lock_acquire+0x70/0x90
[    5.540061]        [<8014169c>] flush_work+0x4c/0x280
[    5.545144]        [<801440f8>] __cancel_work_timer+0x9c/0x1e0
[    5.551008]        [<804acfcc>] watchdog_release+0x3c/0x190
[    5.556612]        [<8022c5e8>] __fput+0x80/0x1c8
[    5.561354]        [<80147b28>] task_work_run+0x94/0xc8
[    5.566613]        [<8010b998>] do_work_pending+0x8c/0xb4
[    5.572049]        [<80107ba8>] slow_work_pending+0xc/0x20
[    5.577568] 
               other info that might help us debug this:

[    5.585585]  Possible unsafe locking scenario:

[    5.591516]        CPU0                    CPU1
[    5.596055]        ----                    ----
[    5.600593]   lock(&wd_data->lock);
[    5.604130]                                lock((&(&wd_data->work)->work));
[    5.611137]                                lock(&wd_data->lock);
[    5.617191]   lock((&(&wd_data->work)->work));
[    5.621681] 
                *** DEADLOCK ***

[    5.627615] 1 lock held by systemd/1:
[    5.631286]  #0:  (&wd_data->lock){+.+...}, at: [<804acfa8>] watchdog_release+0x18/0x190
[    5.639488] 
               stack backtrace:
[    5.643861] CPU: 2 PID: 1 Comm: systemd Not tainted 4.6.0-rc3-00191-gfabf418 #162
[    5.651353] Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
[    5.657909] [<8010f5e4>] (unwind_backtrace) from [<8010c038>] (show_stack+0x10/0x14)
[    5.665671] [<8010c038>] (show_stack) from [<8039d7fc>] (dump_stack+0xa8/0xd4)
[    5.672911] [<8039d7fc>] (dump_stack) from [<80177ee0>] (print_circular_bug+0x214/0x334)
[    5.681012] [<80177ee0>] (print_circular_bug) from [<80179230>] (check_prevs_add+0x4dc/0x8e8)
[    5.689556] [<80179230>] (check_prevs_add) from [<8017b3d8>] (__lock_acquire+0xc6c/0x14ec)
[    5.697831] [<8017b3d8>] (__lock_acquire) from [<8017c4e8>] (lock_acquire+0x70/0x90)
[    5.705584] [<8017c4e8>] (lock_acquire) from [<8014169c>] (flush_work+0x4c/0x280)
[    5.713076] [<8014169c>] (flush_work) from [<801440f8>] (__cancel_work_timer+0x9c/0x1e0)
[    5.721183] [<801440f8>] (__cancel_work_timer) from [<804acfcc>] (watchdog_release+0x3c/0x190)
[    5.729815] [<804acfcc>] (watchdog_release) from [<8022c5e8>] (__fput+0x80/0x1c8)
[    5.737316] [<8022c5e8>] (__fput) from [<80147b28>] (task_work_run+0x94/0xc8)
[    5.744465] [<80147b28>] (task_work_run) from [<8010b998>] (do_work_pending+0x8c/0xb4)
[    5.752401] [<8010b998>] (do_work_pending) from [<80107ba8>] (slow_work_pending+0xc/0x20)

If you can't reproduce it but have an idea how to fix this, I would be
happy to test your idea or patch.

Thanks,
Clemens