linux-kernel - Re: [PATCH] watchdog: Fix lockdep warning

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Date:   Tue, 15 Aug 2023 13:54:07 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Helge Deller <deller@....de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>, Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] watchdog: Fix lockdep warning

Adding workqueue and lockdep people into Cc.

On Fri 2023-08-11 19:11:46, Helge Deller wrote:
> Fully initialize detector_work work struct to avoid this kernel warning
> when lockdep is enabled:
> 
>  =====================================
>  WARNING: bad unlock balance detected!
>  6.5.0-rc5+ #687 Not tainted
>  -------------------------------------
>  swapper/0/1 is trying to release lock (detector_work) at:
>  [<000000004037e554>] __flush_work+0x60/0x658
>  but there are no more locks to release!
> 
>  other info that might help us debug this:
>  no locks held by swapper/0/1.
> 
>  stack backtrace:
>  CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.5.0-rc5+ #687
>  Hardware name: 9000/785/C3700
>  Backtrace:
>   [<0000000041455d5c>] print_unlock_imbalance_bug.part.0+0x20c/0x230
>   [<000000004040d5e8>] lock_release+0x2e8/0x3f8
>   [<000000004037e5cc>] __flush_work+0xd8/0x658
>   [<000000004037eb7c>] flush_work+0x30/0x60
>   [<000000004011f140>] lockup_detector_check+0x54/0x128
>   [<0000000040306430>] do_one_initcall+0x9c/0x408
>   [<0000000040102d44>] kernel_init_freeable+0x688/0x7f0
>   [<000000004146df68>] kernel_init+0x64/0x3a8
>   [<0000000040302020>] ret_from_kernel_thread+0x20/0x28
> 
> Signed-off-by: Helge Deller <deller@....de>
> 
> ---
> 
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index be38276a365f..eab0dfcfa3f9 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -1022,5 +1022,6 @@ void __init lockup_detector_init(void)
>  	else
>  		allow_lockup_detector_init_retry = true;
> 
> +	INIT_WORK(&detector_work, lockup_detector_delay_init);
>  	lockup_detector_setup();
>  }

Strange. The work is initialized when declared:

static struct work_struct detector_work __initdata =
		__WORK_INITIALIZER(detector_work, lockup_detector_delay_init);

, which should initialize the lockdep map as well. I would expect
this is enough.

And I do not see this on x86_64 with CONFIG_PROVE_LOCKING=y
on today's Linus' tree. And the flush_work() is always called
from the lockup_detector_check() late init call.

IMHO, it must be a bug somewhere else. Otherwise, many other
users of DECLARE_WORK() or __WORK_INITIALIZER() would have
the same problem.

Or do I miss something?

Best Regards,
Petr