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

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Date:   Tue, 15 Aug 2023 14:44:25 +0200
From:   Helge Deller <deller@....de>
To:     Petr Mladek <pmladek@...e.com>
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

On 8/15/23 13:54, Petr Mladek wrote:
> 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.

Right. It's 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.

Right. I should have replied earlier to this patch.
Andrew already dropped this patch here from his patch collection.

It turns out the problem is in function static_obj() from
lockdep, which doesn't recognize the __initdata section
for all platforms.

Please look at this patch on the LKML instead, which
fixes the real problem:
[PATCH v2] lockdep: Fix static memory detection even more

Thanks!
Helge