linux-kernel - Re: BUG: MAX_LOCKDEP_CHAIN

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Date:   Thu, 26 Jan 2023 14:07:06 -0500
From:   Waiman Long <longman@...hat.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>, dsterba@...e.cz,
        Btrfs BTRFS <linux-btrfs@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Chris Murphy <lists@...orremedies.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!

On 1/26/23 13:59, Boqun Feng wrote:
> On Thu, Jan 26, 2023 at 01:30:34PM -0500, Waiman Long wrote:
>> On 1/26/23 12:38, Boqun Feng wrote:
>>> [Cc lock folks]
>>>
>>> On Thu, Jan 26, 2023 at 02:47:42PM +0500, Mikhail Gavrilov wrote:
>>>> On Wed, Jan 25, 2023 at 10:21 PM David Sterba <dsterba@...e.cz> wrote:
>>>>> On Wed, Jan 25, 2023 at 01:27:48AM +0500, Mikhail Gavrilov wrote:
>>>>>> On Tue, Jul 26, 2022 at 9:47 PM David Sterba <dsterba@...e.cz> wrote:
>>>>>>> On Tue, Jul 26, 2022 at 05:32:54PM +0500, Mikhail Gavrilov wrote:
>>>>>>>> Hi guys.
>>>>>>>> Always with intensive writing on a btrfs volume, the message "BUG:
>>>>>>>> MAX_LOCKDEP_CHAIN_HLOCKS too low!" appears in the kernel logs.
>>>>>>> Increase the config value of LOCKDEP_CHAINS_BITS, default is 16, 18
>>>>>>> tends to work.
>>>>>> Hi,
>>>>>> Today I was able to get the message "BUG: MAX_LOCKDEP_CHAIN_HLOCKS too
>>>>>> low!" again even with LOCKDEP_CHAINS_BITS=18 and kernel 6.2-rc5.
>>>>>>
>>>>>> ❯ cat /boot/config-`uname -r` | grep LOCKDEP_CHAINS_BITS
>>>>>> CONFIG_LOCKDEP_CHAINS_BITS=18
>>>>>>
>>>>>> [88685.088099] BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!
>>>>>> [88685.088124] turning off the locking correctness validator.
>>>>>> [88685.088133] Please attach the output of /proc/lock_stat to the bug report
>>>>>> [88685.088142] CPU: 14 PID: 1749746 Comm: mv Tainted: G        W    L
>>>>>>     -------  ---  6.2.0-0.rc5.20230123git2475bf0250de.38.fc38.x86_64 #1
>>>>>> [88685.088154] Hardware name: System manufacturer System Product
>>>>>> Name/ROG STRIX X570-I GAMING, BIOS 4408 10/28/2022
>>>>>>
>>>>>> What's next? Increase this value to 19?
>>>>> Yes, though increasing the value is a workaround so you may see the
>>>>> warning again.
>>>> Is there any sense in this WARNING if we would ignore it and every
>>>> time increase the threshold value?
>>> Lockdep uses static allocated array to track lock holdings chains to
>>> avoid dynmaic memory allocation in its own code. So if you see the
>>> warning it means your test has more combination of lock holdings than
>>> the array can record. In other words, you reach the resource limitation,
>>> and in that sense it makes sense to just ignore it and increase the
>>> value: you want to give lockdep enough resource to work, right?
>>>
>>>> May Be set 99 right away? Or remove such a check condition?
>>> That requires having 2^99 * 5 * sizeof(u16) memory for lock holding
>>> chains array..
>> Note that every increment of LOCKDEP_CHAINS_BITS double the storage space.
>> With 99, that will likely exceed the total amount of memory you have in your
>> system.
>>
>> Boqun, where does the 5 figure come from. It is just a simple u16 array of
> 	#define MAX_LOCKDEP_CHAINS_BITS	CONFIG_LOCKDEP_CHAINS_BITS
> 	#define MAX_LOCKDEP_CHAINS	(1UL << MAX_LOCKDEP_CHAINS_BITS)
>
> 	#define MAX_LOCKDEP_CHAIN_HLOCKS (MAX_LOCKDEP_CHAINS*5)
>
> I think the last one means we think the average length of a lock chain
> is 5, in other words, in average, a task hold at most 5 locks. I don't
> know where the 5 came from either, but it's there ;-)

You are right. I missed that when I looked. So 5 is assumed to the 
average length of a lock chain.

Thanks,
Longman