Re: [WARN] LOCKDEP: MAX_LOCKDEP_CHAIN_HLOCKS too low

[Lance Yang <lance.yang@linux.dev>]

Hi Longman,

Thanks for looking into this!

On 2025/5/27 12:48, Waiman Long wrote:
> On 5/26/25 10:02 PM, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> Hi all,
>>
>> With CONFIG_LOCKDEP on, I got this warning during kernel builds:
>>
>> [Tue May 27 00:22:59 2025] BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!

[...]

>>
>> $ cat .config|grep CONFIG_LOCKDEP
>> CONFIG_LOCKDEP_SUPPORT=y
>> CONFIG_LOCKDEP=y
>> CONFIG_LOCKDEP_BITS=15
>> CONFIG_LOCKDEP_CHAINS_BITS=16
>> CONFIG_LOCKDEP_STACK_TRACE_BITS=19
>> CONFIG_LOCKDEP_STACK_TRACE_HASH_BITS=14
>> CONFIG_LOCKDEP_CIRCULAR_QUEUE_BITS=12
>>
>> Is it safe? Or could this be a real locking issue?
> 
> The lock chains store the locking order of nested locks. The default 
> value of 16 may be too low now as the kernel is becoming more complex in 
> term of possible nested locking orders. Anyway, I would suggest upping 
> the CONFIG_LOCKDEP_CHAIN_BITS to 17 or even 18 to prevent this kind of 
> problem. In fact, the latest RHEL debug kernel sets 
> CONFIG_LOCKDEP_CHAINS_BITS to 18.

Yes, makes sense to me. Bumping it to 18 sounds reasonable as the kernel
is getting more complex in terms of possible nested locking orders. It
uses a bit more memory, but keeping LOCKDEP working is worth it ;)

And if there are no objections, I’d be happy to send a patch making the
change.

Thanks,
Lance

> 
> Cheers,
> Longman
>