linux-kernel - Re: [PATCH] LOCKDEP: use depends on LOCKDEP

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Date:   Mon, 24 May 2021 21:18:26 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Waiman Long <llong@...hat.com>, Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org,
        Johannes Berg <johannes@...solutions.net>,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        linux-um@...ts.infradead.org
Subject: Re: [PATCH] LOCKDEP: use depends on LOCKDEP_SUPPORT instead of $ARCH
 list

On 5/24/21 5:05 PM, Waiman Long wrote:
> On 5/24/21 5:41 PM, Randy Dunlap wrote:
>> On 5/24/21 2:31 PM, Randy Dunlap wrote:
>>> On 5/24/21 2:04 PM, Waiman Long wrote:
>>>
>>>> Is it possible to just get rid of the 2nd depends-on statement?
>>>>
>>>> The 2nd depends-on line was introduced by commit 7d37cb2c912d ("lib: fix kconfig dependency on ARCH_WANT_FRAME_POINTER"):
>>> and I should have looked at that history too. Thanks.
>>>
>>> Yes, I agree, we can just delete that line...
>>>
>>> I'll send a v2 and copy the author of commit 7d37cb2c912d as well.
>> Hm, as I review that commit, I have to wonder if the previous 'select'
>> was correct (if we disregard the Kconfig warning).  If so, then
>> FRAME_POINTER is still wanted/needed for some arch-es.
>>
>> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
>> index 2779c29d9981..417c3d3e521b 100644
>> --- a/lib/Kconfig.debug
>> +++ b/lib/Kconfig.debug
>> @@ -1363,7 +1363,7 @@ config LOCKDEP
>>          bool
>>          depends on DEBUG_KERNEL && LOCK_DEBUGGING_SUPPORT
>>          select STACKTRACE
>> -       select FRAME_POINTER if !MIPS && !PPC && !ARM && !S390 && !MICROBLAZE && !ARC && !X86 <<<<<<<<<<<
>>
> AFAICS, enabling FRAME_POINTER is a debugging aid as it enable more precise stacktrace. However, not all archs want to enable FRAME_POINTER because of LOCKDEP. Now you are just letting users decide if they want FRAME_POINTER or not. Maybe you can modify the help text to mention that.

LOCKDEP doesn't have any user help text.

For FRAME_POINTER, when ARCH_WANT_FRAME_POINTERS + a few other conditions
is met, it seems that the FRAME_POINTER help text is good enough IMO.

	help
	  If you say Y here the resulting kernel image will be slightly
	  larger and slower, but it gives very useful debugging information
	  in case of kernel bugs. (precise oopses/stacktraces/warnings)

-- 
~Randy