Re: [PATCH] LOCKDEP: use depends on LOCKDEP_SUPPORT instead of $ARCH list

From: Randy Dunlap <rdunlap@infradead.org>
To: Waiman Long <llong@redhat.com>, Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Chris Zankel <chris@zankel.net>,
	Max Filippov <jcmvbkbc@gmail.com>,
	linux-xtensa@linux-xtensa.org,
	Johannes Berg <johannes@sipsolutions.net>,
	Jeff Dike <jdike@addtoit.com>,
	Richard Weinberger <richard@nod.at>,
	Anton Ivanov <anton.ivanov@cambridgegreys.com>,
	linux-um@lists.infradead.org
Subject: Re: [PATCH] LOCKDEP: use depends on LOCKDEP_SUPPORT instead of $ARCH list
Date: Mon, 24 May 2021 21:18:26 -0700	[thread overview]
Message-ID: <0213cd4e-e832-f768-106a-345e77ce2665@infradead.org> (raw)
In-Reply-To: <3a0fb04c-c640-25db-7e7a-fbdd3b4defb5@redhat.com>

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