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

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Message-ID: <8a4ee5be-ad5c-ca06-dd1a-aa13ccc94906@redhat.com>
Date:   Mon, 24 May 2021 17:04:55 -0400
From:   Waiman Long <llong@...hat.com>
To:     Randy Dunlap <rdunlap@...radead.org>,
        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 3:47 PM, Randy Dunlap wrote:
> On 5/17/21 7:02 AM, Waiman Long wrote:
>> On 5/17/21 3:11 AM, Ingo Molnar wrote:
>>> * Randy Dunlap <rdunlap@...radead.org> wrote:
>>>
>>>> Both arch/um/ and arch/xtensa/ cause a Kconfig warning for LOCKDEP.
>>>> These arch-es select LOCKDEP_SUPPORT but they are not listed as one
>>>> of the arch-es that LOCKDEP depends on.
>>>>
>>>> Since (16) arch-es define the Kconfig symbol LOCKDEP_SUPPORT if they
>>>> intend to have LOCKDEP support, replace the awkward list of
>>>> arch-es that LOCKDEP depends on with the LOCKDEP_SUPPORT symbol.
>>>>
>>>> Fixes this kconfig warning: (for both um and xtensa)
>>>>
>>>> WARNING: unmet direct dependencies detected for LOCKDEP
>>>>     Depends on [n]: DEBUG_KERNEL [=y] && LOCK_DEBUGGING_SUPPORT [=y] && (FRAME_POINTER [=n] || MIPS || PPC || S390 || MICROBLAZE || ARM || ARC || X86)
>>>>     Selected by [y]:
>>>>     - PROVE_LOCKING [=y] && DEBUG_KERNEL [=y] && LOCK_DEBUGGING_SUPPORT [=y]
>>>>     - LOCK_STAT [=y] && DEBUG_KERNEL [=y] && LOCK_DEBUGGING_SUPPORT [=y]
>>>>     - DEBUG_LOCK_ALLOC [=y] && DEBUG_KERNEL [=y] && LOCK_DEBUGGING_SUPPORT [=y]
>>>>
>>>> Signed-off-by: Randy Dunlap <rdunlap@...radead.org>
>>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>>> Cc: Ingo Molnar <mingo@...hat.com>
>>>> Cc: Will Deacon <will@...nel.org>
>>>> Cc: Waiman Long <longman@...hat.com>
>>>> Cc: Boqun Feng <boqun.feng@...il.com>
>>>> Cc: Chris Zankel <chris@...kel.net>
>>>> Cc: Max Filippov <jcmvbkbc@...il.com>
>>>> Cc: linux-xtensa@...ux-xtensa.org
>>>> Cc: Johannes Berg <johannes@...solutions.net>
>>>> Cc: Jeff Dike <jdike@...toit.com>
>>>> Cc: Richard Weinberger <richard@....at>
>>>> Cc: Anton Ivanov <anton.ivanov@...bridgegreys.com>
>>>> Cc: linux-um@...ts.infradead.org
>>>> ---
>>>>    lib/Kconfig.debug |    2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> --- linux-next-20210514.orig/lib/Kconfig.debug
>>>> +++ linux-next-20210514/lib/Kconfig.debug
>>>> @@ -1383,7 +1383,7 @@ config LOCKDEP
>>>>        bool
>>>>        depends on DEBUG_KERNEL && LOCK_DEBUGGING_SUPPORT
>>>>        select STACKTRACE
>>>> -    depends on FRAME_POINTER || MIPS || PPC || S390 || MICROBLAZE || ARM || ARC || X86
>>>> +    depends on FRAME_POINTER || LOCKDEP_SUPPORT
>>> Ok - the FRAME_POINTER bit is weird. Are there any architectures that have
>>> FRAME_POINTER defined but no LOCKDEP_SUPPORT?
>> LOCK_DEBUGGING_SUPPORT depends on LOCKDEP_SUPPORT. So this patch is equivalent to just delete the second depends-on line.
> Yes, if we disregard the FRAME_POINTER part.

My understanding is that the 2 depends-on statements have an implicit 
AND. So it is like

DEBUG_KERNEL && LOCK_DEBUGGING_SUPPORT && (FRAME_POINTER || 
LOCKDEP_SUPPORT). LOCK_DEBUGGING_SUPPORT is true means the 
(FRAME_POINTER || LOCKDEP_SUPPORT) will always be true. FRAME_POINTER is 
true doesn't mean the other dependencies are true. That is why I said it 
is equivalent to just "DEBUG_KERNEL && LOCK_DEBUGGING_SUPPORT". IOW, 
FRAME_POINTER will play no part here.

>
>> Beside LOCKDEP, LATENCYTOP also have exactly the same depends-on line.
> True, but I don't get any implication that the same patch applies there.
> Do you?
It is just an observation that I stumble on. It is not related to your 
patch.
>> So isn't FRAME_POINTER used mainly to support STACK_TRACE? However, LOCK_DEBUGGING_SUPPORT has already included STACK_TRACE_SUPPORT in its dependency. So why there is a FRAME_POINTER dependency?
> FRAME_POINTER is one way but it does not seem to be required
> for STACKTRACE_SUPPORT.
>
> Do you have any patch suggestions?

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"):

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 &&
+       depends on FRAME_POINTER || MIPS || PPC || S390 || MICROBLAZE || 
ARM ||
         select KALLSYMS
         select KALLSYMS_ALL

Since STACKTRACE is selected by lockdep, maybe we can just remove the 
2nd depends-on line to see if anyone complain.

Cheers,
Longman