linux-kernel - Re: [PATCH v2 2/2] powerpc/bug: Provide better flexibility to WARN_ON/__WARN

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Message-ID: <5b0984eb-b933-7ea5-462d-283d0fe09424@csgroup.eu>
Date:   Thu, 26 Aug 2021 16:53:03 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Nathan Chancellor <nathan@...nel.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        clang-built-linux@...glegroups.com, llvm@...ts.linux.dev
Subject: Re: [PATCH v2 2/2] powerpc/bug: Provide better flexibility to
 WARN_ON/__WARN_FLAGS() with asm goto



Le 26/08/2021 à 16:45, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@...roup.eu> writes:
>> Le 26/08/2021 à 05:21, Michael Ellerman a écrit :
>>> Nathan Chancellor <nathan@...nel.org> writes:
>>>> On Tue, Apr 13, 2021 at 04:38:10PM +0000, Christophe Leroy wrote:
>>>>> Using asm goto in __WARN_FLAGS() and WARN_ON() allows more
>>>>> flexibility to GCC.
>>> ...
>>>>
>>>> This patch as commit 1e688dd2a3d6 ("powerpc/bug: Provide better
>>>> flexibility to WARN_ON/__WARN_FLAGS() with asm goto") cause a WARN_ON in
>>>> klist_add_tail to trigger over and over on boot when compiling with
>>>> clang:
>>
>> ...
>>
>>>
>>> This patch seems to fix it. Not sure if that's just papering over it though.
>>>
>>> diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
>>> index 1ee0f22313ee..75fcb4370d96 100644
>>> --- a/arch/powerpc/include/asm/bug.h
>>> +++ b/arch/powerpc/include/asm/bug.h
>>> @@ -119,7 +119,7 @@ __label_warn_on:						\
>>>    								\
>>>    			WARN_ENTRY(PPC_TLNEI " %4, 0",		\
>>>    				   BUGFLAG_WARNING | BUGFLAG_TAINT(TAINT_WARN),	\
>>> -				   __label_warn_on, "r" (x));	\
>>> +				   __label_warn_on, "r" (!!(x))); \
>>>    			break;					\
>>>    __label_warn_on:						\
>>>    			__ret_warn_on = true;			\
>>
>> But for a simple WARN_ON() call:
>>
>> void test(unsigned long b)
>> {
>> 	WARN_ON(b);
>> }
>>
>> Without your change with GCC you get:
>>
>> 00000000000012d0 <.test>:
>>       12d0:	0b 03 00 00 	tdnei   r3,0
>>       12d4:	4e 80 00 20 	blr
>>
>>
>> With the !! change you get:
>>
>> 00000000000012d0 <.test>:
>>       12d0:	31 23 ff ff 	addic   r9,r3,-1
>>       12d4:	7d 29 19 10 	subfe   r9,r9,r3
>>       12d8:	0b 09 00 00 	tdnei   r9,0
>>       12dc:	4e 80 00 20 	blr
> 
> Yeah that's a pity.
> 
> We could do something like below, which is ugly, but would be better
> than having to revert the whole thing.

Yes looks nice, we already had that kind of stuff in the past, even more ugly.

> 
> Although this doesn't fix the strange warning in drivers/net/ethernet/sfc.

What's the warning ?


> 
> So possibly we need a CLANG ifdef around the whole thing, and use the
> old style warn for clang.
> 

Christophe