Re: [PATCH v2 2/2] powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS() with asm goto

Christophe Leroy
2021-08-26T14:53:07Z

Le 26/08/2021 à 16:45, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@csgroup.eu> writes:
>> Le 26/08/2021 à 05:21, Michael Ellerman a écrit :
>>> Nathan Chancellor <nathan@kernel.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