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

Michael Ellerman
2021-08-26T14:45:13Z

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.

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

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

cheers

diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
index 1ee0f22313ee..d978d9004d0d 100644
--- a/arch/powerpc/include/asm/bug.h
+++ b/arch/powerpc/include/asm/bug.h
@@ -106,6 +106,12 @@ __label_warn_on: \
} \
} while (0)

+#ifdef CONFIG_CC_IS_CLANG
+#define __clang_warn_hack(x) (!!(x))
+#else
+#define __clang_warn_hack(x) (x)
+#endif
+
#define WARN_ON(x) ({ \
bool __ret_warn_on = false; \
do { \
@@ -119,7 +125,8 @@ __label_warn_on: \
\
WARN_ENTRY(PPC_TLNEI " %4, 0", \
BUGFLAG_WARNING | BUGFLAG_TAINT(TAINT_WARN), \
- __label_warn_on, "r" (x)); \
+ __label_warn_on, \
+ "r" __clang_warn_hack(x)); \
break; \
__label_warn_on: \
__ret_warn_on = true; \