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

Christophe Leroy
2021-08-26T06:37:14Z

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

Christophe