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

[Christophe Leroy]

Le 26/08/2021 à 05:21, Michael Ellerman a écrit :
> Nathan Chancellor <nathan@xxxxxxxxxx> 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