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

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Date:   Thu, 26 Aug 2021 08:37:09 +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 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

Christophe