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

From: Michael Ellerman <[email-masked]>
Date: Fri, 27 Aug 2021 00:45:06 +1000

Christophe Leroy <[email-masked]> writes:
> Le 26/08/2021 à 05:21, Michael Ellerman a écrit :
>> Nathan Chancellor <[email-masked]> 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;			\