linux-kernel - Re: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN

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Date:   Sat, 25 Jun 2022 06:46:54 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Sathvika Vasireddy <sv@...ux.ibm.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
CC:     "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "aik@...abs.ru" <aik@...abs.ru>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "naveen.n.rao@...ux.vnet.ibm.com" <naveen.n.rao@...ux.vnet.ibm.com>,
        "mbenes@...e.cz" <mbenes@...e.cz>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "paulus@...ba.org" <paulus@...ba.org>
Subject: Re: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON()



Le 24/06/2022 à 20:32, Sathvika Vasireddy a écrit :
> objtool is throwing *unannotated intra-function call*
> warnings with a few instructions that are marked
> unreachable. Remove unreachable() from WARN_ON()
> to fix these warnings, as the codegen remains same
> with and without unreachable() in WARN_ON().

Did you try the two exemples described in commit 1e688dd2a3d6 
("powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS() with 
asm goto") ?

Without your patch:

00000640 <test>:
  640:	81 23 00 84 	lwz     r9,132(r3)
  644:	71 29 40 00 	andi.   r9,r9,16384
  648:	40 82 00 0c 	bne     654 <test+0x14>
  64c:	80 63 00 0c 	lwz     r3,12(r3)
  650:	4e 80 00 20 	blr
  654:	0f e0 00 00 	twui    r0,0

00000658 <test9w>:
  658:	2c 04 00 00 	cmpwi   r4,0
  65c:	41 82 00 0c 	beq     668 <test9w+0x10>
  660:	7c 63 23 96 	divwu   r3,r3,r4
  664:	4e 80 00 20 	blr
  668:	0f e0 00 00 	twui    r0,0
  66c:	38 60 00 00 	li      r3,0
  670:	4e 80 00 20 	blr


With your patch:

00000640 <test>:
  640:	81 23 00 84 	lwz     r9,132(r3)
  644:	71 29 40 00 	andi.   r9,r9,16384
  648:	40 82 00 0c 	bne     654 <test+0x14>
  64c:	80 63 00 0c 	lwz     r3,12(r3)
  650:	4e 80 00 20 	blr
  654:	0f e0 00 00 	twui    r0,0
  658:	4b ff ff f4 	b       64c <test+0xc>		<==

0000065c <test9w>:
  65c:	2c 04 00 00 	cmpwi   r4,0
  660:	41 82 00 0c 	beq     66c <test9w+0x10>
  664:	7c 63 23 96 	divwu   r3,r3,r4
  668:	4e 80 00 20 	blr
  66c:	0f e0 00 00 	twui    r0,0
  670:	38 60 00 00 	li      r3,0			<==
  674:	4e 80 00 20 	blr				<==
  678:	38 60 00 00 	li      r3,0
  67c:	4e 80 00 20 	blr


Christophe

> 
> Signed-off-by: Sathvika Vasireddy <sv@...ux.ibm.com>
> ---
>   arch/powerpc/include/asm/bug.h | 1 -
>   1 file changed, 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
> index ecbae1832de3..df6c11e008b9 100644
> --- a/arch/powerpc/include/asm/bug.h
> +++ b/arch/powerpc/include/asm/bug.h
> @@ -97,7 +97,6 @@
>   	__label__ __label_warn_on;				\
>   								\
>   	WARN_ENTRY("twi 31, 0, 0", BUGFLAG_WARNING | (flags), __label_warn_on); \
> -	unreachable();						\
>   								\
>   __label_warn_on:						\
>   	break;							\