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

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Message-ID: <YsLXkCMtQpxmDhVM@hirez.programming.kicks-ass.net>
Date:   Mon, 4 Jul 2022 14:05:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Sathvika Vasireddy <sv@...ux.ibm.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "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()

On Sat, Jun 25, 2022 at 06:46:54AM +0000, Christophe Leroy wrote:
> 
> 
> 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

Per this construct you should do as x86 does and assume twui terminates
control flow and explicitly annotate the WARN case. That is, given the
fact that BUG as no instructions following it, you can't very well
annotate that.

Alternatively, you can teach objtool to look at __bug_table to
distinguish these cases.