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Date: Mon, 4 Jul 2022 12:44:30 +0000 From: Christophe Leroy <christophe.leroy@...roup.eu> To: Peter Zijlstra <peterz@...radead.org> 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() Le 04/07/2022 à 14:05, Peter Zijlstra a écrit : > 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. That exactly the problem I guess. I'm fine with replacing the unreachable() by __builtin_unreachable() with our __WARN_FLAGS() and BUG() but we will still have a problem with some of the unrachable() that are in core parts of the kernel. Even the ones in arch/powerpc/, they are valid and should remain. The point seems that the generic annotate_unreachable() is wrong for powerpc as is, and activating CONFIG_OBJTOOL lead to bad code generation. By the way, for which functionnalities of objtool is that analysis necessary ? I understand it is not necessary to mcount accounting, so maybe the not empty annotate_unreachable() should be limited to those those functionnalities ? > > Alternatively, you can teach objtool to look at __bug_table to > distinguish these cases. Isn't it enough to tell objtool that execution never go past twui, using INSN_BUG ? By the way, for __WARN_FLAGS, we use the __extable for the continuation. Is objtools able to follow __extable ? Christophe
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