From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 52993C31E40 for ; Mon, 12 Aug 2019 23:23:32 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id EAC3F2075B for ; Mon, 12 Aug 2019 23:23:31 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EAC3F2075B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=perches.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 5B2016B0003; Mon, 12 Aug 2019 19:23:31 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 561616B0005; Mon, 12 Aug 2019 19:23:31 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 4501B6B0006; Mon, 12 Aug 2019 19:23:31 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0169.hostedemail.com [216.40.44.169]) by kanga.kvack.org (Postfix) with ESMTP id 1EAD46B0003 for ; Mon, 12 Aug 2019 19:23:31 -0400 (EDT) Received: from smtpin20.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with SMTP id 83ED1181AC9AE for ; Mon, 12 Aug 2019 23:23:30 +0000 (UTC) X-FDA: 75815354580.20.scene68_1e95e8ff18e3b X-HE-Tag: scene68_1e95e8ff18e3b X-Filterd-Recvd-Size: 6703 Received: from smtprelay.hostedemail.com (smtprelay0182.hostedemail.com [216.40.44.182]) by imf31.hostedemail.com (Postfix) with ESMTP for ; Mon, 12 Aug 2019 23:23:30 +0000 (UTC) Received: from filter.hostedemail.com (clb03-v110.bra.tucows.net [216.40.38.60]) by smtprelay08.hostedemail.com (Postfix) with ESMTP id C1578182CED2A; Mon, 12 Aug 2019 23:23:29 +0000 (UTC) X-Session-Marker: 6A6F6540706572636865732E636F6D X-HE-Tag: leaf22_1e480bf41a219 X-Filterd-Recvd-Size: 5199 Received: from XPS-9350.home (cpe-23-242-196-136.socal.res.rr.com [23.242.196.136]) (Authenticated sender: joe@perches.com) by omf07.hostedemail.com (Postfix) with ESMTPA; Mon, 12 Aug 2019 23:23:27 +0000 (UTC) Message-ID: <058c848ef329fa68ef40ca58fa6bbd65b97de0e1.camel@perches.com> Subject: Re: [PATCH v2] kbuild: Change fallthrough comments to attributes From: Joe Perches To: Nick Desaulniers Cc: Nathan Huckleberry , Masahiro Yamada , Michal Marek , Nathan Chancellor , Linux Kbuild mailing list , LKML , Linux Memory Management List , clang-built-linux , "Gustavo A. R. Silva" Date: Mon, 12 Aug 2019 16:23:26 -0700 In-Reply-To: References: <20190812214711.83710-1-nhuck@google.com> <20190812221416.139678-1-nhuck@google.com> <814c1b19141022946d3e0f7e24d69658d7a512e4.camel@perches.com> Content-Type: text/plain; charset="ISO-8859-1" User-Agent: Evolution 3.30.5-0ubuntu0.18.10.1 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, 2019-08-12 at 16:11 -0700, Nick Desaulniers wrote: > On Mon, Aug 12, 2019 at 3:40 PM Joe Perches wrote: > > On Mon, 2019-08-12 at 15:14 -0700, Nathan Huckleberry wrote: > > > Clang does not support the use of comments to label > > > intentional fallthrough. This patch replaces some uses > > > of comments to attributesto cut down a significant number > > > of warnings on clang (from ~50000 to ~200). Only comments > > > in commonly used header files have been replaced. > > > > > > Since there is still quite a bit of noise, this > > > patch moves -Wimplicit-fallthrough to > > > Makefile.extrawarn if you are compiling with > > > clang. > > > > Unmodified clang does not emit this warning without a patch. > > Correct, Nathan is currently implementing support for attribute > fallthrough in Clang in: > https://reviews.llvm.org/D64838 > > I asked him in person to evaluate how many warnings we'd see in an > arm64 defconfig with his patch applied. There were on the order of > 50k warnings, mostly from these headers. I asked him to send these > patches, then land support in the compiler, that way should our CI > catch fire overnight, we can carry out of tree fixes until they land. > With the changes here to Makefile.extrawarn, we should not need to > carry any out of tree patches. > > > > diff --git a/Makefile b/Makefile > > [] > > > @@ -846,7 +846,11 @@ NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include) > > > KBUILD_CFLAGS += -Wdeclaration-after-statement > > > > > > # Warn about unmarked fall-throughs in switch statement. > > > +# If the compiler is clang, this warning is only enabled if W=1 in > > > +# Makefile.extrawarn > > > +ifndef CONFIG_CC_IS_CLANG > > > KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough,) > > > +endif > > > > It'd be better to remove CONFIG_CC_IS_CLANG everywhere > > eventually as it adds complexity and makes .config files > > not portable to multiple systems. > > > > > diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h > > [] > > > @@ -253,4 +253,8 @@ > > > */ > > > #define __weak __attribute__((__weak__)) > > > > > > +#if __has_attribute(fallthrough) > > > +#define __fallthrough __attribute__((fallthrough)) > > > > This should be __attribute__((__fallthrough__)) > > Agreed. I think the GCC documentation on attributes had a point about > why the __ prefix/suffix was important, which is why we went with that > in Miguel's original patchset. > > > And there is still no agreement about whether this should > > be #define fallthrough or #define __fallthrough > > > > https://lore.kernel.org/patchwork/patch/1108577/ > > > > > diff --git a/include/linux/jhash.h b/include/linux/jhash.h > > [] > > > @@ -86,19 +86,43 @@ static inline u32 jhash(const void *key, u32 length, u32 initval) > > [] > > > + case 12: > > > + c += (u32)k[11]<<24; > > > + __fallthrough; > > > > You might consider trying out the scripted conversion tool > > attached to this email: > > > > https://lore.kernel.org/lkml/61ddbb86d5e68a15e24ccb06d9b399bbf5ce2da7.camel@perches.com/ > > I guess the thing I'm curious about is why /* fall through */ is being > used vs __attribute__((__fallthrough__))? Surely there's some > discussion someone can point me to? AFAIK: It's historic. https://lkml.org/lkml/2019/8/4/83 coverity and lint do not support __attribute__((__fallthrough__)) but do support /* fallthrough */ comments in their analysis output. I prefer converting all the comments to a macro / pseudo keyword. The cvt_style.pl script does a reasonable job of conversion.