From patchwork Wed Jul 18 09:13:36 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Dirk Gouders X-Patchwork-Id: 10531639 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 3C62F6020A for ; Wed, 18 Jul 2018 09:16:25 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 323AA2908D for ; Wed, 18 Jul 2018 09:16:25 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 26B3929131; Wed, 18 Jul 2018 09:16:25 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.0 required=2.0 tests=BAYES_00, DKIM_ADSP_DISCARD, DKIM_SIGNED, MAILING_LIST_MULTI, RCVD_IN_DNSWL_HI, T_DKIM_INVALID autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 61BEE290E9 for ; Wed, 18 Jul 2018 09:16:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730565AbeGRJxT (ORCPT ); Wed, 18 Jul 2018 05:53:19 -0400 Received: from services.gouders.net ([141.101.32.176]:33092 "EHLO services.gouders.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730598AbeGRJxT (ORCPT ); Wed, 18 Jul 2018 05:53:19 -0400 Received: from lena.gouders.net ([193.175.198.193]) (authenticated bits=0) by services.gouders.net (8.14.8/8.14.8) with ESMTP id w6I9DuD7028581 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES128-GCM-SHA256 bits=128 verify=NO); Wed, 18 Jul 2018 11:14:06 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=gouders.net; s=gnet; t=1531905247; bh=Ttig7I4bBJY7gQ+/MLdt4XgYmszpqezZ2QsOmkEC3UU=; h=From:To:Cc:Subject:Date; b=LWTdsitapnQ9qUkw6IhTbQ/YZa+DcUs7dJSLL5Ce13Z3k8dVXuv+hP9dhQSfevtoX Dv7tWF1xQdX7M7NBg1OJ01m2hycSBRf3Z/17ENRExqMBwI7pBefjIPPN8HMBgRPAoV Jg08S+PAcD/hR6x19MiwLDiuz+B5k+cYlsQm1YYo= From: Dirk Gouders To: Masahiro Yamada , Jonathan Corbet Cc: Dirk Gouders , linux-kbuild@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH] kbuild: if_changed: document single use per target limitation Date: Wed, 18 Jul 2018 11:13:36 +0200 Message-Id: <20180718091336.4540-1-dirk@gouders.net> X-Mailer: git-send-email 2.16.1 Sender: linux-kbuild-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kbuild@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Users of if_changed could easily feel invited to use it to divide a recipe into parts like: a: prereq FORCE $(call if_changed,do_a) $(call if_changed,do_b) But this is problematic, because if_changed should not be used more than once per target: in the above example, if_changed stores the command-line of the given command in .a.cmd and when a is up-to-date with respect to prereq, the file .a.cmd contains the command-line for the last command executed, i.e. do_b. When the recipe is then executed again, without any change of prerequisites, the command-line check for do_a will fail, do_a will be executed and stored in .a.cmd. The next check, however, will still see the old content (the file isn't re-read) and if_changed will skip do_b, because the command-line test will not recognize a change. On the next execution of the recipe the roles will flip: do_a is OK but do_b not and it will be executed. And so on... Signed-off-by: Dirk Gouders --- Documentation/kbuild/makefiles.txt | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt index 048fc39a6b91..01fae5a01ce7 100644 --- a/Documentation/kbuild/makefiles.txt +++ b/Documentation/kbuild/makefiles.txt @@ -1105,6 +1105,12 @@ When kbuild executes, the following steps are followed (roughly): target: source(s) FORCE #WRONG!# $(call if_changed, ld/objcopy/gzip/...) + Note: if_changed should not be used more than once per target. + It stores the executed command in a corresponding .cmd + file and multiple calls would result in overwrites and + unwanted results when the target is up to date and only the + tests on changed commands trigger execution of commands. + ld Link target. Often, LDFLAGS_$@ is used to set specific options to ld.