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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 840F2C43603 for ; Thu, 5 Dec 2019 17:35:18 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 521D222525 for ; Thu, 5 Dec 2019 17:35:18 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="RM5kEcsH" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 521D222525 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:58546 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1icv25-0001xH-HJ for qemu-devel@archiver.kernel.org; Thu, 05 Dec 2019 12:35:17 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:60990) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1icv0O-00013D-ES for qemu-devel@nongnu.org; Thu, 05 Dec 2019 12:33:33 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1icv0I-0006UZ-0n for qemu-devel@nongnu.org; Thu, 05 Dec 2019 12:33:29 -0500 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:44130 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1icv0G-0006RY-3e for qemu-devel@nongnu.org; Thu, 05 Dec 2019 12:33:25 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1575567201; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=SXeZyo48H3SZ9vKNo89jpscEKcAmkJ65SmGcOay8P6g=; b=RM5kEcsHTyKGqFH/bUVeQ/6BF21N4sYuxtXzS+eTI3lGX4lGttc7l0Dfb8jq1BNyj1arxD ueUym+D+NjRmW057UswGMBxm0eSloq2SGlaZrSSJcW2F7hzl14akl+LzBXP1y90QfAQgDB 4lf3whKZYtpLj8TMUoUdh3Xh3g0/J8k= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-1-gkHur7j6PiOysEy_2vMLcg-1; Thu, 05 Dec 2019 12:33:20 -0500 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7409E800D41; Thu, 5 Dec 2019 17:33:16 +0000 (UTC) Received: from [10.3.116.171] (ovpn-116-171.phx2.redhat.com [10.3.116.171]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 885356012D; Thu, 5 Dec 2019 17:32:42 +0000 (UTC) Subject: Re: [RFC v5 024/126] error: auto propagated local_err To: Vladimir Sementsov-Ogievskiy , Markus Armbruster References: <20191011160552.22907-1-vsementsov@virtuozzo.com> <20191011160552.22907-25-vsementsov@virtuozzo.com> <87muc8p24w.fsf@dusky.pond.sub.org> <55393c08-5bda-8042-1a95-f350b3781d99@virtuozzo.com> <87d0d3c5k7.fsf@dusky.pond.sub.org> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <54c19813-8a10-1607-6fdc-cd7d930ecac7@redhat.com> Date: Thu, 5 Dec 2019 11:32:41 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-MC-Unique: gkHur7j6PiOysEy_2vMLcg-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Stefan Hajnoczi , Jan Kiszka , Hailiang Zhang , "qemu-block@nongnu.org" , Aleksandar Rikalo , Halil Pasic , =?UTF-8?Q?Herv=c3=a9_Poussineau?= , Anthony Perard , Samuel Thibault , Laszlo Ersek , Jason Wang , Laurent Vivier , Eduardo Habkost , Xie Changlong , Peter Lieven , "Dr. David Alan Gilbert" , Beniamino Galvani , Eric Auger , Alex Williamson , Ronnie Sahlberg , Richard Henderson , Kevin Wolf , Andrew Jeffery , Chris Wulff , Subbaraya Sundeep , Michael Walle , "qemu-ppc@nongnu.org" , Bastian Koppelmann , Igor Mammedov , Fam Zheng , Peter Maydell , "sheepdog@lists.wpkg.org" , Matthew Rosato , David Hildenbrand , Palmer Dabbelt , Max Filippov , Hannes Reinecke , Stefano Stabellini , "Gonglei \(Arei\)" , Liu Yuan , Artyom Tarasenko , Thomas Huth , Amit Shah , Stefan Weil , Greg Kurz , Yuval Shaia , "qemu-s390x@nongnu.org" , "qemu-arm@nongnu.org" , Peter Chubb , =?UTF-8?Q?C=c3=a9dric_Le_Goater?= , Stafford Horne , "qemu-riscv@nongnu.org" , Cornelia Huck , Aleksandar Markovic , Aurelien Jarno , Paul Burton , Sagar Karandikar , Paul Durrant , Anthony Green , Gerd Hoffmann , "Edgar E. Iglesias" , Guan Xuetao , Juan Quintela , Michael Roth , Christian Borntraeger , Joel Stanley , Antony Pavlov , "xen-devel@lists.xenproject.org" , "integration@gluster.org" , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , Andrew Baumann , Max Reitz , Denis Lunev , "Michael S. Tsirkin" , Mark Cave-Ayland , "qemu-devel@nongnu.org" , Vincenzo Maffione , Marek Vasut , =?UTF-8?Q?Marc-Andr=c3=a9_Lureau?= , Alistair Francis , Pavel Dovgalyuk , Giuseppe Lettieri , Luigi Rizzo , David Gibson , Tony Krowiak , =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= , Xiao Guangrong , Pierre Morel , Wen Congyang , Jean-Christophe Dubois , Paolo Bonzini , Stefan Berger Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 12/5/19 8:58 AM, Vladimir Sementsov-Ogievskiy wrote: >>>> What about you provide the examples, and then I try to polish the prose? >>> >>> 1: error_fatal problem >>> >>> Assume the following code flow: >>> >>> int f1(errp) { >>> ... >>> ret = f2(errp); >>> if (ret < 0) { >>> error_append_hint(errp, "very useful hint"); >>> return ret; >>> } >>> ... >>> } >>> >>> Now, if we call f1 with &error_fatal argument and f2 fails, the program >>> will exit immediately inside f2, when setting the errp. User will not >>> see the hint. >>> >>> So, in this case we should use local_err. >> >> How does this example look after the transformation? Without ERRP_AUTO_PROPAGATE(), the transformation is a lot of boilerplate: int f1(errp) { Error *err = NULL; ret = f2(&err); if (ret < 0) { error_append_hint(&err, "very useful hint"); error_propagate(errp, err); return ret; } } what's worse, that boilerplate to solve problem 1 turns out to be... > > Good point. > > int f1(errp) { > ERRP_AUTO_PROPAGATE(); > ... > ret = f2(errp); > if (ret < 0) { > error_append_hint(errp, "very useful hint"); > return ret; > } > ... > } > > - nothing changed, only add macro at start. But now errp is safe, if it was > error_fatal it is wrapped by local error, and will only call exit on automatic > propagation on f1 finish. > >> >>> 2: error_abort problem >>> >>> Now, consider functions without return value. We normally use local_err >>> variable to catch failures: >>> >>> void f1(errp) { >>> Error *local_err = NULL; >>> ... >>> f2(&local_err); >>> if (local_err) { >>> error_propagate(errp, local_err); >>> return; >>> } >>> ... >>> } the very same code as the cause of problem 2. >>> >>> Now, if we call f2 with &error_abort and f2 fails, the stack in resulting >>> crash dump will point to error_propagate, not to the failure point in f2, >>> which complicates debugging. >>> >>> So, we should never wrap error_abort by local_err. >> >> Likewise. > > And here: > > void f1(errp) { > ERRP_AUTO_PROPAGATE(); > ... > f2(errp); > if (*errp) { > return; > } > ... > > - if errp was NULL, it is wrapped, so dereferencing errp is safe. On return, > local error is automatically propagated to original one. So, the use of ERRP_AUTO_PROPAGATE() solves BOTH problems 1 and 2 - we avoid the boilerplate that trades one problem for another, by consolidating ALL of the boilerplate into a single-line macro, such that error_propagate() no longer needs to be called anywhere except inside the ERRP_AUTO_PROPAGATE macro. > >> >>> >>> === >>> >>> Our solution: >>> >>> - Fixes [1.], adding invocation of new macro into functions with error_appen_hint/error_prepend, >>> New macro will wrap error_fatal. >>> - Fixes [2.], by switching from hand-written local_err to smart macro, which never >>> wraps error_abort. >>> - Handles [3.], by switching to macro, which is less code >>> - Additionally, macro doesn't wrap normal non-zero errp, to avoid extra propagations >>> (in fact, error_propagate is called, but returns immediately on first if (!local_err)) >> > > -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org