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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 9C14EC6FA82 for ; Fri, 23 Sep 2022 07:25:06 +0000 (UTC) Received: from localhost ([::1]:35406 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1obd3Z-0002sX-B0 for qemu-devel@archiver.kernel.org; Fri, 23 Sep 2022 03:25:05 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:37762) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1obctP-0007Hb-G9 for qemu-devel@nongnu.org; Fri, 23 Sep 2022 03:14:35 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:33162) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1obctN-0006nG-Fj for qemu-devel@nongnu.org; Fri, 23 Sep 2022 03:14:35 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1663917272; h=from:from:reply-to: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=w9Utm4p3cueEbo2W+RKuLlmPdH2ChoRW9/3f9Y/fqng=; b=IufMurxHr/EkcU7j+/aRm/TTW0GsmGz5Nc/uTLXCaNUn8NfgsFkzT28pEe5K9XN9092G+G zYRAiId4hvMg/XnOrtMjs/PkKQ6I/Msrpuqfxm/1z5hwBq94Sm/05mZ8UfJtgPgZy3EAup JnOCOkYHzcEYdrZKXo+V/ZiEWLpLwxI= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-278-MMcBEX0AMJWX25CFbnHxvA-1; Fri, 23 Sep 2022 03:14:29 -0400 X-MC-Unique: MMcBEX0AMJWX25CFbnHxvA-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 4E2F13802B8E for ; Fri, 23 Sep 2022 07:14:29 +0000 (UTC) Received: from redhat.com (unknown [10.33.36.67]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 370D2C15BA8; Fri, 23 Sep 2022 07:14:27 +0000 (UTC) Date: Fri, 23 Sep 2022 08:14:25 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: qemu-devel@nongnu.org, "Dr . David Alan Gilbert" , Leonardo Bras Soares Passos , Juan Quintela Subject: Re: [PATCH 2/5] migration: Fix race on qemu_file_shutdown() Message-ID: References: <20220920223800.47467-1-peterx@redhat.com> <20220920223800.47467-3-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.6 (2022-06-05) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Sep 22, 2022 at 03:37:11PM -0400, Peter Xu wrote: > On Thu, Sep 22, 2022 at 05:58:25PM +0100, Daniel P. Berrangé wrote: > > On Tue, Sep 20, 2022 at 06:37:57PM -0400, Peter Xu wrote: > > > In qemu_file_shutdown(), there's a possible race if with current order of > > > operation. There're two major things to do: > > > > > > (1) Do real shutdown() (e.g. shutdown() syscall on socket) > > > (2) Update qemufile's last_error > > > > > > We must do (2) before (1) otherwise there can be a race condition like: > > > > > > page receiver other thread > > > ------------- ------------ > > > qemu_get_buffer() > > > do shutdown() > > > returns 0 (buffer all zero) > > > (meanwhile we didn't check this retcode) > > > try to detect IO error > > > last_error==NULL, IO okay > > > install ALL-ZERO page > > > set last_error > > > --> guest crash! > > > > > > To fix this, we can also check retval of qemu_get_buffer(), but not all > > > APIs can be properly checked and ultimately we still need to go back to > > > qemu_file_get_error(). E.g. qemu_get_byte() doesn't return error. > > > > > > Maybe some day a rework of qemufile API is really needed, but for now keep > > > using qemu_file_get_error() and fix it by not allowing that race condition > > > to happen. Here shutdown() is indeed special because the last_error was > > > emulated. For real -EIO errors it'll always be set when e.g. sendmsg() > > > error triggers so we won't miss those ones, only shutdown() is a bit tricky > > > here. > > > > The ultimate answer really is to stop using QEMUFile entirely and just > > do migration with the QIOChannel objects directly. The work I did in the > > last cycle to remove the QEMUFileOps callbacks was another piece of the > > puzzle in moving in that direction, by ensuring that every QEMUFile is > > now backed by a QIOChannel. All QEMUFile is doing now is adding the I/O > > caching layer and some convenience APIs for I/O operations. > > > > So the next step would be to add a QIOChannelCached class that can wrap > > over another QIOChannel, to add the I/O buffering functionality that > > currently exists in QEMUFile. Once that's done, it'd be at the stage > > where we could look at how to use QIOChannel APIs for VMstate. It would > > likely involve wiring up an Error **errp parameter into a great many > > places so we get synchronous error propagation instead of out-of-band > > error checking, so a phased transition would need to be figured out. > > Yes, Error** sounds very good to have. > > So far I'm not satisfied with qemufile api majorly because of that error > handling, especially on *get() interfaces. > > Besides that, do you have anything else in mind that would make > QIOChannelCached better than qemufile (e.g. on how we do caching)? Depends what you mean by better ? I think the caching code would be a bit easier to understand, because QEMUFile gets a bit confusing about which logic is used for read side and which is used for the write side. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|