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=-7.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 54AAAC47076 for ; Fri, 21 May 2021 16:58:08 +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 B6EBE613D9 for ; Fri, 21 May 2021 16:58:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B6EBE613D9 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]:49372 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lk8TO-00062K-Mp for qemu-devel@archiver.kernel.org; Fri, 21 May 2021 12:58:06 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:35260) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lk8Rx-00056h-Mm for qemu-devel@nongnu.org; Fri, 21 May 2021 12:56:37 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:56744) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lk8Ru-0006Nn-Qg for qemu-devel@nongnu.org; Fri, 21 May 2021 12:56:36 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1621616192; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=+R5coJqNf0Y8CNLpPMdpWrfpiuxLzOPDiQQ68p6pYsE=; b=RWQJlYkKlaVU6EWArMe4cWshpwSBGalvs9tpBykmA+r0IftY4o8ve8oE88m6bA1HgIhvCz 4TPkOZAj23P8ThljP1IiJHTH1mmRvb8SqCIAB54d3KbNSXJdOj/QEOnZUNXxQKXpHIuely J0TJuZB108MzVpsqN/WkFYotE+N9UpQ= 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-366-YdOhsffDNUGbiUOR47htjw-1; Fri, 21 May 2021 12:56:25 -0400 X-MC-Unique: YdOhsffDNUGbiUOR47htjw-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id DFCA48015F8; Fri, 21 May 2021 16:56:23 +0000 (UTC) Received: from redhat.com (ovpn-114-5.ams2.redhat.com [10.36.114.5]) by smtp.corp.redhat.com (Postfix) with ESMTPS id CCCA15C1BB; Fri, 21 May 2021 16:56:16 +0000 (UTC) Date: Fri, 21 May 2021 17:56:14 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu , chenjiashang@huawei.com, Markus Armbruster , QEMU , "Gonglei (Arei)" , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , Paolo Bonzini , "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" Subject: Re: A bug of Monitor Chardev ? Message-ID: References: <87lf88pmyn.fsf@dusky.pond.sub.org> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.0.7 (2021-05-04) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit 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: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.374, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, 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: 2.1.23 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 Fri, May 21, 2021 at 05:33:46PM +0100, Daniel P. Berrangé wrote: > On Fri, May 21, 2021 at 10:43:36AM -0400, Peter Xu wrote: > > > > I think the original problem was that if qemu_chr_fe_set_handlers() is called > > in main thread, it can start to race somehow within execution of the function > > qemu_chr_fe_set_handlers() right after we switch context at: > > > > qemu_chr_be_update_read_handlers(s, context); > > > > Then the rest code in qemu_chr_fe_set_handlers() will continue to run in main > > thread for sure, but the should be running with the new iothread context, which > > introduce a race condition. > > > > Running qemu_chr_be_update_read_handlers() in BH resolves that because then all > > things run in the monitor iothread only and natually serialized. > > The first message in this thread, however, claims that it is *not* > in fact serialized, when using the BH. > > > So the new comment looks indeed not fully right, as the chr device should be > > indeed within main thread context before qemu_chr_fe_set_handlers(), it's just > > that the race may start right away if without BH when context switch happens > > for the chr. > > It sounds like both the comment and the code are potentially wrong. I feel like our root cause problem that the original code was trying to workaround, is that the chardev is "active" from the very moment it is created, regardless of whether the frontend is ready to use it. IIUC, in this case the socket chardev is already listen()ing and accept()ing incoming clients off the network, before the monitor has finished configuring its hooks into the chardev. This means that the initial listen()/accept() I/O watches are using the default GMainContext, and the monitor *has* to remove them and put in new watches on the thread private GMainContext. To eliminate any risk of races, we need to make it possible for the monitor to configure the GMainContext on the chardevs *before* any I/O watches are configured. This in turn suggests that we need to split the chardev initialization into two phases. First we have the basic chardev creation, with object creation, option parsing/sanity checking, socket creation, and then second we have the actual activation where the I/O watches are added. IOW, qemu_chr_new() is the former and gets run from generic code in the main() method, or in QMP chardev_add. A new 'qemu_chr_activate' method would be called by whatever frontend is using the chardev, after registering a custom GMainContext. This would involve updating every single existing user of chardevs to add a call to qemu_chr_activate, but that's worth it to eliminate the race by design, rather than workaround it. 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 :|