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=-5.0 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,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 1FC43C47076 for ; Fri, 21 May 2021 17:17:44 +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 8C7FB61163 for ; Fri, 21 May 2021 17:17:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8C7FB61163 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]:44696 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lk8mM-0006Zm-Ob for qemu-devel@archiver.kernel.org; Fri, 21 May 2021 13:17:42 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:40476) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lk8km-0003p1-FG for qemu-devel@nongnu.org; Fri, 21 May 2021 13:16:04 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:30638) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lk8ki-0001G5-Py for qemu-devel@nongnu.org; Fri, 21 May 2021 13:16:04 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1621617360; 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=+Tf8STVDoIjzZ3MuxtZBYhquEQSrJy7cunQ4K5pNV3E=; b=D1Dt8g4zfjrGFpJCn3+p/yy3Hhy/3A7Q5uGO5PCOKixayuqCJcdKE6LVh2raS1FiaI1fp+ A4hoaQRfP5oUuQzIuQh/GTdaoq4fyBJtS4iboHJOa4vUZt91FNevWQ1/xWSX5ETCtbm1qB 4XNAma2DWp9WBITTRB6frWIGlVxL1JU= 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-166-WQw74YfRMyW-PK9er7TYLA-1; Fri, 21 May 2021 13:15:56 -0400 X-MC-Unique: WQw74YfRMyW-PK9er7TYLA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 9ABB1802B4F; Fri, 21 May 2021 17:15:55 +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 7F8B05D6DC; Fri, 21 May 2021 17:15:49 +0000 (UTC) Date: Fri, 21 May 2021 18:15:47 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu 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 10.5.11.15 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?= Cc: chenjiashang@huawei.com, Markus Armbruster , QEMU , "Gonglei \(Arei\)" , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , Paolo Bonzini , "Longpeng \(Mike, Cloud Infrastructure Service Product Dept.\)" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, May 21, 2021 at 01:09:17PM -0400, Peter Xu wrote: > On Fri, May 21, 2021 at 05:56:14PM +0100, Daniel P. Berrangé wrote: > > 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. > > When we are still running the monitor_init() code IIUC the main thread is still > occupied so it won't be able to handle any listen()/accept() even if there's > event already, so IIUC race can only happen when main thread started the event > loop then run concurrently with the BH in the other iothread. > > So, besides above split of chardev init (which sounds like a bigger > surgery)... would it also work if we let the main thread wait until that BH > executed in monitor iothread? Then all pending listen()/accept() event will > directly be done in the monitor thread. My concern is what happens when we add support for monitor hotplug in the future. A client wanting to add a second monitor to a running QEMU will run "chardev-add" followed by a hypothetical "monitor-add" command. Both of these will be processed in the monitor thread, so the main thread will have already started handling I/O events for the chardev before "monitor-add" gets processed. Thus I think the only long term safe solution is to have a design that guarantees no I/O events are registered by *any* chardev impl, until the frontend connects to the chardev. 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 :|