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=-10.0 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham 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 D142BC47076 for ; Fri, 21 May 2021 16:35:10 +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 2D401613E3 for ; Fri, 21 May 2021 16:35:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2D401613E3 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]:34212 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lk87B-0002Y4-4T for qemu-devel@archiver.kernel.org; Fri, 21 May 2021 12:35:09 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:54006) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lk867-0001qo-Jr for qemu-devel@nongnu.org; Fri, 21 May 2021 12:34:03 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:46426) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lk863-0002Dr-QU for qemu-devel@nongnu.org; Fri, 21 May 2021 12:34:02 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1621614838; 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=8ycxINMQnM/7dUcgti9QAQ/4/BqW/3mZBoXPn1WT4vk=; b=IwsO2F4zVQ/PtjQe9wdHyjcpbnnrrZaB3Ub3IzSCe2nRbwNkLBn7Nb+MobT9aWFlzF7+IT 5D4ZzS6g8885wDlA+MKYGixC/mjbodKb0rycyqB3bKY5yFQxARcp8PWJZZ1frDu4q+fy7o Q1j2XGrie/EEP4kfAZUcAklhZr4i3A4= 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-523-92d8LbO9NKSppEQ5K_hVlA-1; Fri, 21 May 2021 12:33:56 -0400 X-MC-Unique: 92d8LbO9NKSppEQ5K_hVlA-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 51BA58015F8; Fri, 21 May 2021 16:33:55 +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 D7ADB5D9CA; Fri, 21 May 2021 16:33:49 +0000 (UTC) Date: Fri, 21 May 2021 17:33:46 +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 10.5.11.14 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=216.205.24.124; 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 10:43:36AM -0400, Peter Xu wrote: > On Fri, May 21, 2021 at 09:25:52AM +0200, Markus Armbruster wrote: > > Daniel P. Berrangé writes: > > > > > On Wed, May 19, 2021 at 08:17:51PM +0400, Marc-André Lureau wrote: > > >> Hi > > >> > > >> On Mon, May 17, 2021 at 11:11 AM Longpeng (Mike, Cloud Infrastructure > > >> Service Product Dept.) wrote: > > >> > > >> > We find a race during QEMU starting, which would case the QEMU process > > >> > coredump. > > >> > > > >> >
| > > >> > | > > >> > [1] create MON chardev | > > >> > qemu_create_early_backends | > > >> > chardev_init_func | > > >> > | > > >> > [2] create MON iothread | > > >> > qemu_create_late_backends | > > >> > mon_init_func | > > >> > aio_bh_schedule-----------------------> > > >> > monitor_qmp_setup_handlers_bh > > >> > [3] enter main loog | tcp_chr_update_read_handler > > >> > (* A client come in, e.g. Libvirt *) | update_ioc_handlers > > >> > > > >> tcp_chr_new_client | > > >> > update_ioc_handlers | > > >> > | > > >> > [4] create new hup_source | > > >> > s->hup_source = *PTR1* | > > >> > g_source_attach(s->hup_source)| > > >> > | [5] > > >> > remove_hup_source(*PTR1*) > > >> > | (create new > > >> > hup_source) > > >> > | s->hup_source = > > >> > *PTR2* > > >> > [6] g_source_attach_unlocked | > > >> > *PTR1* is freed by [5] | > > >> > > > >> > Do you have any suggestion to fix this bug ? Thanks! > > >> > > > >> > > > >> I see.. I think the simplest would be for the chardev to not be dispatched > > >> in the original thread after monitor_init_qmp(). It looks like this should > > >> translate at least to calling qio_net_listener_set_client_func_full() with > > >> NULL handlers. I can't see where we could fit that in the chardev API. > > >> Perhaps add a new qemu_chr_be_disable_handlers() (until > > >> update_read_handlers is called again to enable them)? > > >> > > >> Daniel? Paolo? > > > > > > IIUC, the problem is: > > > > > > - when we first create the chardev, its IO watches are setup with > > > the default (NULL) GMainContext which is processed by the main > > > thread > > > > > > - when we create the monitor, we re-initialize the chardev to > > > attach its IO watches to a custom GMainCOntext associated with > > > the monitor thread. > > > > > > - The re-initialization is happening in a bottom half that runs > > > in the monitor thread, thus the main thread can already start > > > processing an IO event in parallel > > > > > > Looking at the code in qmp.c monitor_init_qmp method it has a > > > comment: > > > > > > /* > > > * We can't call qemu_chr_fe_set_handlers() directly here > > > * since chardev might be running in the monitor I/O > > > * thread. Schedule a bottom half. > > > */ > > > > > > AFAICT, that comment is wrong. monitor_init_qmp is called from > > > monitor_init, which is called from monitor_init_opts, which is > > > called from qemu_create_late_backends, which runs in the main > > > thread. > > > > Goes back to commit a5ed352596a8b7eb2f9acce34371b944ac3056c4 > > Author: Peter Xu > > Date: Fri Mar 9 16:59:52 2018 +0800 > > > > monitor: allow using IO thread for parsing > > > > For each Monitor, add one field "use_io_thr" to show whether it will be > > using the dedicated monitor IO thread to handle input/output. When set, > > monitor IO parsing work will be offloaded to the dedicated monitor IO > > thread, rather than the original main loop thread. > > > > This only works for QMP. HMP will always be run on the main loop > > thread. > > > > Currently we're still keeping use_io_thr off always. Will turn it on > > later at some point. > > > > One thing to mention is that we cannot set use_io_thr for every QMP > > monitor. The problem is that MUXed typed chardevs may not work well > > with it now. When MUX is used, frontend of chardev can be the monitor > > plus something else. The only thing we know would be safe to be run > > outside main thread so far is the monitor frontend. All the rest of the > > frontends should still be run in main thread only. > > > > Signed-off-by: Peter Xu > > Message-Id: <20180309090006.10018-10-peterx@redhat.com> > > Reviewed-by: Eric Blake > > [eblake: squash in Peter's followup patch to avoid test failures] > > Signed-off-by: Eric Blake > > > > Peter, do you remember why you went for a bottom half? > > > > Hmm, back then it was in monitor_init(), which was called from several > > places. Did we manage to lose the need for a bottom half along the way? > > > > Note that the initial comment was a bit different: > > > > if (mon->use_io_thr) { > > /* > > * Make sure the old iowatch is gone. It's possible when > > * e.g. the chardev is in client mode, with wait=on. > > */ > > remove_fd_in_watch(chr); > > /* > > * We can't call qemu_chr_fe_set_handlers() directly here > > * since during the procedure the chardev will be active > > * and running in monitor iothread, while we'll still do > > * something before returning from it, which is a possible > > * race too. To avoid that, we just create a BH to setup > > * the handlers. > > */ > > aio_bh_schedule_oneshot(monitor_get_aio_context(), > > monitor_qmp_setup_handlers_bh, mon); > > /* We'll add this to mon_list in the BH when setup done */ > > return; > > } else { > > qemu_chr_fe_set_handlers(&mon->chr, monitor_can_read, > > monitor_qmp_read, monitor_qmp_event, > > NULL, mon, NULL, true); > > } > > > > I changed it in commit 774a6b67a40. > > 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. 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 :|