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 10531C433B4 for ; Wed, 19 May 2021 17:15:18 +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 709E3611BD for ; Wed, 19 May 2021 17:15:15 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 709E3611BD 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]:55384 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ljPms-0005as-EK for qemu-devel@archiver.kernel.org; Wed, 19 May 2021 13:15:14 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:37434) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ljPFs-0005W9-7t for qemu-devel@nongnu.org; Wed, 19 May 2021 12:41:08 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:43799) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ljPFq-0000pj-9h for qemu-devel@nongnu.org; Wed, 19 May 2021 12:41:07 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1621442465; 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=tCV1Za5Y3GfDGVAVGVzU+TZJ0d4I09mBV8EIyDRaWPA=; b=CMPYX2YZm297j8jqFPHYhsUs+dOfi+Y9YeLbfuB3JQiC32w5V2ODtUHQKamRyGN0KYDpX/ ugtOfCpG/ZS3CsBglNrEK77aA47nUZPnjvTXdmjiHAw1whZx9CJbotCDENchiWF1/BrBYk lwmCjNVIgkU9HsBhB9tYe3r+kJhlxqI= 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-469-GxVTY40MOR-jEfOsl5pzTg-1; Wed, 19 May 2021 12:41:01 -0400 X-MC-Unique: GxVTY40MOR-jEfOsl5pzTg-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 943D71854E26; Wed, 19 May 2021 16:41:00 +0000 (UTC) Received: from redhat.com (ovpn-115-106.ams2.redhat.com [10.36.115.106]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0DC5910074E0; Wed, 19 May 2021 16:40:53 +0000 (UTC) Date: Wed, 19 May 2021 17:40:51 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Subject: Re: A bug of Monitor Chardev ? Message-ID: References: MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.0.7 (2021-05-04) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 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=-0.39, 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 \(Arei\)" , 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 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. I think we should explicitly document that monitor_init_qmp is *required* to be invoked from the main thread and then remove the bottom half usage. If we ever find a need to create a new monitor from a non-main thread, that thread could use an idle callback attached to the default GMainContext to invoke monitor_init_qmp. 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 :|