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 79587CCF9F8 for ; Wed, 12 Nov 2025 09:01:17 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vJ6iO-0006wL-Kj; Wed, 12 Nov 2025 04:00:32 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vJ6hy-0006pp-GN for qemu-devel@nongnu.org; Wed, 12 Nov 2025 04:00:09 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vJ6hu-0004p2-CX for qemu-devel@nongnu.org; Wed, 12 Nov 2025 04:00:04 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1762937995; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=XvjNZzXQZJcAJcVbCJnBYjpYL1HMhEu9sk+EC8shCPI=; b=Gl8eFn/ol+VMq3AN6/OU3w/tMX9uRdEFtvKFiprhEq6rQV5voaiCRiCMtxGAFkwqVxiSvI i2So7mzWnH6E0XiWZbU1ndKjqBqYxwOE0Hnp5Uoa8jahx6U15wlw9aRUR3ZSPqJS9hinXe S0uge3bnX/oBtgOPfXxgq+Wi9+02xso= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-595-vahaQn9dOOKiO1phEqoavQ-1; Wed, 12 Nov 2025 03:59:53 -0500 X-MC-Unique: vahaQn9dOOKiO1phEqoavQ-1 X-Mimecast-MFC-AGG-ID: vahaQn9dOOKiO1phEqoavQ_1762937992 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E11431800650; Wed, 12 Nov 2025 08:59:50 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.18]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 377A01800900; Wed, 12 Nov 2025 08:59:50 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 3CBAE21E6A27; Wed, 12 Nov 2025 09:59:47 +0100 (CET) From: Markus Armbruster To: Jie Song Cc: qemu-devel@nongnu.org, armbru@redhat.com, Jie Song , =?utf-8?Q?Daniel_P=2E_Berrang=C3=A9?= Subject: Re: [PATCH] monitor/qmp: cleanup socket listener sources early to avoid fd handling race In-Reply-To: <20251111150144.76751-1-mail@jiesong.me> (Jie Song's message of "Tue, 11 Nov 2025 23:01:44 +0800") References: <20251111150144.76751-1-mail@jiesong.me> Date: Wed, 12 Nov 2025 09:59:47 +0100 Message-ID: <87a50r8w3g.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Daniel, is this in your area of expertise? Jie Song, can you identify the commit that introduced the bug? Jie Song writes: > From: Jie Song > > When starting a dummy QEMU process with virsh, monitor_init_qmp() enables > IOThread monitoring of the QMP fd by default. However, a race condition > exists during the initialization phase: the IOThread only removes the > main thread's fd watch when it reaches qio_net_listener_set_client_func_full(), > which may be delayed under high system load. > > This creates a window between monitor_qmp_setup_handlers_bh() and > qio_net_listener_set_client_func_full() where both the main thread and > IOThread are simultaneously monitoring the same fd and processing events. > This race can cause either the main thread or the IOThread to hang and > become unresponsive. > > Fix this by proactively cleaning up the listener's IO sources in > monitor_init_qmp() before the IOThread initializes QMP monitoring, > ensuring exclusive fd ownership and eliminating the race condition. > > The fix introduces socket_chr_listener_cleanup() to destroy and unref > all existing IO sources on the socket chardev listener, guaranteeing > that no concurrent fd monitoring occurs during the transition to > IOThread handling. > > Signed-off-by: Jie Song > --- > chardev/char-socket.c | 18 ++++++++++++++++++ > include/chardev/char-socket.h | 2 ++ > monitor/qmp.c | 6 ++++++ > 3 files changed, 26 insertions(+) > > diff --git a/chardev/char-socket.c b/chardev/char-socket.c > index 62852e3caf..073a9da855 100644 > --- a/chardev/char-socket.c > +++ b/chardev/char-socket.c > @@ -656,6 +656,24 @@ static void tcp_chr_telnet_destroy(SocketChardev *s) > } > } > > +void socket_chr_listener_cleanup(Chardev *chr) > +{ > + SocketChardev *s = SOCKET_CHARDEV(chr); > + > + if (s->listener) { > + QIONetListener *listener = s->listener; > + size_t i; > + > + for (i = 0; i < listener->nsioc; i++) { > + if (listener->io_source[i]) { > + g_source_destroy(listener->io_source[i]); > + g_source_unref(listener->io_source[i]); > + listener->io_source[i] = NULL; > + } > + } > + } > +} > + > static void tcp_chr_update_read_handler(Chardev *chr) > { > SocketChardev *s = SOCKET_CHARDEV(chr); > diff --git a/include/chardev/char-socket.h b/include/chardev/char-socket.h > index d6d13ad37f..682440c6de 100644 > --- a/include/chardev/char-socket.h > +++ b/include/chardev/char-socket.h > @@ -84,4 +84,6 @@ typedef struct SocketChardev SocketChardev; > DECLARE_INSTANCE_CHECKER(SocketChardev, SOCKET_CHARDEV, > TYPE_CHARDEV_SOCKET) > > +void socket_chr_listener_cleanup(Chardev *chr); > + > #endif /* CHAR_SOCKET_H */ > diff --git a/monitor/qmp.c b/monitor/qmp.c > index cb99a12d94..d9d1fafa70 100644 > --- a/monitor/qmp.c > +++ b/monitor/qmp.c > @@ -25,6 +25,7 @@ > #include "qemu/osdep.h" > > #include "chardev/char-io.h" > +#include "chardev/char-socket.h" > #include "monitor-internal.h" > #include "qapi/error.h" > #include "qapi/qapi-commands-control.h" > @@ -537,6 +538,11 @@ void monitor_init_qmp(Chardev *chr, bool pretty, Error **errp) > * e.g. the chardev is in client mode, with wait=on. > */ > remove_fd_in_watch(chr); > + /* > + * Clean up listener IO sources early to prevent racy fd > + * handling between the main thread and the I/O thread. > + */ > + socket_chr_listener_cleanup(chr); > /* > * We can't call qemu_chr_fe_set_handlers() directly here > * since chardev might be running in the monitor I/O