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 26129CCF9F8 for ; Wed, 12 Nov 2025 09:06:23 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vJ6nV-0000ZR-Q4; Wed, 12 Nov 2025 04:05:49 -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 1vJ6nM-0000Ut-By for qemu-devel@nongnu.org; Wed, 12 Nov 2025 04:05:40 -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 1vJ6nK-00061L-IO for qemu-devel@nongnu.org; Wed, 12 Nov 2025 04:05:40 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1762938336; 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:in-reply-to:in-reply-to: references:references; bh=8LPpWJ6x4yu8Nrz27PfADQOX74aaMKJgeshxfMxRy/o=; b=T1+KpTrggJ//HuqZHntHg/l1ynt73Kjrr7aHBZRKajSCxw3n3H21B/O+6A++hoLfGm6dNc n/F9n0aXbOLPk25V3XSjXZPuFBgE37NL3S4eHdEGZtZht7FSho9VjUN6NuQIICGztErdVE K7vB4VorhSSoImmwFA0qii/37Jx3vEk= Received: from mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-509-IV6z0QaQNgWTD00nkHW_DA-1; Wed, 12 Nov 2025 04:05:34 -0500 X-MC-Unique: IV6z0QaQNgWTD00nkHW_DA-1 X-Mimecast-MFC-AGG-ID: IV6z0QaQNgWTD00nkHW_DA_1762938333 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (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-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 98D9F1956050; Wed, 12 Nov 2025 09:05:32 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.56]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id B99911955F1A; Wed, 12 Nov 2025 09:05:30 +0000 (UTC) Date: Wed, 12 Nov 2025 09:05:26 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Jie Song Cc: qemu-devel@nongnu.org, armbru@redhat.com, Jie Song Subject: Re: [PATCH] monitor/qmp: cleanup socket listener sources early to avoid fd handling race Message-ID: References: <20251111150144.76751-1-mail@jiesong.me> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20251111150144.76751-1-mail@jiesong.me> User-Agent: Mutt/2.2.14 (2025-02-20) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Nov 11, 2025 at 11:01:44PM +0800, Jie Song wrote: > 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); This is unsafe (may crash) because the chardev used by the monitor may not be a SocketChardev. Having todo back QMP is already calling 'remove_fd_in_watch' to purge the I/O sources. So if there is a flaw, I would expect any fix to be entirely in the chardev code, in a path from remove_fd_in_watch. > /* > * We can't call qemu_chr_fe_set_handlers() directly here > * since chardev might be running in the monitor I/O With 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 :|