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 lists1p.gnu.org (lists1p.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 45C44C43458 for ; Thu, 2 Jul 2026 13:33:07 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wfHXB-0005Hn-1W; Thu, 02 Jul 2026 09:32:53 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wfHX8-0005Gp-ML for qemu-devel@nongnu.org; Thu, 02 Jul 2026 09:32:50 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wfHX6-0007mr-Kv for qemu-devel@nongnu.org; Thu, 02 Jul 2026 09:32:50 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1782999167; 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=KEiiqyCknAK/WhENlqtqHsVylMyOpbn7DqtgvExilNM=; b=RtULRtcKMTMspYinTcAP6/LFu1HUPP/ZbKEwK0FkchNEof7F7vflY4TQdscjJEXisI8ejt 8wgRxvcwUp7eY0qcvHiHUVQBSpWJo8LyNYUPyw+3UG2INxFKx1W1tH1uxtF5pMxkN0luLd 7MUieaOgqPjBzTtS99q8kUbGa3upViE= Received: from mx-prod-mc-08.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-516-fw2krMUsOR-LBVAAdlwHhA-1; Thu, 02 Jul 2026 09:32:44 -0400 X-MC-Unique: fw2krMUsOR-LBVAAdlwHhA-1 X-Mimecast-MFC-AGG-ID: fw2krMUsOR-LBVAAdlwHhA_1782999161 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-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 152B518C1072; Thu, 2 Jul 2026 13:32:41 +0000 (UTC) Received: from redhat.com (unknown [10.44.33.186]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 02A8C180058D; Thu, 2 Jul 2026 13:32:36 +0000 (UTC) Date: Thu, 2 Jul 2026 14:32:33 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Markus Armbruster Cc: marcandre.lureau@redhat.com, qemu-devel@nongnu.org, devel@lists.libvirt.org, Paolo Bonzini , "Dr. David Alan Gilbert" , Alex =?utf-8?Q?Benn=C3=A9e?= , Christian Brauner , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Peter Krempa Subject: Re: [PATCH v5 32/35] monitor: add support for auto-deleting monitors upon close Message-ID: References: <20260624173752.2928717-1-berrange@redhat.com> <20260624173752.2928717-33-berrange@redhat.com> <178233405051.3955748.10990724544083287885.b4-review@b4> <87zf0busnt.fsf@pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <87zf0busnt.fsf@pond.sub.org> User-Agent: Mutt/2.3.2 (2026-04-26) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 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: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.445, 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_H4=0.001, RCVD_IN_MSPIKE_WL=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: qemu development 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 Wed, Jul 01, 2026 at 08:32:06AM +0200, Markus Armbruster wrote: > marcandre.lureau@redhat.com writes: > > >> The default monitor is usually a long lived object that will exist for > >> the entire lifetime of the VM. A monitor can only service a single > >> client at a time though, and so it might be desirable to hotplug > >> additional monitors at runtime for specific tasks. If doing that, > >> however, there is a need to remove the monitor when it is no longer > >> needed. > > Whatever adds the additional monitor can also delete it. The fact that > you propose other means suggests you believe this would be cumbersome in > practice. Why? > > >> > >> Allowing a client to run "object-del" against its own monitor adds > >> complex edge cases, as it would be desirable to send the QMP response > >> despite the monitor sending it being deleted. Doing "object-del" alone > >> will also result in orphaning a character device backend instance, as > >> there is no opportunity to run the companion "chardev-del" command. > >> > >> A simpler way to ensure cleanup is to add the concept of auto-deleting > >> monitor objects. Specifically when the "CHR_EVENT_CLOSED" event is > >> emitted, the equivalent of "object-del" + "chardev-del" can be run > >> internally. Since the transient client has already droppped its > >> monitor connection, there is no synchronization to be concerned about. > > If object-del or chardev-del fail, there's no way to report the error. > Can they fail? > > Do we always want to delete both monitor and character device? > > >> This is implemented via a new "close-action=none|delete" property on > >> the 'monitor-qmp' object. This concept could be extended with further > >> actions in future, for example: > >> > >> * close-action=shutdown - graceful guest shutdown > >> * close-action=terminate - immediate guest poweroff > >> * close-action=stop - pause guest CPUs while the monitor is not > >> connected to any client > > I'm not sure these would be useful and safe. Mentioning them here is > okay regardless. > > >> > >> This is left as an exercise for future interested contributors. > >> > >> Tested-by: Peter Krempa > >> Signed-off-by: Daniel P. Berrangé > >> Message-ID: <20260624173752.2928717-33-berrange@redhat.com> snip > >> diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h > >> index 5522e05464b..23829f32f9a 100644 > >> --- a/monitor/monitor-internal.h > >> +++ b/monitor/monitor-internal.h snip > >> +static void monitor_qmp_self_delete_bh(void *opaque) > >> +{ > >> + MonitorQMP *mon = opaque; > >> + g_autofree char *mon_id = object_property_get_child_name( > >> + object_get_objects_root(), OBJECT(mon)); > >> + g_autofree char *chardev_id = g_strdup(mon->parent_obj.chardev_id); > >> + Error *local_error = NULL; > >> + > >> + if (!mon_id) { > >> + /* Another monitor raced & ran 'object-del' on 'mon' > >> + * before this BH got scheduled, so we have a ref on > >> + * mon but it is already unparented. > >> + */ > >> + object_unref(mon); > >> + return; > >> + } > >> + > >> + user_creatable_del(mon_id, &local_error); > >> + object_unref(mon); > > This object_unref() and the one above pair with the object_ref() in > monitor_qmp_event() case MONITOR_QMP_CLOSE_ACTION_DELETE. Correct? > > Is this worth a comment? > > >> + if (local_error != NULL) { > >> + error_report_err(local_error); > >> + } else { > >> + qmp_chardev_remove(chardev_id, NULL); > >> + } > >> +} > >> + > >> static void monitor_qmp_event(void *opaque, QEMUChrEvent event) > >> { > >> QDict *data; > >> MonitorQMP *mon = opaque; > >> > >> + /* Protect against race if a client drops & quickly > >> + * reconnects - we'll have the delete BH scheduled > >> + * so must not honour a new open request > >> + */ > >> + if (mon->delete_pending) { > >> + return; > >> + } > > Consider "something happens between setting @delete_pending and monitor > destruction that triggers the event" and "same thing happens after > destruction". Any observable differences in behavior? > > Are there any other monitor interactions that might need similar > special-casing when @delete_pending? When the BH deletes the monitor object, it will disconnect the chardev handlers. So there is no possibility of monitor_qmp_event being called after the monitor QOM destruction is complete. The only race is between this method and the start of the BH. With regards, Daniel -- |: https://berrange.com ~~ https://hachyderm.io/@berrange :| |: https://libvirt.org ~~ https://entangle-photo.org :| |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :|