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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, 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 E734DC54E4A for ; Mon, 11 May 2020 12:08:25 +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 B2DD8206F5 for ; Mon, 11 May 2020 12:08:25 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="DsXxxIUa" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B2DD8206F5 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]:37638 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jY7EO-0005vy-VR for qemu-devel@archiver.kernel.org; Mon, 11 May 2020 08:08:24 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:35072) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jY7DY-000579-Mb for qemu-devel@nongnu.org; Mon, 11 May 2020 08:07:32 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:32473 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1jY7DX-0007kP-Qh for qemu-devel@nongnu.org; Mon, 11 May 2020 08:07:32 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1589198850; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Np+spSlUQoSRWDqNQKklEPYXjcFeyGOTCLfQi6UZtlA=; b=DsXxxIUa2a1oCAlYduYxizbs6sQXkcQIhIX9lM1Md9r1FQBgMBdHFKQl7EkO3na+5NgW+v QGlJRuK2qC/SPTyGDwC+0veJkFoOM+Yn+E4QQm+I1RSN0nhmz+b/Tpd0cEKJR1Ig5VeW8C NpKPx1tRQYKlMpVEVfvcsSSaVrlBAnw= 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-304-k07uXTsaNNy5Jeu16Mtv8Q-1; Mon, 11 May 2020 08:07:26 -0400 X-MC-Unique: k07uXTsaNNy5Jeu16Mtv8Q-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id BBB3C8005AD; Mon, 11 May 2020 12:07:25 +0000 (UTC) Received: from work-vm (ovpn-114-150.ams2.redhat.com [10.36.114.150]) by smtp.corp.redhat.com (Postfix) with ESMTPS id BF0435C1B2; Mon, 11 May 2020 12:07:20 +0000 (UTC) Date: Mon, 11 May 2020 13:07:18 +0100 From: "Dr. David Alan Gilbert" To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Subject: Re: [PATCH 0/5] Introduce 'yank' oob qmp command to recover from hanging qemu Message-ID: <20200511120718.GD2811@work-vm> References: <20200511114947.GJ1135885@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200511114947.GJ1135885@redhat.com> User-Agent: Mutt/1.13.4 (2020-02-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.120; envelope-from=dgilbert@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/11 00:05:06 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] 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_H2=-0.001, SPF_PASS=-0.001, T_HK_NAME_DR=0.01, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN 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: , Cc: Kevin Wolf , Lukas Straub , qemu-block , Juan Quintela , qemu-devel , Max Reitz , Paolo Bonzini , =?iso-8859-1?Q?Marc-Andr=E9?= Lureau Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Daniel P. Berrangé (berrange@redhat.com) wrote: > On Mon, May 11, 2020 at 01:14:34PM +0200, Lukas Straub wrote: > > Hello Everyone, > > In many cases, if qemu has a network connection (qmp, migration, chardev, etc.) > > to some other server and that server dies or hangs, qemu hangs too. > > If qemu as a whole hangs due to a stalled network connection, that is a > bug in QEMU that we should be fixing IMHO. QEMU should be doing non-blocking > I/O in general, such that if the network connection or remote server stalls, > we simply stop sending I/O - we shouldn't ever hang the QEMU process or main > loop. > > There are places in QEMU code which are not well behaved in this respect, > but many are, and others are getting fixed where found to be important. > > Arguably any place in QEMU code which can result in a hang of QEMU in the > event of a stalled network should be considered a security flaw, because > the network is untrusted in general. That's not really true of the 'management network' - people trust that and I don't see a lot of the qemu code getting fixed safely for all of them. > > These patches introduce the new 'yank' out-of-band qmp command to recover from > > these kinds of hangs. The different subsystems register callbacks which get > > executed with the yank command. For example the callback can shutdown() a > > socket. This is intended for the colo use-case, but it can be used for other > > things too of course. > > IIUC, invoking the "yank" command unconditionally kills every single > network connection in QEMU that has registered with the "yank" subsystem. > IMHO this is way too big of a hammer, even if we accept there are bugs in > QEMU not handling stalled networking well. But isn't this hammer conditional - I see that it's a migration capabiltiy for the migration socket, and a flag in nbd - so it only yanks things you've told it to. > eg if a chardev hangs QEMU, and we tear down everything, killing the NBD > connection used for the guest disk, we needlessly break I/O. > > eg doing this in the chardev backend is not desirable, because the bugs > with hanging QEMU are typically caused by the way the frontend device > uses the chardev blocking I/O calls, instead of non-blocking I/O calls. > Having a way to get out of any of these problems from a single point is quite nice. To be useful in COLO you need to know for sure you can get out of any network screwup. We already use shutdown(2) in migrate_cancel and migrate-pause for basically the same reason; I don't think we've got anything similar for NBD, and we probably should have (I think I asked for it fairly recently). Dave > 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 :| -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK