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=-8.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 97863C433B4 for ; Thu, 8 Apr 2021 09:23:28 +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 230786113A for ; Thu, 8 Apr 2021 09:23:28 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 230786113A 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]:60212 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lUQsp-0007bE-3G for qemu-devel@archiver.kernel.org; Thu, 08 Apr 2021 05:23:27 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:55406) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lUQrM-0006dM-2B for qemu-devel@nongnu.org; Thu, 08 Apr 2021 05:21:56 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:28672) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lUQrJ-0002vY-1I for qemu-devel@nongnu.org; Thu, 08 Apr 2021 05:21:54 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1617873711; 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=HyjOR8ymtCieNfnVX0qTVO8lSPdEWmnIjxYyug8wBjc=; b=h0tkokKMqU0276/n8MYH/TEIYE+FOd+HeYn0/YYhqyCAbtZaYoDzwKCu+nxMvM365mIzMe a3WJrXBBsQedYcfrjGP95QBkQl2nRBAgHibxL9uOSWe4RhLwLtPAK6MDnVQqr13hqpr8Pk EpaSP2hgBgMrcq4jPuT+gsrUmHLvdvk= 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-102-XTE6MtQvOwamxc-u7FNytw-1; Thu, 08 Apr 2021 05:21:47 -0400 X-MC-Unique: XTE6MtQvOwamxc-u7FNytw-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 6E029107ACE8; Thu, 8 Apr 2021 09:21:46 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-114-17.ams2.redhat.com [10.36.114.17]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C1F6C6B8DD; Thu, 8 Apr 2021 09:21:45 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 2E0F2113865F; Thu, 8 Apr 2021 11:21:42 +0200 (CEST) From: Markus Armbruster To: Kevin Wolf Subject: Re: [PATCH v2] monitor/qmp: fix race on CHR_EVENT_CLOSED without OOB References: <20210322154024.15011-1-s.reiter@proxmox.com> Date: Thu, 08 Apr 2021 11:21:42 +0200 In-Reply-To: (Kevin Wolf's message of "Wed, 7 Apr 2021 15:19:12 +0200") Message-ID: <87lf9tces9.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 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_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: , Cc: Paolo Bonzini , Stefan Reiter , qemu-devel@nongnu.org, Wolfgang Bumiller , Thomas Lamprecht Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Kevin Wolf writes: > Am 22.03.2021 um 16:40 hat Stefan Reiter geschrieben: >> The QMP dispatcher coroutine holds the qmp_queue_lock over a yield >> point, where it expects to be rescheduled from the main context. If a >> CHR_EVENT_CLOSED event is received just then, it can race and block the >> main thread on the mutex in monitor_qmp_cleanup_queue_and_resume. >> >> monitor_resume does not need to be called from main context, so we can >> call it immediately after popping a request from the queue, which allows >> us to drop the qmp_queue_lock mutex before yielding. >> >> Suggested-by: Wolfgang Bumiller >> Signed-off-by: Stefan Reiter >> --- >> v2: >> * different approach: move everything that needs the qmp_queue_lock mutex before >> the yield point, instead of moving the event handling to a different context > > The interesting new case here seems to be that new requests could be > queued and the dispatcher coroutine could be kicked before yielding. > This is safe because &qmp_dispatcher_co_busy is accessed with atomics > on both sides. > > The important part is just that the first (conditional) yield stays > first, so that the aio_co_wake() in handle_qmp_command() won't reenter > the coroutine while it is expecting to be reentered from somewhere else. > This is still the case after the patch. > > Reviewed-by: Kevin Wolf Thanks for saving me from an ugly review headache. Should this go into 6.0?