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=-8.6 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 55BD2C43461 for ; Wed, 7 Apr 2021 13:21:32 +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 7C4B361242 for ; Wed, 7 Apr 2021 13:21:31 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7C4B361242 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]:53486 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lU87e-0006qe-6b for qemu-devel@archiver.kernel.org; Wed, 07 Apr 2021 09:21:30 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:38726) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lU85j-0006FP-FF for qemu-devel@nongnu.org; Wed, 07 Apr 2021 09:19:31 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:36305) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lU85b-00053A-C1 for qemu-devel@nongnu.org; Wed, 07 Apr 2021 09:19:28 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1617801559; 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=HpnSpOWQ36RrKI7unWAGW5I3lGgrhO3FfpdGO0jVwWo=; b=ViVWSDBQia3D5n41ckqa5sgkJ/YtVtFS4Vc4c2BWbJZfBCeIHPS+V72qk29LyfpGf+CQYp GVG1EJRkqrb3xCuMV8gjxayhh1XIz+YuV3MXJxZqhAJJFpdXXiv+u83rvz0gUiBBY/+vZo p5/5HGo2Ji6GbFNEXni+KYjxFSLmCZA= 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-462-rweiGcwRND-WcGMSdCA9jA-1; Wed, 07 Apr 2021 09:19:16 -0400 X-MC-Unique: rweiGcwRND-WcGMSdCA9jA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A3018BBEF1; Wed, 7 Apr 2021 13:19:15 +0000 (UTC) Received: from merkur.fritz.box (ovpn-115-47.ams2.redhat.com [10.36.115.47]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0CAD460C04; Wed, 7 Apr 2021 13:19:13 +0000 (UTC) Date: Wed, 7 Apr 2021 15:19:12 +0200 From: Kevin Wolf To: Stefan Reiter Subject: Re: [PATCH v2] monitor/qmp: fix race on CHR_EVENT_CLOSED without OOB Message-ID: References: <20210322154024.15011-1-s.reiter@proxmox.com> MIME-Version: 1.0 In-Reply-To: <20210322154024.15011-1-s.reiter@proxmox.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=kwolf@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@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: qemu-devel@nongnu.org, Paolo Bonzini , Thomas Lamprecht , Markus Armbruster , Wolfgang Bumiller Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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