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=-6.5 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS 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 50672C433DF for ; Thu, 9 Jul 2020 10:26:29 +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 21DCA206DF for ; Thu, 9 Jul 2020 10:26:29 +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="Vdd3TT/s" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 21DCA206DF 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]:59746 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jtTl6-0006wi-D4 for qemu-devel@archiver.kernel.org; Thu, 09 Jul 2020 06:26:28 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:57202) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jtTjo-0005sy-Kk for qemu-devel@nongnu.org; Thu, 09 Jul 2020 06:25:08 -0400 Received: from us-smtp-1.mimecast.com ([205.139.110.61]:31418 helo=us-smtp-delivery-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 1jtTjm-0007YU-B5 for qemu-devel@nongnu.org; Thu, 09 Jul 2020 06:25:08 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1594290305; 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=VYjJpbLmDRg8oBOiPHHX0aZz4NC0y0Ul0MEpxaJ8L4E=; b=Vdd3TT/selbZaIcroE1hl0MCmixa1Y3WqzthqCU6mcdVtWKylHeUmnNzzge+HL5yLj8NUH 9nyvE6fm8+wSyHJKOBUiII5QxKAs6PW9+XW34uqyz0C20YI0J5Pfh6OksAMqimc4026ale zvKFWXpjVVmfRw58OTvATF9BG35TznE= 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-52-Sm_zLCILOiqE8dZQ5O25MA-1; Thu, 09 Jul 2020 06:25:01 -0400 X-MC-Unique: Sm_zLCILOiqE8dZQ5O25MA-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 B94EF19253C5; Thu, 9 Jul 2020 10:25:00 +0000 (UTC) Received: from starship (unknown [10.35.206.232]) by smtp.corp.redhat.com (Postfix) with ESMTP id 144225C1C3; Thu, 9 Jul 2020 10:24:55 +0000 (UTC) Message-ID: Subject: Re: [PATCH v2 4/7] device-core: use atomic_set on .realized property From: Maxim Levitsky To: Stefan Hajnoczi Date: Thu, 09 Jul 2020 13:24:54 +0300 In-Reply-To: <20200527150053.GJ29137@stefanha-x1.localdomain> References: <20200511160951.8733-1-mlevitsk@redhat.com> <20200511160951.8733-5-mlevitsk@redhat.com> <20200527150053.GJ29137@stefanha-x1.localdomain> User-Agent: Evolution 3.34.4 (3.34.4-1.fc31) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mlevitsk@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=205.139.110.61; envelope-from=mlevitsk@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/09 04:20:09 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, 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=-1, 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: Fam Zheng , "Daniel P. =?ISO-8859-1?Q?Berrang=E9?=" , Eduardo Habkost , "Michael S. Tsirkin" , qemu-devel@nongnu.org, Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, 2020-05-27 at 16:00 +0100, Stefan Hajnoczi wrote: > On Mon, May 11, 2020 at 07:09:48PM +0300, Maxim Levitsky wrote: > > Some code might race with placement of new devices on a bus. > > We currently first place a (unrealized) device on the bus > > and then realize it. > > > > As a workaround, users that scan the child device list, can > > check the realized property to see if it is safe to access such a device. > > Use an atomic write here too to aid with this. > > > > A separate discussion is what to do with devices that are unrealized: > > It looks like for this case we only call the hotplug handler's unplug > > callback and its up to it to unrealize the device. > > An atomic operation doesn't cause harm for this code path though. > > > > Signed-off-by: Maxim Levitsky > > --- > > hw/core/qdev.c | 15 ++++++++++++++- > > 1 file changed, 14 insertions(+), 1 deletion(-) > > Please add a comment to struct DeviceState saying the realized field > must be accessed with atomic_load_acquire() when used outside the QEMU > global mutex. > > > diff --git a/hw/core/qdev.c b/hw/core/qdev.c > > index 732789e2b7..d530c5922f 100644 > > --- a/hw/core/qdev.c > > +++ b/hw/core/qdev.c > > @@ -964,7 +964,20 @@ static void device_set_realized(Object *obj, bool value, Error **errp) > > } > > } > > > > + atomic_store_release(&dev->realized, value); > > + > > } else if (!value && dev->realized) { > > + > > + /* > > + * Change the value so that any concurrent users are aware > > + * that the device is going to be unrealized > > + * > > + * TODO: change .realized property to enum that states > > + * each phase of the device realization/unrealization > > + */ > > + > > + atomic_store_release(&dev->realized, value); > > I'm not sure if atomic_store_release() is strong enough in the true -> > false case: > > Operations coming after ``atomic_store_release()`` can still be > reordered before it. > > A reader may already seen changes made to unrealize the DeviceState even > though realized still appears to be true. A full write memory barrier > seems safer here. Done. Best regards, Maxim Levitsky