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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 3052DC2BA2B for ; Thu, 16 Apr 2020 20:37:20 +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 EC9BA206A2 for ; Thu, 16 Apr 2020 20:37:19 +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="K39XJpsV" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EC9BA206A2 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]:39082 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jPBGB-0000LX-1Y for qemu-devel@archiver.kernel.org; Thu, 16 Apr 2020 16:37:19 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:60478) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jPBFV-00080T-Il for qemu-devel@nongnu.org; Thu, 16 Apr 2020 16:36:38 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jPBFT-00040R-GM for qemu-devel@nongnu.org; Thu, 16 Apr 2020 16:36:36 -0400 Received: from us-smtp-1.mimecast.com ([207.211.31.81]:32886 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jPBFT-0003zp-9z for qemu-devel@nongnu.org; Thu, 16 Apr 2020 16:36:35 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1587069394; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=PMs7kMb35HuCdMRBCSKzXefdpwtPTgWF2vukymgQgys=; b=K39XJpsVvIRviLy0anEM7AntPp34bsRLNflmfaSR5ECAkqUh7RXPiq0aeFMpJ8b7BiKQRL pveiFT8rGy4WfAZ+9JoLc9W3hgJf7CY0RU5p5jUUaxkaqG0pUCIXXgVQlpdikHJ4KrxXhw k2IX0VgBCWaQdD1wOSqje8738aA6GuI= 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-23-tA_sbelSNwW7W2MjZZGntw-1; Thu, 16 Apr 2020 16:36:32 -0400 X-MC-Unique: tA_sbelSNwW7W2MjZZGntw-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 65A9A800D53; Thu, 16 Apr 2020 20:36:31 +0000 (UTC) Received: from maximlenovopc.usersys.redhat.com (unknown [10.35.206.56]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4335CA09A5; Thu, 16 Apr 2020 20:36:25 +0000 (UTC) From: Maxim Levitsky To: qemu-devel@nongnu.org Subject: [PATCH 0/4] RFC/WIP: Fix scsi devices plug/unplug races w.r.t virtio-scsi iothread Date: Thu, 16 Apr 2020 23:36:20 +0300 Message-Id: <20200416203624.32366-1-mlevitsk@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 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 , =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= , Eduardo Habkost , "Michael S. Tsirkin" , Maxim Levitsky , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Hi! This is a patch series that is a result of my discussion with Paulo on how to correctly fix the root cause of the BZ #1812399. The root cause of this bug is the fact that IO thread is running mostly unlocked versus main thread on which device hotplug is done. qdev_device_add first creates the device object, then places it on the bus, and only then realizes it. However some drivers and currently only virtio-scsi enumerate its child bus devices on each request that is received from the guest and that can happen= on the IO thread. Thus we have a window when new device is on the bus but not realized and ca= n be accessed by the virtio-scsi driver in that state. Fix that by doing two things: 1. Add partial RCU protection to the list of a bus's child devices. This allows the scsi IO thread to safely enumerate the child devices while it races with the hotplug placing the device on the bus. 2. Make the virtio-scsi driver check .realized property of the scsi device and avoid touching the device if it isn't I don't think that this is very pretty way to solve this, we discussed this with Paulo and it kind of looks like the lesser evil. I am open to your tho= ughts about this. Note that this patch series doesn't pass some unit tests and in particular = qtest 'drive_del-test' I did some light debug of this test and I see that the reason for this is t= hat now child device deletion can be delayed due to RCU. This is also something I would like to discuss i= n this RFC. Note also that I might have some code style errors and bugs in this since I= haven't tested the code in depth yet, because I am not yet sure that this is the ri= ght way to fix that bug Also note that in the particular bug report the issue wasn't a race but rat= her due to combination of things, the .realize code in the middle managed to trigge= r IO on the virtqueue which caused the virtio-scsi driver to access the half realized device. How= ever since this can happen as well with real IO thread, this patch series was do= ne, which fixes this as well. Best regards, =09Maxim Levitsky Maxim Levitsky (4): scsi/scsi_bus: switch search direction in scsi_device_find device-core: use RCU for list of childs of a bus device-core: use atomic_set on .realized property virtio-scsi: don't touch scsi devices that are not yet realized hw/core/bus.c | 43 ++++++++++++++++++++---------- hw/core/qdev.c | 48 ++++++++++++++++++++++------------ hw/scsi/scsi-bus.c | 27 ++++++++++++++++--- hw/scsi/virtio-scsi.c | 24 +++++++++++++++-- include/hw/qdev-core.h | 3 +++ include/hw/virtio/virtio-bus.h | 7 +++-- 6 files changed, 114 insertions(+), 38 deletions(-) --=20 2.17.2