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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 6ACF7C433EF for ; Thu, 27 Jan 2022 14:29:52 +0000 (UTC) Received: from localhost ([::1]:59314 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nD5mZ-0006e9-JV for qemu-devel@archiver.kernel.org; Thu, 27 Jan 2022 09:29:51 -0500 Received: from eggs.gnu.org ([209.51.188.92]:52618) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nD5Xk-0007yA-NN for qemu-devel@nongnu.org; Thu, 27 Jan 2022 09:14:33 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:22631) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nD5Xj-00028V-7i for qemu-devel@nongnu.org; Thu, 27 Jan 2022 09:14:32 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1643292870; 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=/4YQBd0EOyjDHGoIozj1n0eDeMIx6cbGfzYD1y0BC0g=; b=W9cW4kyZV5j6szb+mw5Te/A0HL0twRu0skqMY9xvjYjcS0X45hA7iRhztfr7n7BIVtw4vf BSbuYCUq+YCIjLGGXTdZDkVa+vp5OHkPE6SGI4k2tpBQr4xILiIxMP6Ogou4di/ros2Mpc t3jrRcBaaAqF1Xp7oxdGtUx5FmTWStk= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-180-sbATfuhMO76XMICttMkaDQ-1; Thu, 27 Jan 2022 09:14:27 -0500 X-MC-Unique: sbATfuhMO76XMICttMkaDQ-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 31CAE84B9A5; Thu, 27 Jan 2022 14:14:26 +0000 (UTC) Received: from redhat.com (unknown [10.39.195.252]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B6D3584651; Thu, 27 Jan 2022 14:14:15 +0000 (UTC) Date: Thu, 27 Jan 2022 15:14:14 +0100 From: Kevin Wolf To: Vladimir Sementsov-Ogievskiy Subject: Re: [PATCH] block: bdrv_set_backing_hd(): use drained section Message-ID: References: <20220124173741.2984056-1-vsementsov@virtuozzo.com> MIME-Version: 1.0 In-Reply-To: <20220124173741.2984056-1-vsementsov@virtuozzo.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 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: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.159, 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: den@openvz.org, hreitz@redhat.com, qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 24.01.2022 um 18:37 hat Vladimir Sementsov-Ogievskiy geschrieben: > Graph modifications should be done in drained section. stream_prepare() > handler of block stream job call bdrv_set_backing_hd() without using > drained section and it's theoretically possible that some IO request > will interleave with graph modification and will use outdated pointers > to removed block nodes. > > Some other callers use bdrv_set_backing_hd() not caring about drained > sections too. So it seems good to make a drained section exactly in > bdrv_set_backing_hd(). > > Signed-off-by: Vladimir Sementsov-Ogievskiy Thanks, applied to the block branch. > Hi all! > > We faced the following bug in our Rhel7-based downstream: > read request crashes because backing bs is NULL unexpectedly (honestly, > it crashes inside bdrv_is_allocated(), which is called during read and > it's a downstream-only code, but that doesn't make real sense). > > In gdb I also see block-stream job in state > "refcnt = 0, status = JOB_STATUS_NULL", but it's still in jobs list. > > So, I assume that backing file was disappeared exactly as final step of > block-stream job. And the problem is that this step should be done in > drained section, but seems that it isn't. > > If we have a drained section, we'd wait for finish of read request > before removing the backing node. > > I don't have a reproducer. I spent some time to write a test, but there > are problems that makes hard to use blkdebug's break-points: we have > drained section at block-stream start, and we do have drained section at > block-stream finish: bdrv_cor_filter_drop() called from stream_prepare() > does drained section (unlike bdrv_set_backing_hd()). Maybe a unit test would be easier to write for this kind of thing than an iotest? > So, the fix is intuitive. I think, it's correct) > > Note also, that alternative would be to make a drained section in > stream_prepare() and don't touch bdrv_set_backing_hd() function. But it > seems good to make a public graph-modification function more safe. Yes, makes sense to me. Kevin