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 B9749C54E58 for ; Mon, 25 Mar 2024 20:06:56 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1roqaG-0001tE-VQ; Mon, 25 Mar 2024 16:06:17 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1roqaE-0001sZ-Dc for qemu-devel@nongnu.org; Mon, 25 Mar 2024 16:06:15 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1roqaB-0007QE-Sf for qemu-devel@nongnu.org; Mon, 25 Mar 2024 16:06:13 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1711397168; 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=AVODJGUYd/VsvF2tPl6vDThtr/kvmRd+0KaDEHHmwRU=; b=bKLI3kVKM6g0KL8HrHQHvcB43KfEgh5Lgkl81dxOlQi84EQsC+g/0lsArjf+5vdozpV1av 6lKG4TnC13hYuS5LnoF4bhYK775uBxbJKyAc3CbZQTWOQE7A+RscYz39xfWTSS9o9fZMA4 VSn+Vw+6WSAf5v1wLC3JSMinGb2oxAE= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-374-URHS6XKtNRiQEaS5ymVGaA-1; Mon, 25 Mar 2024 16:06:06 -0400 X-MC-Unique: URHS6XKtNRiQEaS5ymVGaA-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 277478007A1; Mon, 25 Mar 2024 20:06:06 +0000 (UTC) Received: from localhost (unknown [10.39.192.114]) by smtp.corp.redhat.com (Postfix) with ESMTP id 97863492BD6; Mon, 25 Mar 2024 20:06:05 +0000 (UTC) Date: Mon, 25 Mar 2024 16:06:00 -0400 From: Stefan Hajnoczi To: Fiona Ebner Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, qemu-stable@nongnu.org, hreitz@redhat.com, kwolf@redhat.com, fam@euphon.net, t.lamprecht@proxmox.com, w.bumiller@proxmox.com Subject: Re: [PATCH v3 2/4] block-backend: fix edge case in bdrv_next() where BDS associated to BB changes Message-ID: <20240325200600.GB1944176@fedora> References: <20240322095009.346989-1-f.ebner@proxmox.com> <20240322095009.346989-3-f.ebner@proxmox.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="LVRyxxsVAdelT9ON" Content-Disposition: inline In-Reply-To: <20240322095009.346989-3-f.ebner@proxmox.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.9 Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.065, 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_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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --LVRyxxsVAdelT9ON Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Mar 22, 2024 at 10:50:07AM +0100, Fiona Ebner wrote: > The old_bs variable in bdrv_next() is currently determined by looking > at the old block backend. However, if the block graph changes before > the next bdrv_next() call, it might be that the associated BDS is not > the same that was referenced previously. In that case, the wrong BDS > is unreferenced, leading to an assertion failure later: >=20 > > bdrv_unref: Assertion `bs->refcnt > 0' failed. >=20 > In particular, this can happen in the context of bdrv_flush_all(), > when polling for bdrv_co_flush() in the generated co-wrapper leads to > a graph change (for example with a stream block job [0]). >=20 > A racy reproducer: >=20 > > #!/bin/bash > > rm -f /tmp/backing.qcow2 > > rm -f /tmp/top.qcow2 > > ./qemu-img create /tmp/backing.qcow2 -f qcow2 64M > > ./qemu-io -c "write -P42 0x0 0x1" /tmp/backing.qcow2 > > ./qemu-img create /tmp/top.qcow2 -f qcow2 64M -b /tmp/backing.qcow2 -F = qcow2 > > ./qemu-system-x86_64 --qmp stdio \ > > --blockdev qcow2,node-name=3Dnode0,file.driver=3Dfile,file.filename=3D/= tmp/top.qcow2 \ > > < > {"execute": "qmp_capabilities"} > > {"execute": "block-stream", "arguments": { "job-id": "stream0", "device= ": "node0" } } > > {"execute": "quit"} > > EOF >=20 > [0]: >=20 > > #0 bdrv_replace_child_tran (child=3D..., new_bs=3D..., tran=3D...) > > #1 bdrv_replace_node_noperm (from=3D..., to=3D..., auto_skip=3D..., tr= an=3D..., errp=3D...) > > #2 bdrv_replace_node_common (from=3D..., to=3D..., auto_skip=3D..., de= tach_subchain=3D..., errp=3D...) > > #3 bdrv_drop_filter (bs=3D..., errp=3D...) > > #4 bdrv_cor_filter_drop (cor_filter_bs=3D...) > > #5 stream_prepare (job=3D...) > > #6 job_prepare_locked (job=3D...) > > #7 job_txn_apply_locked (fn=3D..., job=3D...) > > #8 job_do_finalize_locked (job=3D...) > > #9 job_exit (opaque=3D...) > > #10 aio_bh_poll (ctx=3D...) > > #11 aio_poll (ctx=3D..., blocking=3D...) > > #12 bdrv_poll_co (s=3D...) > > #13 bdrv_flush (bs=3D...) > > #14 bdrv_flush_all () > > #15 do_vm_stop (state=3D..., send_stop=3D...) > > #16 vm_shutdown () >=20 > Signed-off-by: Fiona Ebner > --- >=20 > No changes in v3. > New in v2. >=20 > block/block-backend.c | 7 +++---- > 1 file changed, 3 insertions(+), 4 deletions(-) Reviewed-by: Stefan Hajnoczi --LVRyxxsVAdelT9ON Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmYB2SgACgkQnKSrs4Gr c8joawgAqw7wNEZfMdxjhOrtJQP7wwBN/gRZc+6359Pwx/3p7eyYp0++QJDXnopi 25cPPY6pn2JybQnUfFEWMv3Gwk0Z49uJUlh2By1TPxYAVwm60GaiaG5wv8swnDE1 zl0XxAmqVdMIycfyHQoVr6kMCZqn0wxVnb0AxsaUAJGsZCFE0AC+5FKqfh4SbDiF 98IaoLyxRCcHKTQm79OijBEDB9+oXGao5ZOu8QGjiK8S/Ym64g6udHZzpdzPfgZ6 uNRhkAjh77wypfuq4M0t4/LOSIqsZh034/b/6+RgylJaF71Ou2sKtGDODXHKmfIR oNnRl/7kUyLFrAavHz4V7/D+rMl3Eg== =dFG8 -----END PGP SIGNATURE----- --LVRyxxsVAdelT9ON--