From nobody Fri Oct 24 22:17:01 2025 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) client-ip=208.118.235.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zohomail.com; spf=pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=fail(p=none dis=none) header.from=redhat.com Return-Path: Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) by mx.zohomail.com with SMTPS id 1519841290183341.55719872845305; Wed, 28 Feb 2018 10:08:10 -0800 (PST) Received: from localhost ([::1]:45949 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1er699-0005nV-RN for importer@patchew.org; Wed, 28 Feb 2018 13:08:07 -0500 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41750) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1er670-0004Ts-T2 for qemu-devel@nongnu.org; Wed, 28 Feb 2018 13:05:56 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1er66z-0004K2-P3 for qemu-devel@nongnu.org; Wed, 28 Feb 2018 13:05:54 -0500 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:38032 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1er66u-0004HU-VI; Wed, 28 Feb 2018 13:05:49 -0500 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 88A7E8D6D6; Wed, 28 Feb 2018 18:05:48 +0000 (UTC) Received: from localhost (unknown [10.40.205.159]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 29E302026612; Wed, 28 Feb 2018 18:05:48 +0000 (UTC) From: Max Reitz To: qemu-block@nongnu.org Date: Wed, 28 Feb 2018 19:05:00 +0100 Message-Id: <20180228180507.3964-10-mreitz@redhat.com> In-Reply-To: <20180228180507.3964-1-mreitz@redhat.com> References: <20180228180507.3964-1-mreitz@redhat.com> X-Scanned-By: MIMEDefang 2.78 on 10.11.54.4 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.2]); Wed, 28 Feb 2018 18:05:48 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.2]); Wed, 28 Feb 2018 18:05:48 +0000 (UTC) for IP:'10.11.54.4' DOMAIN:'int-mx04.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'mreitz@redhat.com' RCPT:'' X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 66.187.233.73 Subject: [Qemu-devel] [PATCH v3 09/16] block: Generalize should_update_child() rule X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Fam Zheng , qemu-devel@nongnu.org, Max Reitz , Stefan Hajnoczi , John Snow Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RSF_0 Z_629925259 SPT_0 Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Currently, bdrv_replace_node() refuses to create loops from one BDS to itself if the BDS to be replaced is the backing node of the BDS to replace it: Say there is a node A and a node B. Replacing B by A means making all references to B point to A. If B is a child of A (i.e. A has a reference to B), that would mean we would have to make this reference point to A itself -- so we'd create a loop. bdrv_replace_node() (through should_update_child()) refuses to do so if B is the backing node of A. There is no reason why we should create loops if B is not the backing node of A, though. The BDS graph should never contain loops, so we should always refuse to create them. If B is a child of A and B is to be replaced by A, we should simply leave B in place there because it is the most sensible choice. A more specific argument would be: Putting filter drivers into the BDS graph is basically the same as appending an overlay to a backing chain. But the main child BDS of a filter driver is not "backing" but "file", so restricting the no-loop rule to backing nodes would fail here. Signed-off-by: Max Reitz --- include/block/block_int.h | 2 ++ block.c | 44 ++++++++++++++++++++++++++++++++++---------- 2 files changed, 36 insertions(+), 10 deletions(-) diff --git a/include/block/block_int.h b/include/block/block_int.h index 5ae7738cf8..7767fc28d3 100644 --- a/include/block/block_int.h +++ b/include/block/block_int.h @@ -601,6 +601,8 @@ struct BdrvChild { QLIST_ENTRY(BdrvChild) next_parent; }; =20 +typedef QLIST_HEAD(BdrvChildList, BdrvChild) BdrvChildList; + /* * Note: the function bdrv_append() copies and swaps contents of * BlockDriverStates, so if you add new fields to this struct, please diff --git a/block.c b/block.c index 814e5a02da..d41193025f 100644 --- a/block.c +++ b/block.c @@ -3323,16 +3323,39 @@ static bool should_update_child(BdrvChild *c, Block= DriverState *to) return false; } =20 - if (c->role =3D=3D &child_backing) { - /* If @from is a backing file of @to, ignore the child to avoid - * creating a loop. We only want to change the pointer of other - * parents. */ - QLIST_FOREACH(to_c, &to->children, next) { - if (to_c =3D=3D c) { - break; - } - } - if (to_c) { + /* If the child @c belongs to the BDS @to, replacing the current + * c->bs by @to would mean to create a loop. + * + * Such a case occurs when appending a BDS to a backing chain. + * For instance, imagine the following chain: + * + * guest device -> node A -> further backing chain... + * + * Now we create a new BDS B which we want to put on top of this + * chain, so we first attach A as its backing node: + * + * node B + * | + * v + * guest device -> node A -> further backing chain... + * + * Finally we want to replace A by B. When doing that, we want to + * replace all pointers to A by pointers to B -- except for the + * pointer from B because (1) that would create a loop, and (2) + * that pointer should simply stay intact: + * + * guest device -> node B + * | + * v + * node A -> further backing chain... + * + * In general, when replacing a node A (c->bs) by a node B (@to), + * if A is a child of B, that means we cannot replace A by B there + * because that would create a loop. Silently detaching A from B + * is also not really an option. So overall just leaving A in + * place there is the most sensible choice. */ + QLIST_FOREACH(to_c, &to->children, next) { + if (to_c =3D=3D c) { return false; } } @@ -3358,6 +3381,7 @@ void bdrv_replace_node(BlockDriverState *from, BlockD= riverState *to, =20 /* Put all parents into @list and calculate their cumulative permissio= ns */ QLIST_FOREACH_SAFE(c, &from->parents, next_parent, next) { + assert(c->bs =3D=3D from); if (!should_update_child(c, to)) { continue; } --=20 2.14.3