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bh=/FiHgfxsqyDyfxEXB1QurImZiK6ee8uiHeIHcWP8gCg=; b=P2hnViXodUydoxTGg5RqH0qM3Tu6iPxld5Zz5L6BeVT+FUXktwHI99/+STMEO5CG5cOPRW Qhpm/UWDGVl3PSBJ0BFEEe4zU7DnXQQjHoxgMl8Ezf1V7mFjwqgKs7NwnZXJSoSQmqaszB YYoEyWB0XnWr8tB/Cb1q8/6E5Iudp68= X-MC-Unique: 5o6xs8wXMs-BxSFhdkKsYg-1 From: Max Reitz To: qemu-block@nongnu.org Subject: [PULL 15/15] block: make bdrv_drop_intermediate() less wrong Date: Mon, 9 Nov 2020 18:38:39 +0100 Message-Id: <20201109173839.2135984-16-mreitz@redhat.com> In-Reply-To: <20201109173839.2135984-1-mreitz@redhat.com> References: <20201109173839.2135984-1-mreitz@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=216.205.24.124; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/11/09 00:04:29 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Peter Maydell , qemu-devel@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail-DKIM: fail (Header signature does not verify) Content-Type: text/plain; charset="utf-8" From: Vladimir Sementsov-Ogievskiy First, permission update loop tries to do iterations transactionally, but the whole update is not transactional: nobody roll-back successful loop iterations when some iteration fails. Second, in the iteration we have nested permission update: c->klass->update_filename may point to bdrv_child_cb_update_filename() which calls bdrv_backing_update_filename(), which may do node reopen to RW. Permission update system is not prepared to nested updates, at least it has intermediate permission-update state stored in BdrvChild structures: has_backup_perm, backup_perm and backup_shared_perm. So, let's first do bdrv_replace_node_common() (which is more transactional than open-coded update in bdrv_drop_intermediate()) and then call update_filename() in separate. We still do not rollback changes in case of update_filename() failure but it's not much worse than pre-patch behavior. Note that bdrv_replace_node_common() does check for frozen children, so corresponding check is dropped in bdrv_drop_intermediate(). Signed-off-by: Vladimir Sementsov-Ogievskiy Message-Id: <20201106124241.16950-4-vsementsov@virtuozzo.com> Reviewed-by: Alberto Garcia Signed-off-by: Max Reitz --- block.c | 54 ++++++++++++++++++++++++------------------------------ 1 file changed, 24 insertions(+), 30 deletions(-) diff --git a/block.c b/block.c index 9a945a058d..f1cedac362 100644 --- a/block.c +++ b/block.c @@ -4910,9 +4910,11 @@ int bdrv_drop_intermediate(BlockDriverState *top, Bl= ockDriverState *base, { BlockDriverState *explicit_top =3D top; bool update_inherits_from; - BdrvChild *c, *next; + BdrvChild *c; Error *local_err =3D NULL; int ret =3D -EIO; + g_autoptr(GSList) updated_children =3D NULL; + GSList *p; =20 bdrv_ref(top); bdrv_subtree_drained_begin(top); @@ -4926,14 +4928,6 @@ int bdrv_drop_intermediate(BlockDriverState *top, Bl= ockDriverState *base, goto exit; } =20 - /* This function changes all links that point to top and makes - * them point to base. Check that none of them is frozen. */ - QLIST_FOREACH(c, &top->parents, next_parent) { - if (c->frozen) { - goto exit; - } - } - /* If 'base' recursively inherits from 'top' then we should set * base->inherits_from to top->inherits_from after 'top' and all * other intermediate nodes have been dropped. @@ -4950,36 +4944,36 @@ int bdrv_drop_intermediate(BlockDriverState *top, B= lockDriverState *base, backing_file_str =3D base->filename; } =20 - QLIST_FOREACH_SAFE(c, &top->parents, next_parent, next) { - /* Check whether we are allowed to switch c from top to base */ - GSList *ignore_children =3D g_slist_prepend(NULL, c); - ret =3D bdrv_check_update_perm(base, NULL, c->perm, c->shared_perm, - ignore_children, NULL, &local_err); - g_slist_free(ignore_children); - if (ret < 0) { - error_report_err(local_err); - goto exit; - } + QLIST_FOREACH(c, &top->parents, next_parent) { + updated_children =3D g_slist_prepend(updated_children, c); + } + + bdrv_replace_node_common(top, base, false, &local_err); + if (local_err) { + error_report_err(local_err); + goto exit; + } + + for (p =3D updated_children; p; p =3D p->next) { + c =3D p->data; =20 - /* If so, update the backing file path in the image file */ if (c->klass->update_filename) { ret =3D c->klass->update_filename(c, base, backing_file_str, &local_err); if (ret < 0) { - bdrv_abort_perm_update(base); + /* + * TODO: Actually, we want to rollback all previous iterat= ions + * of this loop, and (which is almost impossible) previous + * bdrv_replace_node()... + * + * Note, that c->klass->update_filename may lead to permis= sion + * update, so it's a bad idea to call it inside permission + * update transaction of bdrv_replace_node. + */ error_report_err(local_err); goto exit; } } - - /* - * Do the actual switch in the in-memory graph. - * Completes bdrv_check_update_perm() transaction internally. - * c->frozen is false, we have checked that above. - */ - bdrv_ref(base); - bdrv_replace_child(c, base); - bdrv_unref(top); } =20 if (update_inherits_from) { --=20 2.28.0