From patchwork Thu Mar 24 12:57:56 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Hanna Czenczek X-Patchwork-Id: 12790669 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 DA727C433EF for ; Thu, 24 Mar 2022 12:58:59 +0000 (UTC) Received: from localhost ([::1]:35442 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nXN3K-0006lc-RG for qemu-devel@archiver.kernel.org; Thu, 24 Mar 2022 08:58:58 -0400 Received: from eggs.gnu.org ([209.51.188.92]:43448) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nXN2X-0005wK-3V for qemu-devel@nongnu.org; Thu, 24 Mar 2022 08:58:09 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:22275) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nXN2S-0005GM-VN for qemu-devel@nongnu.org; Thu, 24 Mar 2022 08:58:06 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1648126683; 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: content-transfer-encoding:content-transfer-encoding; bh=Y6mr8A+8rOJKKxvrF7yvXuTBbdquq4Pj/NlF3t1wm58=; b=WEk1iLXB0sEAg19KuvJLP3C8hO2UbpeMIrpN/qfhOzPo52ACj8FVfG4Tmi6a26OlX8J+ks m/9C0Bg/XUD5j1/dI27yvT4F1xQhlX/Y6VfCQAOWUH9Q8yD51nXv5+SZnIt6BAvyLstQuq URftAIeIDYTzdnJHJpueMo1Zevar9Go= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-542-QgcRz9-FMdmojz38x2f58A-1; Thu, 24 Mar 2022 08:58:02 -0400 X-MC-Unique: QgcRz9-FMdmojz38x2f58A-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id EC1A7106655B; Thu, 24 Mar 2022 12:58:01 +0000 (UTC) Received: from localhost (unknown [10.39.194.123]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 204C1111F3C7; Thu, 24 Mar 2022 12:57:57 +0000 (UTC) From: Hanna Reitz To: qemu-block@nongnu.org Subject: [PATCH] block/stream: Drain subtree around graph change Date: Thu, 24 Mar 2022 13:57:56 +0100 Message-Id: <20220324125756.9950-1-hreitz@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.78 on 10.11.54.3 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=hreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Received-SPF: pass client-ip=170.10.133.124; envelope-from=hreitz@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=-0.082, 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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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: , Cc: Kevin Wolf , Hanna Reitz , Vladimir Sementsov-Ogievskiy , John Snow , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" When the stream block job cuts out the nodes between top and base in stream_prepare(), it does not drain the subtree manually; it fetches the base node, and tries to insert it as the top node's backing node with bdrv_set_backing_hd(). bdrv_set_backing_hd() however will drain, and so the actual base node might change (because the base node is actually not part of the stream job) before the old base node passed to bdrv_set_backing_hd() is installed. This has two implications: First, the stream job does not keep a strong reference to the base node. Therefore, if it is deleted in bdrv_set_backing_hd()'s drain (e.g. because some other block job is drained to finish), we will get a use-after-free. We should keep a strong reference to that node. Second, even with such a strong reference, the problem remains that the base node might change before bdrv_set_backing_hd() actually runs and as a result the wrong base node is installed. Both effects can be seen in 030's TestParallelOps.test_overlapping_5() case, which has five nodes, and simultaneously streams from the middle node to the top node, and commits the middle node down to the base node. As it is, this will sometimes crash, namely when we encounter the above-described use-after-free. Taking a strong reference to the base node, we no longer get a crash, but the resuling block graph is less than ideal: The expected result is obviously that all middle nodes are cut out and the base node is the immediate backing child of the top node. However, if stream_prepare() takes a strong reference to its base node (the middle node), and then the commit job finishes in bdrv_set_backing_hd(), supposedly dropping that middle node, the stream job will just reinstall it again. Therefore, we need to keep the whole subtree drained in stream_prepare(), so that the graph modification it performs is effectively atomic, i.e. that the base node it fetches is still the base node when bdrv_set_backing_hd() sets it as the top node's backing node. Verify this by asserting in said 030's test case that the base node is always the top node's immediate backing child when both jobs are done. Signed-off-by: Hanna Reitz --- block/stream.c | 11 ++++++++++- tests/qemu-iotests/030 | 5 +++++ 2 files changed, 15 insertions(+), 1 deletion(-) diff --git a/block/stream.c b/block/stream.c index 3acb59fe6a..5c08208db9 100644 --- a/block/stream.c +++ b/block/stream.c @@ -64,7 +64,11 @@ static int stream_prepare(Job *job) bdrv_cor_filter_drop(s->cor_filter_bs); s->cor_filter_bs = NULL; + bdrv_subtree_drained_begin(s->above_base); + base = bdrv_filter_or_cow_bs(s->above_base); + bdrv_ref(base); + unfiltered_base = bdrv_skip_filters(base); if (bdrv_cow_child(unfiltered_bs)) { @@ -75,14 +79,19 @@ static int stream_prepare(Job *job) base_fmt = unfiltered_base->drv->format_name; } } + bdrv_set_backing_hd(unfiltered_bs, base, &local_err); ret = bdrv_change_backing_file(unfiltered_bs, base_id, base_fmt, false); if (local_err) { error_report_err(local_err); - return -EPERM; + ret = -EPERM; + goto out; } } +out: + bdrv_unref(base); + bdrv_subtree_drained_end(s->above_base); return ret; } diff --git a/tests/qemu-iotests/030 b/tests/qemu-iotests/030 index 567bf1da67..14112835ed 100755 --- a/tests/qemu-iotests/030 +++ b/tests/qemu-iotests/030 @@ -436,6 +436,11 @@ class TestParallelOps(iotests.QMPTestCase): self.vm.run_job(job='node4', auto_dismiss=True) self.assert_no_active_block_jobs() + # Assert that node0 is now the backing node of node4 + result = self.vm.qmp('query-named-block-nodes') + node4 = next(node for node in result['return'] if node['node-name'] == 'node4') + self.assertEqual(node4['image']['backing-image']['filename'], self.imgs[0]) + # Test a block-stream and a block-commit job in parallel # Here the stream job is supposed to finish quickly in order to reproduce # the scenario that triggers the bug fixed in 3d5d319e1221 and 1a63a907507