From patchwork Tue Dec 11 10:19:45 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Filipe Manana X-Patchwork-Id: 10723401 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id CB2941759 for ; Tue, 11 Dec 2018 10:19:55 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id B8E0229C6E for ; Tue, 11 Dec 2018 10:19:55 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id AC5C029CBE; Tue, 11 Dec 2018 10:19:55 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.0 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 3440929C6E for ; Tue, 11 Dec 2018 10:19:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726573AbeLKKTv (ORCPT ); Tue, 11 Dec 2018 05:19:51 -0500 Received: from mail.kernel.org ([198.145.29.99]:33092 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726397AbeLKKTt (ORCPT ); Tue, 11 Dec 2018 05:19:49 -0500 Received: from localhost.localdomain (bl8-197-74.dsl.telepac.pt [85.241.197.74]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 72E5F20811 for ; Tue, 11 Dec 2018 10:19:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1544523588; bh=AMeTwxsY6Gsb4QvI1hVZh2yIsuqw7rId0rnVv7V1bvU=; h=From:To:Subject:Date:In-Reply-To:References:From; b=JBAY3zHOgf9pV/5kWGJ0Io4vCtdMkSnz9u3EuJPpfSJBJhi22ZrKrVEIR1WPDt6/z J6P9GvyuOPsmM6ufmYMaNtCt+cLFur5Hb92jRw2os+2tHAW28ithDY3gAe64th6VA6 ngj6LsGUGIEN92cXXR33WcIWTIFYbKvAHM7tk48g= From: fdmanana@kernel.org To: linux-btrfs@vger.kernel.org Subject: [PATCH v2] Btrfs: send, fix race with transaction commits that create snapshots Date: Tue, 11 Dec 2018 10:19:45 +0000 Message-Id: <20181211101945.28695-1-fdmanana@kernel.org> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20181210175502.16620-1-fdmanana@kernel.org> References: <20181210175502.16620-1-fdmanana@kernel.org> Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP From: Filipe Manana If we create a snapshot of a snapshot currently being used by a send operation, we can end up with send failing unexpectedly (returning -ENOENT error to user space for example). The following diagram shows how this happens. CPU 1 CPU2 CPU3 btrfs_ioctl_send() (...) create_snapshot() -> creates snapshot of a root used by the send task btrfs_commit_transaction() create_pending_snapshot() __get_inode_info() btrfs_search_slot() btrfs_search_slot_get_root() down_read commit_root_sem get reference on eb of the commit root -> eb with bytenr == X up_read commit_root_sem btrfs_cow_block(root node) btrfs_free_tree_block() -> creates delayed ref to free the extent btrfs_run_delayed_refs() -> runs the delayed ref, adds extent to fs_info->pinned_extents btrfs_finish_extent_commit() unpin_extent_range() -> marks extent as free in the free space cache transaction commit finishes btrfs_start_transaction() (...) btrfs_cow_block() btrfs_alloc_tree_block() btrfs_reserve_extent() -> allocates extent at bytenr == X btrfs_init_new_buffer(bytenr X) btrfs_find_create_tree_block() alloc_extent_buffer(bytenr X) find_extent_buffer(bytenr X) -> returns existing eb, which the send task got (...) -> modifies content of the eb with bytenr == X -> uses an eb that now belongs to some other tree and no more matches the commit root of the snapshot, resuts will be unpredictable The consequences of this race can be various, and can lead to searches in the commit root performed by the send task failing unexpectedly (unable to find inode items, returning -ENOENT to user space, for example) or not failing because an inode item with the same number was added to the tree that reused the metadata extent, in which case send can behave incorrectly in the worst case or just fail later for some reason. Fix this by performing a copy of the commit root's extent buffer when doing a search in the context of a send operation. Signed-off-by: Filipe Manana --- V2: Updated comment and made it clone the extent buffer really only in the send context, and not for every search on the commit root (all searches other from the ones done by send are well behaved hold the commit_root_sem). fs/btrfs/ctree.c | 29 +++++++++++++++++++++++------ 1 file changed, 23 insertions(+), 6 deletions(-) diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c index 539901fb5165..99e7645ad94e 100644 --- a/fs/btrfs/ctree.c +++ b/fs/btrfs/ctree.c @@ -2584,14 +2584,27 @@ static struct extent_buffer *btrfs_search_slot_get_root(struct btrfs_root *root, root_lock = BTRFS_READ_LOCK; if (p->search_commit_root) { - /* The commit roots are read only so we always do read locks */ - if (p->need_commit_sem) + /* + * The commit roots are read only so we always do read locks, + * and we always must hold the commit_root_sem when doing + * searches on them, the only exception is send where we don't + * want to block transaction commits for a long time, so + * we need to clone the commit root in order to avoid races + * with transaction commits that create a snapshot of one of + * the roots used by a send operation. + */ + if (p->need_commit_sem) { down_read(&fs_info->commit_root_sem); - b = root->commit_root; - extent_buffer_get(b); - level = btrfs_header_level(b); - if (p->need_commit_sem) + b = btrfs_clone_extent_buffer(root->commit_root); up_read(&fs_info->commit_root_sem); + if (!b) + return ERR_PTR(-ENOMEM); + + } else { + b = root->commit_root; + extent_buffer_get(b); + } + level = btrfs_header_level(b); /* * Ensure that all callers have set skip_locking when * p->search_commit_root = 1. @@ -2717,6 +2730,10 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root, again: prev_cmp = -1; b = btrfs_search_slot_get_root(root, p, write_lock_level); + if (IS_ERR(b)) { + ret = PTR_ERR(b); + goto done; + } while (b) { level = btrfs_header_level(b);