From patchwork Tue Jan 20 18:05:33 2015 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: David Sterba X-Patchwork-Id: 5671441 Return-Path: X-Original-To: patchwork-linux-btrfs@patchwork.kernel.org Delivered-To: patchwork-parsemail@patchwork2.web.kernel.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.136]) by patchwork2.web.kernel.org (Postfix) with ESMTP id 9CE6BC058D for ; Tue, 20 Jan 2015 18:09:07 +0000 (UTC) Received: from mail.kernel.org (localhost [127.0.0.1]) by mail.kernel.org (Postfix) with ESMTP id 78CF9203C4 for ; Tue, 20 Jan 2015 18:09:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7C51E203AC for ; Tue, 20 Jan 2015 18:09:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753112AbbATSGu (ORCPT ); Tue, 20 Jan 2015 13:06:50 -0500 Received: from cantor2.suse.de ([195.135.220.15]:37169 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751621AbbATSFf (ORCPT ); Tue, 20 Jan 2015 13:05:35 -0500 Received: from relay1.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 37A25ADEA; Tue, 20 Jan 2015 18:05:34 +0000 (UTC) Received: by ds.suse.cz (Postfix, from userid 10065) id F1809DA84E; Tue, 20 Jan 2015 19:05:33 +0100 (CET) From: David Sterba To: linux-btrfs@vger.kernel.org Cc: guihc.fnst@cn.fujitsu.com, quwenruo@cn.fujitsu.com, miaoxie@huawei.com, David Sterba Subject: [PATCH 2/3] btrfs: Don't call btrfs_start_transaction() on frozen fs to avoid deadlock. Date: Tue, 20 Jan 2015 19:05:33 +0100 Message-Id: X-Mailer: git-send-email 2.1.3 In-Reply-To: References: Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org X-Spam-Status: No, score=-6.9 required=5.0 tests=BAYES_00, RCVD_IN_DNSWL_HI, T_RP_MATCHES_RCVD, UNPARSEABLE_RELAY autolearn=unavailable version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on mail.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP From: Qu Wenruo Commit 6b5fe46dfa52 (btrfs: do commit in sync_fs if there are pending changes) will call btrfs_start_transaction() in sync_fs(), to handle some operations needed to be done in next transaction. However this can cause deadlock if the filesystem is frozen, with the following sys_r+w output: [ 143.255932] Call Trace: [ 143.255936] [] schedule+0x29/0x70 [ 143.255939] [] __sb_start_write+0xb3/0x100 [ 143.255971] [] start_transaction+0x2e6/0x5a0 [btrfs] [ 143.255992] [] btrfs_start_transaction+0x1b/0x20 [btrfs] [ 143.256003] [] btrfs_sync_fs+0xca/0xd0 [btrfs] [ 143.256007] [] sync_fs_one_sb+0x20/0x30 [ 143.256011] [] iterate_supers+0xe1/0xf0 [ 143.256014] [] sys_sync+0x55/0x90 [ 143.256017] [] system_call_fastpath+0x12/0x17 [ 143.256111] Call Trace: [ 143.256114] [] schedule+0x29/0x70 [ 143.256119] [] rwsem_down_write_failed+0x1c5/0x2d0 [ 143.256123] [] call_rwsem_down_write_failed+0x13/0x20 [ 143.256131] [] thaw_super+0x28/0xc0 [ 143.256135] [] do_vfs_ioctl+0x3f5/0x540 [ 143.256187] [] SyS_ioctl+0x91/0xb0 [ 143.256213] [] system_call_fastpath+0x12/0x17 The reason is like the following: (Holding s_umount) VFS sync_fs staff: |- btrfs_sync_fs() |- btrfs_start_transaction() |- sb_start_intwrite() (Waiting thaw_fs to unfreeze) VFS thaw_fs staff: thaw_fs() (Waiting sync_fs to release s_umount) So deadlock happens. This can be easily triggered by fstest/generic/068 with inode_cache mount option. The fix is to check if the fs is frozen, if the fs is frozen, just return and waiting for the next transaction. Cc: David Sterba Reported-by: Gui Hecheng Signed-off-by: Qu Wenruo [enhanced comment, changed to SB_FREEZE_WRITE] Signed-off-by: David Sterba --- fs/btrfs/super.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c index c86fb5438454..6f49b2872a64 100644 --- a/fs/btrfs/super.c +++ b/fs/btrfs/super.c @@ -1000,6 +1000,16 @@ int btrfs_sync_fs(struct super_block *sb, int wait) */ if (fs_info->pending_changes == 0) return 0; + /* + * A non-blocking test if the fs is frozen. We must not + * start a new transaction here otherwise a deadlock + * happens. The pending operations are delayed to the + * next commit after thawing. + */ + if (__sb_start_write(sb, SB_FREEZE_WRITE, false)) + __sb_end_write(sb, SB_FREEZE_WRITE); + else + return 0; trans = btrfs_start_transaction(root, 0); } if (IS_ERR(trans))