From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cn.fujitsu.com ([59.151.112.132]:26740 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1751627AbcG0IgQ (ORCPT ); Wed, 27 Jul 2016 04:36:16 -0400 Received: from G08CNEXCHPEKD01.g08.fujitsu.local (unknown [10.167.33.80]) by cn.fujitsu.com (Postfix) with ESMTP id 344AB4295FA0 for ; Wed, 27 Jul 2016 16:36:08 +0800 (CST) Subject: Re: [PATCH v2] btrfs: fix fsfreeze hang caused by delayed iputs deal To: References: <20160727081036.5321-1-wangxg.fnst@cn.fujitsu.com> From: Wang Xiaoguang Message-ID: <579871CC.8000103@cn.fujitsu.com> Date: Wed, 27 Jul 2016 16:33:16 +0800 MIME-Version: 1.0 In-Reply-To: <20160727081036.5321-1-wangxg.fnst@cn.fujitsu.com> Content-Type: text/plain; charset="windows-1252"; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: hello, On 07/27/2016 04:10 PM, Wang Xiaoguang wrote: > When running fstests generic/068, sometimes we got below deadlock: > xfs_io D ffff8800331dbb20 0 6697 6693 0x00000080 > ffff8800331dbb20 ffff88007acfc140 ffff880034d895c0 ffff8800331dc000 > ffff880032d243e8 fffffffeffffffff ffff880032d24400 0000000000000001 > ffff8800331dbb38 ffffffff816a9045 ffff880034d895c0 ffff8800331dbba8 > Call Trace: > [] schedule+0x35/0x80 > [] rwsem_down_read_failed+0xf2/0x140 > [] ? __filemap_fdatawrite_range+0xd1/0x100 > [] call_rwsem_down_read_failed+0x18/0x30 > [] ? btrfs_alloc_block_rsv+0x2c/0xb0 [btrfs] > [] percpu_down_read+0x35/0x50 > [] __sb_start_write+0x2c/0x40 > [] start_transaction+0x2a5/0x4d0 [btrfs] > [] btrfs_join_transaction+0x17/0x20 [btrfs] > [] btrfs_evict_inode+0x3c4/0x5d0 [btrfs] > [] evict+0xba/0x1a0 > [] iput+0x196/0x200 > [] btrfs_run_delayed_iputs+0x70/0xc0 [btrfs] > [] btrfs_commit_transaction+0x928/0xa80 [btrfs] > [] btrfs_freeze+0x30/0x40 [btrfs] > [] freeze_super+0xf0/0x190 > [] do_vfs_ioctl+0x4a5/0x5c0 > [] ? do_audit_syscall_entry+0x66/0x70 > [] ? syscall_trace_enter_phase1+0x11f/0x140 > [] SyS_ioctl+0x79/0x90 > [] do_syscall_64+0x62/0x110 > [] entry_SYSCALL64_slow_path+0x25/0x25 > > From this warning, freeze_super() already holds SB_FREEZE_FS, but > btrfs_freeze() will call btrfs_commit_transaction() again, if > btrfs_commit_transaction() finds that it has delayed iputs to handle, > it'll start_transaction(), which will try to get SB_FREEZE_FS lock > again, then deadlock occurs. > > The root cause is that in btrfs, sync_filesystem(sb) does not make > sure all metadata is updated. There still maybe some codes adding > delayed iputs, see below sample race window: > > CPU1 | CPU2 > |-> freeze_super() | > |-> sync_filesystem(sb); | > | |-> cleaner_kthread() > | | |-> btrfs_delete_unused_bgs() > | | |-> btrfs_remove_chunk() > | | |-> btrfs_remove_block_group() > | | |-> btrfs_add_delayed_iput() > | | > |-> sb->s_writers.frozen = SB_FREEZE_FS; | > |-> sb_wait_write(sb, SB_FREEZE_FS); | > | acquire SB_FREEZE_FS lock. | > | | > |-> btrfs_freeze() | > |-> btrfs_commit_transaction() | > |-> btrfs_run_delayed_iputs() | > | will handle delayed iputs, | > | that means start_transaction() | > | will be called, which will try | > | to get SB_FREEZE_FS lock. | > > To fix this issue, in btrfs_commit_transaction(), if fs already is in SB_FREEZE_FS, > we do not handle delayed iputs. > > Signed-off-by: Wang Xiaoguang > --- > fs/btrfs/transaction.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c > index 948aa18..e6931d9 100644 > --- a/fs/btrfs/transaction.c > +++ b/fs/btrfs/transaction.c > @@ -2277,8 +2277,13 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans, > > kmem_cache_free(btrfs_trans_handle_cachep, trans); > > + /* > + * If s_writers.frozen >= SB_FREEZE_FS, we can not handle delayed > + * iputs, otherwise it'll result in deallock about SB_FREEZE_FS. > + */ > if (current != root->fs_info->transaction_kthread && > - current != root->fs_info->cleaner_kthread) > + current != root->fs_info->cleaner_kthread && > + root->fs_info->sb->s_writers.frozen >= SB_FREEZE_FS) > btrfs_run_delayed_iputs(root); Sorry, I sent this too quickly... Above deadlock still occurs, please wait a while. Regards, Xiaoguang Wang > > return ret;