Re: [PATCH 1/2] btrfs: fix fsfreeze hang caused by delayed iputs deal

[Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>]

hello,

On 07/05/2016 01:35 AM, David Sterba wrote:
> On Wed, Jun 29, 2016 at 01:15:10PM +0800, Wang Xiaoguang wrote:
>> When running fstests generic/068, sometimes we got below WARNING:
>>    xfs_io          D ffff8800331dbb20     0  6697   6693 0x00000080
>>    ffff8800331dbb20 ffff88007acfc140 ffff880034d895c0 ffff8800331dc000
>>    ffff880032d243e8 fffffffeffffffff ffff880032d24400 0000000000000001
>>    ffff8800331dbb38 ffffffff816a9045 ffff880034d895c0 ffff8800331dbba8
>>    Call Trace:
>>    [<ffffffff816a9045>] schedule+0x35/0x80
>>    [<ffffffff816abab2>] rwsem_down_read_failed+0xf2/0x140
>>    [<ffffffff8118f5e1>] ? __filemap_fdatawrite_range+0xd1/0x100
>>    [<ffffffff8134f978>] call_rwsem_down_read_failed+0x18/0x30
>>    [<ffffffffa06631fc>] ? btrfs_alloc_block_rsv+0x2c/0xb0 [btrfs]
>>    [<ffffffff810d32b5>] percpu_down_read+0x35/0x50
>>    [<ffffffff81217dfc>] __sb_start_write+0x2c/0x40
>>    [<ffffffffa067f5d5>] start_transaction+0x2a5/0x4d0 [btrfs]
>>    [<ffffffffa067f857>] btrfs_join_transaction+0x17/0x20 [btrfs]
>>    [<ffffffffa068ba34>] btrfs_evict_inode+0x3c4/0x5d0 [btrfs]
>>    [<ffffffff81230a1a>] evict+0xba/0x1a0
>>    [<ffffffff812316b6>] iput+0x196/0x200
>>    [<ffffffffa06851d0>] btrfs_run_delayed_iputs+0x70/0xc0 [btrfs]
>>    [<ffffffffa067f1d8>] btrfs_commit_transaction+0x928/0xa80 [btrfs]
>>    [<ffffffffa0646df0>] btrfs_freeze+0x30/0x40 [btrfs]
>>    [<ffffffff81218040>] freeze_super+0xf0/0x190
>>    [<ffffffff81229275>] do_vfs_ioctl+0x4a5/0x5c0
>>    [<ffffffff81003176>] ? do_audit_syscall_entry+0x66/0x70
>>    [<ffffffff810038cf>] ? syscall_trace_enter_phase1+0x11f/0x140
>>    [<ffffffff81229409>] SyS_ioctl+0x79/0x90
>>    [<ffffffff81003c12>] do_syscall_64+0x62/0x110
>>    [<ffffffff816acbe1>] 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. See below race window in freeze_super():
>> 	sync_filesystem(sb);
>> 		|
>> 		| race window
>> 		| In this period, cleaner_kthread() may be scheduled to
>> 		| run, and it call btrfs_delete_unused_bgs() which will
>> 		| add some delayed iputs.
>> 		|
>> 	sb->s_writers.frozen = SB_FREEZE_FS;
>> 	sb_wait_write(sb, SB_FREEZE_FS);
>> 	if (sb->s_op->freeze_fs) {
>> 		/* freeze_fs will call btrfs_commit_transaction() */
>> 		ret = sb->s_op->freeze_fs(sb);
>>
>> So if btrfs is doing freeze job, we should block
>> btrfs_delete_unused_bgs(), to avoid add delayed iputs.
>>
>> Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
>> ---
>>   fs/btrfs/disk-io.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
>> index 863bf7a..fdbe0df 100644
>> --- a/fs/btrfs/disk-io.c
>> +++ b/fs/btrfs/disk-io.c
>> @@ -1846,8 +1846,11 @@ static int cleaner_kthread(void *arg)
>>   		 * after acquiring fs_info->delete_unused_bgs_mutex. So we
>>   		 * can't hold, nor need to, fs_info->cleaner_mutex when deleting
>>   		 * unused block groups.
>> +		 *
> Extra line, but I think you intended to write a comment that explains
> why the freeze protection is required here :)
Yes, but forgot to... :)
>
>>   		 */
>> +		__sb_start_write(root->fs_info->sb, SB_FREEZE_WRITE, true);
> There's opencoding an existing wrapper sb_start_write, please use it
> instead.
OK, I can submit a new version using this wrapper.
Also could you please have a look at my reply to Filipe Manana in
last mail? I suggest another solution, thanks.

Regards,
Xiaoguang Wang
>
>>   		btrfs_delete_unused_bgs(root->fs_info);
>> +		__sb_end_write(root->fs_info->sb, SB_FREEZE_WRITE);
>>   sleep:
>>   		if (!again) {
>>   			set_current_state(TASK_INTERRUPTIBLE);
>> -- 
>> 2.9.0
>>
>>
>>
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