Date:   Thu, 26 Aug 2021 13:56:16 +0800
From:   Gang He <ghe@...e.com>
To:     Joseph Qi <joseph.qi@...ux.alibaba.com>, mark@...heh.com,
        jlbec@...lplan.org, Wengang Wang <wen.gang.wang@...cle.com>
CC:     linux-kernel@...r.kernel.org, ocfs2-devel@....oracle.com
Subject: Re: [Ocfs2-devel] [PATCH] ocfs2: reflink deadlock when clone file to
 the same directory simultaneously



On 2021/8/19 14:31, Gang He wrote:
> 
> 
> On 2021/8/19 10:02, Joseph Qi wrote:
>>
>>
>> On 8/19/21 9:51 AM, Gang He wrote:
>>>
>>>
>>> On 2021/8/18 19:20, Joseph Qi wrote:
>>>>
>>>>
>>>> On 8/18/21 5:20 PM, Gang He wrote:
>>>>>
>>>>>
>>>>> On 2021/8/13 17:54, Joseph Qi wrote:
>>>>>>
>>>>>>
>>>>>> On 8/9/21 6:08 PM, Gang He wrote:
>>>>>>> Hi Joseph and All,
>>>>>>>
>>>>>>> The deadlock is caused by self-locking on one node.
>>>>>>> There is three node cluster (mounted to /mnt/shared), the user run reflink command to clone the file to the same directory repeatedly,
>>>>>>> e.g.
>>>>>>>      reflink "/mnt/shared/test" \
>>>>>>>      "/mnt/shared/.snapshots/test.`date +%m%d%H%M%S`.`hostname`"
>>>>>>>
>>>>>>> After a while, the reflink process on each node is hung, the file system cannot be listed.
>>>>>>> The problematic reflink command process is blocked by itself, e.g. the reflink process is hung at ghe-sle15sp2-nd2,
>>>>>>> kernel: task:reflink         state:D stack:    0 pid:16992 ppid:  4530
>>>>>>> kernel: Call Trace:
>>>>>>> kernel:  __schedule+0x2fd/0x750
>>>>>>> kernel:  ? try_to_wake_up+0x17b/0x4e0
>>>>>>> kernel:  schedule+0x2f/0xa0
>>>>>>> kernel:  schedule_timeout+0x1cc/0x310
>>>>>>> kernel:  ? __wake_up_common+0x74/0x120
>>>>>>> kernel:  wait_for_completion+0xba/0x140
>>>>>>> kernel:  ? wake_up_q+0xa0/0xa0
>>>>>>> kernel:  __ocfs2_cluster_lock.isra.41+0x3b5/0x820 [ocfs2]
>>>>>>> kernel:  ? ocfs2_inode_lock_full_nested+0x1fc/0x960 [ocfs2]
>>>>>>> kernel:  ocfs2_inode_lock_full_nested+0x1fc/0x960 [ocfs2]
>>>>>>> kernel:  ocfs2_init_security_and_acl+0xbe/0x1d0 [ocfs2]
>>>>>>> kernel:  ocfs2_reflink+0x436/0x4c0 [ocfs2]
>>>>>>> kernel:  ? ocfs2_reflink_ioctl+0x2ca/0x360 [ocfs2]
>>>>>>> kernel:  ocfs2_reflink_ioctl+0x2ca/0x360 [ocfs2]
>>>>>>> kernel:  ocfs2_ioctl+0x25e/0x670 [ocfs2]
>>>>>>> kernel:  do_vfs_ioctl+0xa0/0x680
>>>>>>> kernel:  ksys_ioctl+0x70/0x80
>>>>>>>
>>>>>>> In fact, the destination directory(.snapshots) inode dlm lock was acquired by ghe-sle15sp2-nd2, next there is bast message from other nodes to ask ghe-sle15sp2-nd2 downconvert lock, but the operation failed, the kernel message is printed like,
>>>>>>> kernel: (ocfs2dc-AA35DD9,2560,3):ocfs2_downconvert_lock:3660 ERROR: DLM error -16 while calling ocfs2_dlm_lock on resource M0000000000000000046e0200000000
>>>>>>> kernel: (ocfs2dc-AA35DD9,2560,3):ocfs2_unblock_lock:3904 ERROR: status = -16
>>>>>>> kernel: (ocfs2dc-AA35DD9,2560,3):ocfs2_process_blocked_lock:4303 ERROR: status = -16
>>>>>>>
>>>>>>> Then, the reflink process tries to acquire this directory inode dlm lock, the process is blocked, the dlm lock resource in memory looks like
>>>>>>>
>>>>>>>         l_name = "M0000000000000000046e0200000000",
>>>>>>>         l_ro_holders = 0,
>>>>>>>         l_ex_holders = 0,
>>>>>>>         l_level = 5 '\005',
>>>>>>>         l_requested = 0 '\000',
>>>>>>>         l_blocking = 5 '\005',
>>>>>>>         l_type = 0 '\000',
>>>>>>>         l_action = 0 '\000',
>>>>>>>         l_unlock_action = 0 '\000',
>>>>>>>         l_pending_gen = 645948,
>>>>>>>
>>>>>>>
>>>>>>> So far, I do not know what makes dlm lock function failed, it also looks we do not handle this failure case in dlmglue layer, but I always reproduce this hang with my test script, e.g.
>>>>>>>
>>>>>>>       loop=1
>>>>>>>       while ((loop++)) ; do
>>>>>>>             for i in `seq 1 100`; do
>>>>>>>               reflink "/mnt/shared/test" "/mnt/shared/.snapshots /test.${loop}.${i}.`date +%m%d%H%M%S`.`hostname`"
>>>>>>>             done
>>>>>>>             usleep 500000
>>>>>>>             rm -f /mnt/shared/.snapshots/testnode1.qcow2.*.`hostname`
>>>>>>>       done
>>>>>>>
>>>>>>> My patch changes multiple acquiring dest directory inode dlm lock during in ocfs2_reflink function, it avoids the hang issue happen again.The code change also can improve reflink performance in this case.
>>>>>>>
>>>>>>> Thanks
>>>>>>> Gang
>>>>>>
>>>>>> 'status = -16' implies DLM_CANCELGRANT.
>>>>>> Do you use stack user instead of o2cb? If yes, can you try o2cb with
>>>>>> your reproducer?
>>>>>
>>>>> I setup o2cb based ocfs2 clusters with sle15sp2 and oracleLinux8u4.
>>>>> After two day testing with the same script, I did not encounter dlm_lock downconvert failure, the hang issue did not happen.
>>>>> After my patch was applied, there was not any side effect, the reflink performance was doubled in the case.
>>>>>
>>>>
>>>> Do you mean the hang only happens on stack user?
>>> Yes.
>>> Why? since o2cb based dlm_lock did not return error -16 when downcovert dlm lock during the whole testing.
>>> But pmck based dlm_lock retuned error -16 during the testing, then we did not handle this error further in dlmglue layer, next encounter the hang issue when dlm_lock acquire the lock. Maybe there is a race condition when using dlm_lock/dlm_unlock(cancel) in dlmglue layer.
>>> Anyway, the problem belongs to ocfs2 own parts.
>>>
>> I meant if DLM_CANCELGRANT is not the expected return code, we'd
>> better fix the issue in stack_user.c but not dlmglue, e.g. some specific
>> wrapper.
> We cannot wrapper(or ignore) this error in stack_user, otherwise it will
> lead to a hang problem when the next dlm_lock is invoked.
> Based on comments from fs/dlm maintainer, the error -16 is returned by
> dlm_lock in case ocfs2 calls dlm_unlock(CANCEL) to cancel an in-progress
> dlm_lock() request.
> In fact, if you read the code comments in dlmglue.c, it also talked
> about the similar situation, but I feel the current code should still
> has a race condition, then trigger dlm_lock return -16 error.
> For o2cb stack, it's dlm_lock did not expose this error, maybe it is
> different in dlm implementation.

Based on my further analysis and BASTS traces, I found there are some 
differences in behavior between o2dlm and fsdlm.
Usually, ocfs2_downconvert_lock() function always downconverts
dlm lock to the expected level for satisfy dlm bast requests
from the other nodes.
But there is a rare situation. When dlm lock conversion is being
canceled, ocfs2_downconvert_lock() function will return -EBUSY.
You need to be aware that ocfs2_cancel_convert() function is
asynchronous in fsdlm implementation, that means ocfs2_cancel_convert()
will return directly, then the actual dlm lock cancel is executed in 
background.
For o2dlm implementation, ocfs2_cancel_convert() function is 
synchronous, that means this function will return after the dlm lock is
cancelled and ast callback function is invoked.

If we does not requeue this lockres entry, ocfs2 downconvert
thread no longer handles this dlm lock bast request. Then, the
other nodes will not get the dlm lock again, the current node's
process will be blocked when acquire this dlm lock again.

So, I will send a new patch to fix this deadlock problem via dlmglue layer.
For this patch, I want to change the patch comments as a reflink 
improvement patch.

Thanks
Gang



> 
> Thanks
> Gang
> 
>>
>> Thanks,
>> Joseph
>>
> 
> 
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