Re: [PATCH] ocfs2: reflink deadlock when clone file to the same directory simultaneously

[Joseph Qi]

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.

Thanks,
Joseph