[Ocfs2-devel] ocfs2: A race between refmap setting and clearing

[Junxiao Bi]

On 01/13/2016 02:21 PM, xuejiufei wrote:
> Hi Junxiao,
> I have not describe the issue clearly.
> 
> Node 1                               Node 2(master)
> dlmlock
> dlm_do_master_request
>                                 dlm_master_request_handler
>                                 -> dlm_lockres_set_refmap_bit
> dlmlock succeed
> dlmunlock succeed
> 
> dlm_purge_lockres
>                                 dlm_deref_handler
>                                 -> find lock resource is in
>                                    DLM_LOCK_RES_SETREF_INPROG state,
>                                    so dispatch a deref work
> dlm_purge_lockres succeed.
> 
> call dlmlock again
> dlm_do_master_request
>                                 dlm_master_request_handler
>                                 -> dlm_lockres_set_refmap_bit
> 
>                                 deref work trigger, call
>                                 dlm_lockres_clear_refmap_bit
>                                 to clear Node 1 from refmap
> 
>                                 dlm_purge_lockres succeed
> 
> dlm_send_remote_lock_request
>                                 return DLM_IVLOCKID because
>                                 the lockres is not exist
More clear now. Thank you.
This is a very complicated race. I didn't have a good solution to fix it
now. Your fix looks work, but I am afraid if we keep going fix this
kinds of races case by case, we will make dlm harder to understand and
easy to involve bugs, maybe we should think about refactor dlm.

Thanks,
Junxiao.

> BUG if the lockres is $RECOVERY
> 
> On 2016/1/13 10:46, Junxiao Bi wrote:
>> On 01/12/2016 03:16 PM, xuejiufei wrote:
>>> Hi, Junxiao
>>>
>>> On 2016/1/12 12:03, Junxiao Bi wrote:
>>>> Hi Jiufei,
>>>>
>>>> On 01/11/2016 10:46 AM, xuejiufei wrote:
>>>>> Hi all,
>>>>> We have found a race between refmap setting and clearing which
>>>>> will cause the lock resource on master is freed before other nodes
>>>>> purge it.
>>>>>
>>>>> Node 1                               Node 2(master)
>>>>> dlm_do_master_request
>>>>>                                 dlm_master_request_handler
>>>>>                                 -> dlm_lockres_set_refmap_bit
>>>>> call dlm_purge_lockres after unlock
>>>>>                                 dlm_deref_handler
>>>>>                                 -> find lock resource is in
>>>>>                                    DLM_LOCK_RES_SETREF_INPROG state,
>>>>>                                    so dispatch a deref work
>>>>> dlm_purge_lockres succeed.
>>>>>
>>>>> dlm_do_master_request
>>>>>                                 dlm_master_request_handler
>>>>>                                 -> dlm_lockres_set_refmap_bit
>>>>>
>>>>>                                 deref work trigger, call
>>>>>                                 dlm_lockres_clear_refmap_bit
>>>>>                                 to clear Node 1 from refmap
>>>>>
>>>>> Now Node 2 can purge the lock resource but the owner of lock resource
>>>>> on Node 1 is still Node 2 which may trigger BUG if the lock resource
>>>>> is $RECOVERY or other problems.
>>>>>
>>>>> We have discussed 2 solutions:
>>>>> 1)The master return error to Node 1 if the DLM_LOCK_RES_SETREF_INPROG
>>>>> is set. Node 1 will not retry and master send another message to Node 1
>>>>> after clearing the refmap. Node 1 can purge the lock resource after the
>>>>> refmap on master is cleared.
>>>>> 2) The master return error to Node 1 if the DLM_LOCK_RES_SETREF_INPROG
>>>>> is set, and Node 1 will retry to deref the lockres.
>>>>>
>>>>> Does anybody has better ideas?
>>>>>
>>>> dlm_purge_lockres() will wait to drop ref until
>>>> DLM_LOCK_RES_SETREF_INPROG cleared. So if set this flag when find the
>>>> master during doing master request. And then this flag was cleared when
>>>> receiving assert master message, can this fix the issue?
>>>>
>>> I don't think this can fix. Before doing master request, the lock resource is
>>> already purged. The master should clear the refmap before client purge it.
>> inflight_locks is increased in dlm_get_lock_resource() which will stop
>> lockres purged? Set DLM_LOCK_RES_SETREF_INPROG when found lockres owner
>> during master request, then this will stop lockres purged after unlock?
>>
>> Thanks,
>> Junxiao.
>>
>>>
>>> Thanks,
>>> Jiufei
>>>
>>>> Thanks,
>>>> Junxiao.
>>>>> Thanks,
>>>>> --Jiufei
>>>>>
>>>>
>>>>
>>>> .
>>>>
>>>
>>
>>
>> .
>>
>