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

[xuejiufei <xuejiufei@huawei.com>]

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.

Thanks,
Jiufei

> Thanks,
> Junxiao.
>> Thanks,
>> --Jiufei
>>
> 
> 
> .
>