[Ocfs2-devel] [PATCH v2] ocfs2/dlm: fix race between convert and recovery

[Joseph Qi <joseph.qi@huawei.com>]

On 2015/9/23 9:59, Junxiao Bi wrote:
> On 09/23/2015 09:47 AM, Joseph Qi wrote:
>> On 2015/9/23 9:25, Junxiao Bi wrote:
>>> On 09/22/2015 08:23 PM, Joseph Qi wrote:
>>>> Hi Junxiao,
>>>>
>>>> On 2015/9/22 8:55, Junxiao Bi wrote:
>>>>> On 09/21/2015 05:23 PM, Joseph Qi wrote:
>>>>>> Hi Junxiao,
>>>>>> This solution may have problem in the following scenario:
>>>>>> Consider dlm_send_remote_convert_request has taken too much time, and
>>>>>> dlm_move_lockres_to_recovery_list runs first and new master will see
>>>>>> this node is currently in convert list after recovery.
>>>>>> Then dlm_send_remote_convert_request returns other than DLM_NORMAL and
>>>>>> it will revert it to grant list, then retry convert. This will makes
>>>>>> this node and master inconsistent.
>>>>>> I will try to find another solution to resolve the race issue.
>>>>>
>>>>> If master is down, no need retry convert. May check the return value of
>>>>> dlm_send_remote_convert_request(), if DLM_RECOVERING, don't retry,
>>>>> otherwise retry?
>>>>
>>>> I want to keep the original logic. And for fixing the race case I
>>>> described, how about the following idea?
>>>>
>>>> Check the status DLM_NORMAL. If res->state is currently
>>>> DLM_LOCK_RES_RECOVERING (set in dlm_move_lockres_to_recovery_list, means
>>>> still in recovery) or res master changed (means new master has finished
>>>> the recovery), reset the status to DLM_RECOVERING, just like the check
>>>> at the beginning of dlmconvert_remote. Then it is now in grant list and
>>>> outer will retry.
>>> How this idea fix the race windows you described in patch log? Lock is
>>> reverted to granted list but dlm_send_remote_convert_request() return
>>> DLM_NORMAL.
>>>
>> As I described in the former mail, status is now reset to DLM_RECOVERING,
>> caller will retry. And the original way will just wait forever.
> I mean convert request was sent to res master successfully,
> dlm_send_remote_convert_request() have return DLM_NORMAL, but before res
> master send ast it panic. Looks convert will not retry in this case.
> 
The original way is just what you said.
And my idea is letting dlmlock retry by resetting status to DLM_RECOVERING in
such a case.

> Thanks,
> Junxiao.
>>
>> Thanks
>> Joseph
>>
>>> Thanks,
>>> Junxiao.
>>>>
>>>>>
>>>>> Thanks,
>>>>> Junxiao.
>>>>>
>>>>>>
>>>>>> On 2015/9/20 15:20, Junxiao Bi wrote:
>>>>>>> Reviewed-by: Junxiao Bi <junxiao.bi@oracle.com>
>>>>>>>
>>>>>>>>> ? 2015?9?18????7:25?Joseph Qi <joseph.qi@huawei.com> ???
>>>>>>>>>
>>>>>>>>> There is a race window between dlmconvert_remote and
>>>>>>>>> dlm_move_lockres_to_recovery_list, which will cause a lock with
>>>>>>>>> OCFS2_LOCK_BUSY in grant list, thus system hangs.
>>>>>>>>>
>>>>>>>>> dlmconvert_remote
>>>>>>>>> {	
>>>>>>>>>        spin_lock(&res->spinlock);
>>>>>>>>>        list_move_tail(&lock->list, &res->converting);
>>>>>>>>>        lock->convert_pending = 1;
>>>>>>>>>        spin_unlock(&res->spinlock);
>>>>>>>>>
>>>>>>>>>        status = dlm_send_remote_convert_request();
>>>>>>>>>>>>>>>>>>>>> race window, master has queued ast and return DLM_NORMAL,
>>>>>>>>>               and then down before sending ast.
>>>>>>>>>               this node detects master down and calls
>>>>>>>>>               dlm_move_lockres_to_recovery_list, which will revert the
>>>>>>>>>               lock to grant list.
>>>>>>>>>               Then OCFS2_LOCK_BUSY won't be cleared as new master won't
>>>>>>>>>               send ast any more because it thinks already be authorized.
>>>>>>>>>
>>>>>>>>>        spin_lock(&res->spinlock);
>>>>>>>>>        lock->convert_pending = 0;
>>>>>>>>>        if (status != DLM_NORMAL)
>>>>>>>>>                dlm_revert_pending_convert(res, lock);
>>>>>>>>>        spin_unlock(&res->spinlock);
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> In this case, just leave it in convert list and new master will take care
>>>>>>>>> of it after recovery.  And if convert request returns other than
>>>>>>>>> DLM_NORMAL, convert thread will do the revert itself. So remove the
>>>>>>>>> revert logic in dlm_move_lockres_to_recovery_list.
>>>>>>>>>
>>>>>>>>> changelog since v1:
>>>>>>>>> Clean up convert_pending since it is now useless.
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>> .
>>>>>
>>>>
>>>>
>>>
>>>
>>> .
>>>
>>
>>
> 
> 
> .
>