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

[Junxiao Bi <junxiao.bi@oracle.com>]

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?

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