[Ocfs2-devel] [patch 1/2] ocfs2/dlm: protect 'tracking_list' by 'track_lock'

[piaojun <piaojun@huawei.com>]

On 2017/9/27 9:32, Joseph Qi wrote:
> 
> 
> On 17/9/26 08:39, piaojun wrote:
>>
>>
>> On 2017/9/25 18:35, Joseph Qi wrote:
>>>
>>>
>>> On 17/9/23 11:39, piaojun wrote:
>>>> 'dlm->tracking_list' need to be protected by 'dlm->track_lock'.
>>>>
>>>> Signed-off-by: Jun Piao <piaojun@huawei.com>
>>>> Reviewed-by: Alex Chen <alex.chen@huawei.com>
>>>> ---
>>>>  fs/ocfs2/dlm/dlmdomain.c | 7 ++++++-
>>>>  fs/ocfs2/dlm/dlmmaster.c | 4 ++--
>>>>  2 files changed, 8 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/fs/ocfs2/dlm/dlmdomain.c b/fs/ocfs2/dlm/dlmdomain.c
>>>> index a2b19fb..b118525 100644
>>>> --- a/fs/ocfs2/dlm/dlmdomain.c
>>>> +++ b/fs/ocfs2/dlm/dlmdomain.c
>>>> @@ -726,12 +726,17 @@ void dlm_unregister_domain(struct dlm_ctxt *dlm)
>>>>  		}
>>>>
>>>>  		/* This list should be empty. If not, print remaining lockres */
>>>> +		spin_lock(&dlm->track_lock);
>>>>  		if (!list_empty(&dlm->tracking_list)) {
>>>>  			mlog(ML_ERROR, "Following lockres' are still on the "
>>>>  			     "tracking list:\n");
>>>> -			list_for_each_entry(res, &dlm->tracking_list, tracking)
>>>> +			list_for_each_entry(res, &dlm->tracking_list, tracking) {
>>>> +				spin_unlock(&dlm->track_lock);
>>>
>>> Um... If we unlock here, the iterator still has chance to be corrupted.
>>>
>>> Thanks,
>>> Joseph
>>>
>>
>> we don't need care much about the corrupted 'tracking_list' because we
>> have already picked up 'res' from 'tracking_list'. then we will get
>> 'track_lock' again to prevent 'tracking_list' from being corrupted. but
>> I'd better make sure that 'res' is not NULL before printing, just like:
>>
>> list_for_each_entry(res, &dlm->tracking_list, tracking) {
>> 		spin_unlock(&dlm->track_lock);
>> 		if (res)
>> 			dlm_print_one_lock_resource(res);
>> 		spin_lock(&dlm->track_lock);
>> }
>>
>> Thanks
>> Jun
> 
> IIUC, your intent to add track lock here is to protect tracking list
> when iterate the loop, right? I am saying that if unlock track lock
> here, the loop is still unsafe.
> Checking res here is meaningless. Maybe list_for_each_entry_safe
> could work here.
> BTW, how this race case happens? The above code is during umount,
> what is the other flow?
> 
> Thanks,
> Joseph
> .
> 

I have not caught the race case yet, and the code rarely enter this
branch, because 'tracking list' is always empty here. the key problem
is we try to protect 'tracking list' under 'res->spinlock' and
'dlm->track_lock', but we could not get 'res->spinlock' before
iterating 'tracking list'. I have to figured it out further.

thanks
Jun