Re: [PATCH v5 2/2] decrement static keys on real destroy time

[KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>]

(2012/05/17 19:22), Glauber Costa wrote:

> On 05/17/2012 02:18 PM, KAMEZAWA Hiroyuki wrote:
>> (2012/05/17 18:52), Glauber Costa wrote:
>>
>>> On 05/17/2012 09:37 AM, Andrew Morton wrote:
>>>>>>   If that happens, locking in static_key_slow_inc will prevent any damage.
>>>>>>   My previous version had explicit code to prevent that, but we were
>>>>>>   pointed out that this is already part of the static_key expectations, so
>>>>>>   that was dropped.
>>>> This makes no sense.  If two threads run that code concurrently,
>>>> key->enabled gets incremented twice.  Nobody anywhere has a record that
>>>> this happened so it cannot be undone.  key->enabled is now in an
>>>> unknown state.
>>>
>>> Kame, Tejun,
>>>
>>> Andrew is right. It seems we will need that mutex after all. Just this
>>> is not a race, and neither something that should belong in the
>>> static_branch interface.
>>>
>>
>>
>> Hmm....how about having
>>
>> res_counter_xchg_limit(res,&old_limit, new_limit);
>>
>> if (!cg_proto->updated&&  old_limit == RESOURCE_MAX)
>> 	....update labels...
>>
>> Then, no mutex overhead maybe and activated will be updated only once.
>> Ah, but please fix in a way you like. Above is an example.
> 
> I think a mutex is a lot cleaner than adding a new function to the 
> res_counter interface.
> 
> We could do a counter, and then later decrement the key until the 
> counter reaches zero, but between those two, I still think a mutex here 
> is preferable.
> 
> Only that, instead of coming up with a mutex of ours, we could export 
> and reuse set_limit_mutex from memcontrol.c
> 


ok, please.

thx,
-Kame

> 
>> Thanks,
>> -Kame
>> (*) I'm sorry I won't be able to read e-mails, tomorrow.
>>
> Ok Kame. I am not in a terrible hurry to fix this, it doesn't seem to be 
> hurting any real workload.
> 
>