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

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Date:	Thu, 17 May 2012 14:22:05 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	<cgroups@...r.kernel.org>, <linux-mm@...ck.org>,
	<devel@...nvz.org>, <netdev@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH v5 2/2] decrement static keys on real destroy time

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


> 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.
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