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

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Message-ID: <4FB4D061.10406@jp.fujitsu.com>
Date:	Thu, 17 May 2012 19:18:09 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Glauber Costa <glommer@...allels.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

(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.

Thanks,
-Kame
(*) I'm sorry I won't be able to read e-mails, tomorrow.



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