linux-kernel - Re: [PATCH] cgroup: Fixes the un-paired cgroup lock problem

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Date:	Thu, 5 Nov 2009 17:05:52 -0800
From:	Matt Helsley <matthltc@...ibm.com>
To:	Bill Davidsen <davidsen@....com>
Cc:	Li Zefan <lizf@...fujitsu.com>, containers@...ts.osdl.org,
	Paul Menage <menage@...gle.com>,
	Liu Aleaxander <aleaxander@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cgroup: Fixes the un-paired cgroup lock problem

On Thu, Nov 05, 2009 at 07:14:11PM -0500, Bill Davidsen wrote:
> Li Zefan wrote:
> > Bill Davidsen wrote:
> >   
> >> Li Zefan wrote:
> >>     
> >>> Liu Aleaxander wrote:
> >>>       
> >>>> From: Liu Aleaxander <Aleaxander@...il.com>
> >>>> Date: Wed, 4 Nov 2009 09:27:06 +0800
> >>>> Subject: [PATCH] Fixes the un-paired cgroup lock problem
> >>>>
> >>>> In cgroup_lock_live_group, it locks the cgroup by mutex_lock, while
> >>>> in the
> >>>> cgroup_tasks_write, it unlock it by cgroup_unlock. Even though they are
> >>>> equal, but I do think we should make it pair.
> >>>>
> >>>> BTW, should we replace others with cgroup_lock and cgroup_unlock?
> >>>> Since we already have a wrapper one and it's meaningful.
> >>>>
> >>>>         
> >>> Before I read the email body, I thought there is a bug where
> >>> there is a lock without unlock or vise versa.
> >>>
> >>> I agree the case here can be called "unpaired", but I'm not
> >>> convinced this patch is needed. The code is not buggy or
> >>> confusing. So the patch neither fixes a bug nor make the code
> >>> more readable.
> >>>
> >>>       
> >> I would say it fixes a bug, the one that would be introduced when the
> >> two methods are no longer compatible and essentially two names for the
> >> same thing. And while you may know the code so well that you knew
> >> without looking that this was (currently) okay, there will be lots of
> >> eyes on this code over the years, I think most people would find use of
> >> cgroup_lock to lock the cgroup a LOT more readable.
> >>
> >> While you can't go back in time to murder your grandfather, it creates
> >> no paradox to fix a bug before someone writes it.
> >>
> >>     
> >
> > cgroup_lock() is not necessarily more readable than mutex_lock(&cgroup_mutex),
> > at least the former doesn't tell you the lock is a spin_lock or a mutex.
> >
> >   
> That's the point, cgroup_lock() is an abstraction, you want to lock the 
> cgroup, you call the macro, the macro handles the details, and if 
> thinking (or the most common cache configurations) change, the code 
> still works.

Except it doesn't really "lock the cgroup" as you've been saying -- else
it would take the cgroup to lock as a parameter. Instead it locks
"all cgroups". Clearly there's room for misunderstanding even with
cgroup_lock().

	-Matt Helsley

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