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

[Bill Davidsen]

Li Zefan wrote:
> Bill Davidsen wrote:
>   
> > Li Zefan wrote:
> >     
> > > Liu Aleaxander wrote:
> > >       
> > > > From: Liu Aleaxander <Aleaxander@xxxxxxxxx>
> > > > 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.

> In fact, Ingo showed his distaste towards cgroup_lock():
> 	http://lkml.org/lkml/2009/1/18/39
>
> And I won't worry about the issue you mentioned above. If It does
> happen, the one, who makes the 2 mehtods no long compatible, will
> definitely find out all the places where cgroup_mutex is used and
> make proper change.
>
>   


--
Bill Davidsen <davidsen@xxxxxxx>
 Unintended results are the well-earned reward for incompetence.

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