linux-kernel - Re: [RFC] [PATCH v3 1/2] cgroups: read-write lock CLONE

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Message-Id: <20100728182813.7c2826ae.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 28 Jul 2010 18:28:13 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Ben Blum <bblum@...rew.cmu.edu>
Cc:	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, akpm@...ux-foundation.org,
	ebiederm@...ssion.com, lizf@...fujitsu.com, matthltc@...ibm.com,
	menage@...gle.com, oleg@...hat.com
Subject: Re: [RFC] [PATCH v3 1/2] cgroups: read-write lock CLONE_THREAD
 forking per threadgroup

On Wed, 28 Jul 2010 04:29:53 -0400
Ben Blum <bblum@...rew.cmu.edu> wrote:

> On Mon, Jun 28, 2010 at 11:43:59AM -0400, Ben Blum wrote:
> > On Mon, Jun 28, 2010 at 04:10:31PM +0900, KAMEZAWA Hiroyuki wrote:
> > > By the way, IMHO, hiding lock in cgroup_fork() and cgroup_post_fork() doesn't
> > > seem good idea. How about a code like this ?
> > > 
> > >   read_lock_thread_clone(current);
> > >   cgroup_fork();
> > >  .....
> > >   cgroup_post_fork();
> > >   read_unlock_thrad_clone(current);
> > > 
> > > We may have chances to move these lock to better position if cgroup is
> > > an only user.
> > 
> > I didn't do that out of a desire to change fork.c as little as possible,
> > but that does look better than what I've got. Those two functions should
> > be in fork.c under #ifdef CONFIG_CGROUPS.
> 
> I'm looking at this now and am not sure where the best place to put
> these is:
> 
> 1) Don't make new functions, just put:
> 
>     #ifdef CONFIG_CGROUPS
>         if (clone_flags & CLONE_THREAD)
>             down/up_read(...);
>     #endif
> 
> directly in copy_process() in fork.c. Simplest, but uglifies the code.
> 
> 2) Make static helper functions in fork.c. Good, but not consistent with
> directly using the lock in write-side (attach_proc).
> 
> 3) Define inline functions under #ifdef CONFIG_CGROUPS in sched.h, just
> under the declaration of the lock. Most robust, but I'm hesitant to add
> unneeded stuff to such a popular header file.
> 
> Any opinions?
> 

My point was simple. Because copy_process() is very important path,
the new lock should be visible in copy_process() or kernek/fork.c.
"If the lock is visible in copy_process(), the reader can notice it".

Then, I offer you 2 options.

rename cgroup_fork() and cgroup_post_fork() as
       cgroup_fork_lock() and cgroup_post_fork_unlock() 

Now, the lock is visible and the change is minimum.

Or
       add the definition of lock/unlock to cgroup.h and include it 
       from kernel/fork.c

Thanks,
-Kame

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