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

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Message-Id: <20100628161031.a8c71fce.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Mon, 28 Jun 2010 16:10:31 +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 Fri, 25 Jun 2010 01:46:54 -0400
Ben Blum <bblum@...rew.cmu.edu> wrote:

> Adds functionality to read/write lock CLONE_THREAD fork()ing per-threadgroup
> 
> From: Ben Blum <bblum@...rew.cmu.edu>
> 
> This patch adds an rwsem that lives in a threadgroup's signal_struct that's
> taken for reading in the fork path, under CONFIG_CGROUPS. If another part of
> the kernel later wants to use such a locking mechanism, the CONFIG_CGROUPS
> ifdefs should be changed to a higher-up flag that CGROUPS and the other system
> would both depend on.
> 
> This is a pre-patch for cgroups-procs-write.patch.
> 

Hmm, at adding a new lock, please describe its semantics.
Following is my understanding, right ?

Lock range:
This rwsem is read-locked from cgroup_fork() to cgroup_post_fork(). 
Most of works for fork() are between cgroup_fork() and cgroup_post_for().
This means if sig->threadgroup_fork_lock is held, no new do_work() can
make progress in this process groups. This rwsem is held only when
CLONE_THREAD is in clone_flags. IOW, this rwsem is taken only at creating
new thread.

What we can do with this:
By locking sig->threadgroup_fork_lock, a code can visit _all_ threads
in a process group witout any races. So, if you want to implement an
atomic operation against a process, taking this lock is an idea.

For what:
To implement an atomic process move in cgroup, we need this lock.

Why this implemantation:
Considering cgroup, threads in a cgroup can be under several different
cgroup. So, we can't implement lock in cgroup-internal, we use signal
struct.

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.

Thanks,
-Kame

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