linux-kernel - Re: [PATCH v4 2/2] cgroups: make procs file writable

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Message-ID: <AANLkTikxYHOCbuh=6R43BjzZ5QCA2kfTpt7h_ZrEbNoU@mail.gmail.com>
Date:	Tue, 3 Aug 2010 21:46:29 -0700
From:	Paul Menage <menage@...gle.com>
To:	Ben Blum <bblum@...rew.cmu.edu>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, akpm@...ux-foundation.org,
	ebiederm@...ssion.com, lizf@...fujitsu.com, matthltc@...ibm.com,
	oleg@...hat.com
Subject: Re: [PATCH v4 2/2] cgroups: make procs file writable

On Tue, Aug 3, 2010 at 9:38 PM, Ben Blum <bblum@...rew.cmu.edu> wrote:
>
> rcu_read_lock();
> for_each_subsys(...) {
>        can_attach(...);
> }
> rcu_read_unlock();

Sorry, I was misreading this, and didn't notice that it was already
inside an "if (threadgroup) {}" test.

>
> Which forces all can_attaches to not sleep. So by dropping
> rcu_read_lock(), we allow the possibility of the exec race I described
> in my last email, and therefore we have to check each time we re-acquire
> rcu_read to iterate thread_group.

Agreed.

>
> Yeah, it is not pretty. I call it "double-double-toil-and-trouble-check
> locking". But it is safe.

As a cleanup, I'd be inclined to have a wrapper in cgroup.c, something like

cgroup_can_attach_threadgroup(struct cgroup_subsys *ss, struct cgroup
*cg, struct task_struct *leader, int (*cb)(struct task_struct *t,
struct cgroup *cg))

which handles the RCU section, checking threadgroup_leader(), and
looping through each thread. The the subsystem just has to define a
callback which will be called for each thread.

Paul
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