linux-kernel - Q: cgroup: Questions about possible issues in cgroup locking

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Date:	Wed, 21 Dec 2011 04:43:36 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Li Zefan <lizf@...fujitsu.com>, Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Mandeep Singh Baines <msb@...omium.org>,
	Containers <containers@...ts.linux-foundation.org>,
	Cgroups <cgroups@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul Menage <paul@...lmenage.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Q: cgroup: Questions about possible issues in cgroup locking

Hi,

Starring at some parts of cgroups, I have a few questions:

- Is cgroup_enable_task_cg_list()'s while_each_thread() safe
against concurrent exec()? The leader may change in de_thread()
and invalidate the test done in while_each_thread().

- do_each_thread() also needs RCU and cgroup_enable_task_cg_list()
seems to remind it. But it seems there is at least one caller that
doesn't call rcu_read_lock(): update_cpu_mask() -> update_tasks_cpumask() -> cgroup_scan_tasks()

- By the time we call cgroup_post_fork(), it is ready to be woken up
and usable by the scheduler. I was thinking about a possible race
where somebody kills the child just before we call cgroup_post_fork()
and then it ends up calling cgroup_exit(). In this case should we perhaps
check for PF_EXITING. Of course I'm talking about a race that will certainly
never happen.
If I'm mistaken and the task can not exit concurrently, probably we don't need
the task_lock() there.

- Is the check for use_task_css_set_links in cgroup_post_fork() safe? given
it is checked outside css_set_lock?

Imagine this:

CPU 0                                                        CPU 1
----                                                         -----

cgroup_enable_task_cg() {                            
	uset_tasks_css_set_links = 1
	for_each_thread() {
		add tasks in the list
	}
}
                                                           do_fork() {
                                                               cgroup_post_fork() {
                                                                     use_tasks_css_set_links appears
                                                                     to be equal to 0 due to write/read
                                                                      not flushed. New task won't
                                                                      appear to the list.


- Is the read_lock() in cgroup_attach_proc() still necessary? It was added to protect
against exec() but now I think it is useless. rcu_read_lock() would be necessary to protect
against group_entry removal though I think.

Thanks.
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