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* Q: cgroup: Questions about possible issues in cgroup locking
@ 2011-12-21  3:43 Frederic Weisbecker
  2011-12-21 13:08 ` Oleg Nesterov
  0 siblings, 1 reply; 29+ messages in thread
From: Frederic Weisbecker @ 2011-12-21  3:43 UTC (permalink / raw)
  To: Li Zefan, Tejun Heo
  Cc: LKML, Mandeep Singh Baines, Containers, Cgroups,
	KAMEZAWA Hiroyuki, Oleg Nesterov, Paul Menage, Andrew Morton,
	Paul E. McKenney

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.

^ permalink raw reply	[flat|nested] 29+ messages in thread

end of thread, other threads:[~2012-03-23 17:59 UTC | newest]

Thread overview: 29+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2011-12-21  3:43 Q: cgroup: Questions about possible issues in cgroup locking Frederic Weisbecker
2011-12-21 13:08 ` Oleg Nesterov
2011-12-21 17:56   ` Frederic Weisbecker
2011-12-21 19:01     ` Mandeep Singh Baines
2011-12-21 19:08       ` Frederic Weisbecker
2011-12-21 19:24         ` Mandeep Singh Baines
2011-12-21 20:04           ` Frederic Weisbecker
2011-12-22 15:30             ` Oleg Nesterov
2012-01-04 19:36               ` Mandeep Singh Baines
2012-01-06 15:23                 ` Oleg Nesterov
2012-01-06 18:25                   ` Mandeep Singh Baines
2012-01-11 16:07                     ` Oleg Nesterov
2012-01-12  0:31                       ` Mandeep Singh Baines
2012-01-12 17:07                         ` Oleg Nesterov
2012-01-12 17:57                           ` Mandeep Singh Baines
2012-01-13 15:20                             ` Oleg Nesterov
2012-01-13 18:27                               ` Mandeep Singh Baines
2012-01-14 17:36                                 ` Oleg Nesterov
2012-01-18 23:17                                   ` Mandeep Singh Baines
2012-01-19 15:45                                     ` Oleg Nesterov
2012-01-19 18:18                                       ` Mandeep Singh Baines
2012-01-20 15:06                                         ` Oleg Nesterov
2012-03-20 19:34                                       ` Oleg Nesterov
2012-03-21 18:59                                         ` Mandeep Singh Baines
2012-03-23 17:51                                           ` Oleg Nesterov
2011-12-21 17:59   ` Frederic Weisbecker
2011-12-21 18:11     ` Oleg Nesterov
2011-12-21 18:23       ` Frederic Weisbecker
2012-02-01 16:28   ` Frederic Weisbecker

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