Commit 3ce3230a authored Feb 08, 2012 by Frederic Weisbecker Committed by Tejun Heo Feb 21, 2012

cgroup: Walk task list under tasklist_lock in cgroup_enable_task_cg_list



Walking through the tasklist in cgroup_enable_task_cg_list() inside
an RCU read side critical section is not enough because:

- RCU is not (yet) safe against while_each_thread()

- If we use only RCU, a forking task that has passed cgroup_post_fork()
  without seeing use_task_css_set_links == 1 is not guaranteed to have
  its child immediately visible in the tasklist if we walk through it
  remotely with RCU. In this case it will be missing in its css_set's
  task list.

Thus we need to traverse the list (unfortunately) under the
tasklist_lock. It makes us safe against while_each_thread() and also
make sure we see all forked task that have been added to the tasklist.

As a secondary effect, reading and writing use_task_css_set_links are
now well ordered against tasklist traversing and modification. The new
layout is:

CPU 0                                      CPU 1

use_task_css_set_links = 1                write_lock(tasklist_lock)
read_lock(tasklist_lock)                  add task to tasklist
do_each_thread() {                        write_unlock(tasklist_lock)
	add thread to css set links       if (use_task_css_set_links)
} while_each_thread()                         add thread to css set links
read_unlock(tasklist_lock)

If CPU 0 traverse the list after the task has been added to the tasklist
then it is correctly added to the css set links. OTOH if CPU 0 traverse
the tasklist before the new task had the opportunity to be added to the
tasklist because it was too early in the fork process, then CPU 1
catches up and add the task to the css set links after it added the task
to the tasklist. The right value of use_task_css_set_links is guaranteed
to be visible from CPU 1 due to the LOCK/UNLOCK implicit barrier properties:
the read_unlock on CPU 0 makes the write on use_task_css_set_links happening
and the write_lock on CPU 1 make the read of use_task_css_set_links that comes
afterward to return the correct value.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Mandeep Singh Baines <msb@chromium.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

parent 9a4b4304

kernel/cgroup.c

+20 −0

Original line number	Diff line number	Diff line
		@@ -2707,6 +2707,14 @@ static void cgroup_enable_task_cg_lists(void)
		struct task_struct p, g;
		write_lock(&css_set_lock);
		use_task_css_set_links = 1;
		/*
		* We need tasklist_lock because RCU is not safe against
		* while_each_thread(). Besides, a forking task that has passed
		* cgroup_post_fork() without seeing use_task_css_set_links = 1
		* is not guaranteed to have its child immediately visible in the
		* tasklist if we walk through it with RCU.
		*/
		read_lock(&tasklist_lock);
		do_each_thread(g, p) {
		task_lock(p);
		/*
		@@ -2718,6 +2726,7 @@ static void cgroup_enable_task_cg_lists(void)
		list_add(&p->cg_list, &p->cgroups->tasks);
		task_unlock(p);
		} while_each_thread(g, p);
		read_unlock(&tasklist_lock);
		write_unlock(&css_set_lock);
		}

		@@ -4522,6 +4531,17 @@ void cgroup_fork_callbacks(struct task_struct *child)
		*/
		void cgroup_post_fork(struct task_struct *child)
		{
		/*
		* use_task_css_set_links is set to 1 before we walk the tasklist
		* under the tasklist_lock and we read it here after we added the child
		* to the tasklist under the tasklist_lock as well. If the child wasn't
		* yet in the tasklist when we walked through it from
		* cgroup_enable_task_cg_lists(), then use_task_css_set_links value
		* should be visible now due to the paired locking and barriers implied
		* by LOCK/UNLOCK: it is written before the tasklist_lock unlock
		* in cgroup_enable_task_cg_lists() and read here after the tasklist_lock
		* lock on fork.
		*/
		if (use_task_css_set_links) {
		write_lock(&css_set_lock);
		if (list_empty(&child->cg_list)) {

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