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>

1 files changed, 20 insertions, 0 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 6e4eb4312571..c6877fe9a831 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -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)) {