[PATCH 2/2] cgroup: Walk task list under tasklist_lock in cgroup_enable_task_cg_list

From: Frederic Weisbecker
Date: Tue Feb 07 2012 - 21:37:43 EST

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:


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

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@xxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Li Zefan <lizf@xxxxxxxxxxxxxx>
Cc: Mandeep Singh Baines <msb@xxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
kernel/cgroup.c | 20 ++++++++++++++++++++
1 files changed, 20 insertions(+), 0 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 6e4eb43..c6877fe 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;
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) {
@@ -2718,6 +2726,7 @@ static void cgroup_enable_task_cg_lists(void)
list_add(&p->cg_list, &p->cgroups->tasks);
} while_each_thread(g, p);
+ read_unlock(&tasklist_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) {
if (list_empty(&child->cg_list)) {

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