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-rw-r--r--kernel/cgroup.c20
1 files changed, 20 insertions, 0 deletions
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 6e4eb431257..c6877fe9a83 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)) {