cgroup: Drop task_lock(parent) on cgroup_fork()

[deliverable/linux.git] / kernel / cgroup.c

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index bc3caff138d83c53421271211b8287714ddbe181..dae50d0d8e4bc89af0027530d19409354ec38ee7 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4556,20 +4556,31 @@ static const struct file_operations proc_cgroupstats_operations = {
  *
  * A pointer to the shared css_set was automatically copied in
  * fork.c by dup_task_struct().  However, we ignore that copy, since
- * it was not made under the protection of RCU or cgroup_mutex, so
- * might no longer be a valid cgroup pointer.  cgroup_attach_task() might
- * have already changed current->cgroups, allowing the previously
- * referenced cgroup group to be removed and freed.
+ * it was not made under the protection of RCU, cgroup_mutex or
+ * threadgroup_change_begin(), so it might no longer be a valid
+ * cgroup pointer.  cgroup_attach_task() might have already changed
+ * current->cgroups, allowing the previously referenced cgroup
+ * group to be removed and freed.
+ *
+ * Outside the pointer validity we also need to process the css_set
+ * inheritance between threadgoup_change_begin() and
+ * threadgoup_change_end(), this way there is no leak in any process
+ * wide migration performed by cgroup_attach_proc() that could otherwise
+ * miss a thread because it is too early or too late in the fork stage.
  *
  * At the point that cgroup_fork() is called, 'current' is the parent
  * task, and the passed argument 'child' points to the child task.
  */
 void cgroup_fork(struct task_struct *child)
 {
-       task_lock(current);
+       /*
+        * We don't need to task_lock() current because current->cgroups
+        * can't be changed concurrently here. The parent obviously hasn't
+        * exited and called cgroup_exit(), and we are synchronized against
+        * cgroup migration through threadgroup_change_begin().
+        */
        child->cgroups = current->cgroups;
        get_css_set(child->cgroups);
-       task_unlock(current);
        INIT_LIST_HEAD(&child->cg_list);
 }