cgroups: Add an API to attach a task to current task's cgroup

Add a new kernel API to attach a task to current task's cgroup
in all the active hierarchies.

Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paul Menage <menage@google.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>

include/linux/cgroup.h

@@ -570,6 +570,7 @@ struct task_struct *cgroup_iter_next(struct cgroup *cgrp,
 void cgroup_iter_end(struct cgroup *cgrp, struct cgroup_iter *it);
 int cgroup_scan_tasks(struct cgroup_scanner *scan);
 int cgroup_attach_task(struct cgroup *, struct task_struct *);
+int cgroup_attach_task_current_cg(struct task_struct *);
 
 /*
  * CSS ID is ID for cgroup_subsys_state structs under subsys. This only works
@@ -626,6 +627,12 @@ static inline int cgroupstats_build(struct cgroupstats *stats,
 	return -EINVAL;
 }
 
+/* No cgroups - nothing to do */
+static inline int cgroup_attach_task_current_cg(struct task_struct *t)
+{
+	return 0;
+}
+
 #endif /* !CONFIG_CGROUPS */
 
 #endif /* _LINUX_CGROUP_H */

kernel/cgroup.c

@@ -1788,6 +1788,29 @@ out:
 	return retval;
 }
 
+/**
+ * cgroup_attach_task_current_cg - attach task 'tsk' to current task's cgroup
+ * @tsk: the task to be attached
+ */
+int cgroup_attach_task_current_cg(struct task_struct *tsk)
+{
+	struct cgroupfs_root *root;
+	struct cgroup *cur_cg;
+	int retval = 0;
+
+	cgroup_lock();
+	for_each_active_root(root) {
+		cur_cg = task_cgroup_from_root(current, root);
+		retval = cgroup_attach_task(cur_cg, tsk);
+		if (retval)
+			break;
+	}
+	cgroup_unlock();
+
+	return retval;
+}
+EXPORT_SYMBOL_GPL(cgroup_attach_task_current_cg);
+
 /*
  * Attach task with pid 'pid' to cgroup 'cgrp'. Call with cgroup_mutex
  * held. May take task_lock of task