cgroup: introduce task_css_is_root()
Determining the css of a task usually requires RCU read lock as that's
the only thing which keeps the returned css accessible till its
reference is acquired; however, testing whether a task belongs to the
root can be performed without dereferencing the returned css by
comparing the returned pointer against the root one in init_css_set[]
which never changes.
Implement task_css_is_root() which can be invoked in any context.
This will be used by the scheduled cgroup_freezer change.
v2: cgroup no longer supports modular controllers. No need to export
init_css_set. Pointed out by Li.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index c251585..d60904b 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -473,6 +473,7 @@
};
extern struct cgroup_root cgrp_dfl_root;
+extern struct css_set init_css_set;
static inline bool cgroup_on_dfl(const struct cgroup *cgrp)
{
@@ -700,6 +701,20 @@
return task_css_check(task, subsys_id, false);
}
+/**
+ * task_css_is_root - test whether a task belongs to the root css
+ * @task: the target task
+ * @subsys_id: the target subsystem ID
+ *
+ * Test whether @task belongs to the root css on the specified subsystem.
+ * May be invoked in any context.
+ */
+static inline bool task_css_is_root(struct task_struct *task, int subsys_id)
+{
+ return task_css_check(task, subsys_id, true) ==
+ init_css_set.subsys[subsys_id];
+}
+
static inline struct cgroup *task_cgroup(struct task_struct *task,
int subsys_id)
{
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 11a03d6..3f1ca93 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -348,7 +348,7 @@
* reference-counted, to improve performance when child cgroups
* haven't been created.
*/
-static struct css_set init_css_set = {
+struct css_set init_css_set = {
.refcount = ATOMIC_INIT(1),
.cgrp_links = LIST_HEAD_INIT(init_css_set.cgrp_links),
.tasks = LIST_HEAD_INIT(init_css_set.tasks),