>From fc4ef71935b1effe8612bfb39c7e3a41a2e2346c Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@xxxxxxxxxx>
Date: Mon, 5 May 2014 18:35:12 -0400
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.
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
include/linux/cgroup.h | 15 +++++++++++++++
kernel/cgroup.c | 3 ++-
2 files changed, 17 insertions(+), 1 deletion(-)
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 @@ struct cftype {
};
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 @@ static inline struct cgroup_subsys_state *task_css(struct task_struct *task,
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..7030b37 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -348,7 +348,7 @@ struct cgrp_cset_link {
* 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),
@@ -356,6 +356,7 @@ static struct css_set init_css_set = {
.mg_preload_node = LIST_HEAD_INIT(init_css_set.mg_preload_node),
.mg_node = LIST_HEAD_INIT(init_css_set.mg_node),
};
+EXPORT_SYMBOL_GPL(init_css_set);
static int css_set_count = 1; /* 1 for init_css_set */
--
1.9.0
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