[PATCH v3 3/8] cgroup: implement cgroup_from_id()

[Li Zefan <lizefan@huawei.com>]

This will be used as a replacement for css_lookup().

There's a difference with cgroup id and css id. cgroup id starts with 0,
while css id starts with 1.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
---
 include/linux/cgroup.h |  2 ++
 kernel/cgroup.c        | 16 ++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 8c107e9..e8eb361 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -553,6 +553,8 @@ int task_cgroup_path_from_hierarchy(struct task_struct *task, int hierarchy_id,
 
 int cgroup_task_count(const struct cgroup *cgrp);
 
+struct cgroup *cgroup_from_id(struct cgroup_subsys *ss, int id);
+
 /*
  * Control Group taskset, used to pass around set of tasks to cgroup_subsys
  * methods.
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index b7c7c68..dc4a749 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -5536,6 +5536,22 @@ struct cgroup_subsys_state *cgroup_css_from_dir(struct file *f, int id)
 	return css ? css : ERR_PTR(-ENOENT);
 }
 
+/**
+ * cgroup_from_id - lookup cgroup by id
+ * @ss: cgroup subsys to be looked into
+ * @id: the cgroup id
+ *
+ * Returns the cgroup if there's valid one with @id, otherwise returns NULL.
+ * Should be called under rcu_readlock().
+ */
+struct cgroup *cgroup_from_id(struct cgroup_subsys *ss, int id)
+{
+	rcu_lockdep_assert(rcu_read_lock_held(),
+			   "cgroup_from_id() needs rcu_read_lock()"
+			   " protection");
+	return idr_find(&ss->root->cgroup_idr, id);
+}
+
 #ifdef CONFIG_CGROUP_DEBUG
 static struct cgroup_subsys_state *debug_css_alloc(struct cgroup *cgrp)
 {
-- 
1.8.0.2

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