[RFC v2 01/18] cgroup: Added cgroup_get_from_kernfs_id

[Muneendra]

Added a new function cgroup_get_from_kernfs_id  to retrieve the cgroup
associated with cgroup id.
It takes cgroupid as an argument and returns cgrp and on failure
it returns NULL.
Exported the same as this can be used by blk-cgorup.c

Added function declaration of cgroup_get_from_kernfs_id in cgorup.h

Signed-off-by: Muneendra <muneendra.kumar at broadcom.com>

---
v2:
New patch
---
 include/linux/cgroup.h |  6 ++++++
 kernel/cgroup/cgroup.c | 25 +++++++++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 618838c48313..1741f02a41d6 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -696,6 +696,7 @@ static inline void cgroup_kthread_ready(void)
 }
 
 void cgroup_path_from_kernfs_id(u64 id, char *buf, size_t buflen);
+struct cgroup *cgroup_get_from_kernfs_id(u64 id);
 #else /* !CONFIG_CGROUPS */
 
 struct cgroup_subsys_state;
@@ -743,6 +744,11 @@ static inline bool task_under_cgroup_hierarchy(struct task_struct *task,
 
 static inline void cgroup_path_from_kernfs_id(u64 id, char *buf, size_t buflen)
 {}
+
+static struct cgroup *cgroup_get_from_kernfs_id(u64 id)
+{
+	return NULL;
+}
 #endif /* !CONFIG_CGROUPS */
 
 #ifdef CONFIG_CGROUPS
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index dd247747ec14..f34ca94b04c0 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5803,6 +5803,31 @@ void cgroup_path_from_kernfs_id(u64 id, char *buf, size_t buflen)
 	kernfs_put(kn);
 }
 
+/*
+ * cgroup_get_from_kernfs_id : get the cgroup associated with cgroup id
+ * @id: cgroup id
+ * On success it returns the cgrp on failure it returns NULL
+ */
+struct cgroup *cgroup_get_from_kernfs_id(u64 id)
+{
+	struct kernfs_node *kn;
+	struct cgroup *cgrp = NULL;
+
+	mutex_lock(&cgroup_mutex);
+	kn = kernfs_find_and_get_node_by_id(cgrp_dfl_root.kf_root, id);
+	if (!kn)
+		goto out_unlock;
+
+	cgrp = kn->priv;
+	if (cgroup_is_dead(cgrp) || !cgroup_tryget(cgrp))
+		cgrp = NULL;
+	kernfs_put(kn);
+out_unlock:
+	mutex_unlock(&cgroup_mutex);
+	return cgrp;
+}
+EXPORT_SYMBOL_GPL(cgroup_get_from_kernfs_id);
+
 /*
  * proc_cgroup_show()
  *  - Print task's cgroup paths into seq_file, one line for each hierarchy
-- 
2.26.2

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