cgroup: deal with dummp_top in cgroup_name() and cgroup_path()

My kernel fails to boot, because blkcg calls cgroup_path() while
cgroupfs is not mounted.

Fix both cgroup_name() and cgroup_path().

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 10 insertions, 2 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index ef0b3af0e61c..8c283a910b91 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -487,13 +487,21 @@ struct cgroup_subsys_state *seq_css(struct seq_file *seq);
 
 static inline int cgroup_name(struct cgroup *cgrp, char *buf, size_t buflen)
 {
-        return kernfs_name(cgrp->kn, buf, buflen);
+        /* dummy_top doesn't have a kn associated */
+        if (cgrp->kn)
+                return kernfs_name(cgrp->kn, buf, buflen);
+        else
+                return strlcpy(buf, "/", buflen);
 }
 
 static inline char * __must_check cgroup_path(struct cgroup *cgrp, char *buf,
                                               size_t buflen)
 {
-        return kernfs_path(cgrp->kn, buf, buflen);
+        /* dummy_top doesn't have a kn associated */
+        if (cgrp->kn)
+                return kernfs_path(cgrp->kn, buf, buflen);
+        strlcpy(buf, "/", buflen);
+        return (buflen <= 2) ? NULL : buf;
 }
 
 static inline void pr_cont_cgroup_name(struct cgroup *cgrp)