cgroup: Fix an RCU warning in cgroup_path()

with CONFIG_PROVE_RCU=y, a warning can be triggered:

  # mount -t cgroup -o debug xxx /mnt
  # cat /proc/$$/cgroup

...
kernel/cgroup.c:1649 invoked rcu_dereference_check() without protection!
...

This is a false-positive, because cgroup_path() can be called
with either rcu_read_lock() held or cgroup_mutex held.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index e2769e13980c..4ca928db890c 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1646,7 +1646,9 @@ static inline struct cftype *__d_cft(struct dentry *dentry)
 int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
 {
         char *start;
-        struct dentry *dentry = rcu_dereference(cgrp->dentry);
+        struct dentry *dentry = rcu_dereference_check(cgrp->dentry,
+                                                      rcu_read_lock_held() ||
+                                                      cgroup_lock_is_held());
 
         if (!dentry || cgrp == dummytop) {
                 /*
@@ -1662,13 +1664,17 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
         *--start = '\0';
         for (;;) {
                 int len = dentry->d_name.len;
+
                 if ((start -= len) < buf)
                         return -ENAMETOOLONG;
-                memcpy(start, cgrp->dentry->d_name.name, len);
+                memcpy(start, dentry->d_name.name, len);
                 cgrp = cgrp->parent;
                 if (!cgrp)
                         break;
-                dentry = rcu_dereference(cgrp->dentry);
+
+                dentry = rcu_dereference_check(cgrp->dentry,
+                                               rcu_read_lock_held() ||
+                                               cgroup_lock_is_held());
                 if (!cgrp->parent)
                         continue;
                 if (--start < buf)