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>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index e2769e1..4ca928d 100644 (file)
--- 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)