cgroup: cgroup->dentry isn't a RCU pointer

cgroup->dentry is marked and used as a RCU pointer; however, it isn't
one - the final dentry put doesn't go through call_rcu().  cgroup and
dentry share the same RCU freeing rule via synchronize_rcu() in
cgroup_diput() (kfree_rcu() used on cgrp is unnecessary).  If cgrp is
accessible under RCU read lock, so is its dentry and dereferencing
cgrp->dentry doesn't need any further RCU protection or annotation.

While not being accurate, before the previous patch, the RCU accessors
served a purpose as memory barriers - cgroup->dentry used to be
assigned after the cgroup was made visible to cgroup_path(), so the
assignment and dereferencing in cgroup_path() needed the memory
barrier pair.  Now that list_add_tail_rcu() happens after
cgroup->dentry is assigned, this no longer is necessary.

Remove the now unnecessary and misleading RCU annotations from
cgroup->dentry.  To make up for the removal of rcu_dereference_check()
in cgroup_path(), add an explicit rcu_lockdep_assert(), which asserts
the dereference rule of @cgrp, not cgrp->dentry.

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

1 files changed, 6 insertions, 5 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index d62a529db2f7..affc76d7f739 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1756,9 +1756,11 @@ static struct kobject *cgroup_kobj;
  */
 int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
 {
+        struct dentry *dentry = cgrp->dentry;
         char *start;
-        struct dentry *dentry = rcu_dereference_check(cgrp->dentry,
-                                                      cgroup_lock_is_held());
+
+        rcu_lockdep_assert(rcu_read_lock_held() || cgroup_lock_is_held(),
+                           "cgroup_path() called without proper locking");
 
         if (!dentry || cgrp == dummytop) {
                 /*
@@ -1782,8 +1784,7 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
                 if (!cgrp)
                         break;
 
-                dentry = rcu_dereference_check(cgrp->dentry,
-                                               cgroup_lock_is_held());
+                dentry = cgrp->dentry;
                 if (!cgrp->parent)
                         continue;
                 if (--start < buf)
@@ -4124,7 +4125,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 
         /* allocation complete, commit to creation */
         dentry->d_fsdata = cgrp;
-        rcu_assign_pointer(cgrp->dentry, dentry);
+        cgrp->dentry = dentry;
         list_add_tail(&cgrp->allcg_node, &root->allcg_list);
         list_add_tail_rcu(&cgrp->sibling, &cgrp->parent->children);
         root->number_of_cgroups++;