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@xxxxxxxxxx>
---
include/linux/cgroup.h | 2 +-
kernel/cgroup.c | 11 ++++++-----
2 files changed, 7 insertions(+), 6 deletions(-)
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 8f64b45..d605857 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -165,7 +165,7 @@ struct cgroup {
struct list_head files; /* my files */
struct cgroup *parent; /* my parent */
- struct dentry __rcu *dentry; /* cgroup fs entry, RCU protected */
+ struct dentry *dentry; /* cgroup fs entry, RCU protected */
/* Private pointers for each registered subsystem */
struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 40707cd..278752e 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 (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++;
--
1.7.11.7
--
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