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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>
This commit is contained in:
Tejun Heo 2012-11-19 08:13:36 -08:00
parent 4e139afc22
commit febfcef60d
2 changed files with 7 additions and 6 deletions
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2
include/linux/cgroup.h
View File

@ -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];

11
kernel/cgroup.c
View File

@ -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++;