cgroups: fix probable race with put_css_set[_taskexit] and find_css_set
put_css_set_taskexit may be called when find_css_set is called on other
cpu. And the race will occur:
put_css_set_taskexit side find_css_set side
|
atomic_dec_and_test(&kref->refcount) |
/* kref->refcount = 0 */ |
....................................................................
| read_lock(&css_set_lock)
| find_existing_css_set
| get_css_set
| read_unlock(&css_set_lock);
....................................................................
__release_css_set |
....................................................................
| /* use a released css_set */
|
[put_css_set is the same. But in the current code, all put_css_set are
put into cgroup mutex critical region as the same as find_css_set.]
[akpm@linux-foundation.org: repair comments]
[menage@google.com: eliminate race in css_set refcounting]
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 8c6e1c1..1e49218 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -241,7 +241,6 @@
struct cg_cgroup_link *link;
struct cg_cgroup_link *saved_link;
- write_lock(&css_set_lock);
hlist_del(&cg->hlist);
css_set_count--;
@@ -251,16 +250,25 @@
list_del(&link->cgrp_link_list);
kfree(link);
}
-
- write_unlock(&css_set_lock);
}
-static void __release_css_set(struct kref *k, int taskexit)
+static void __put_css_set(struct css_set *cg, int taskexit)
{
int i;
- struct css_set *cg = container_of(k, struct css_set, ref);
-
+ /*
+ * Ensure that the refcount doesn't hit zero while any readers
+ * can see it. Similar to atomic_dec_and_lock(), but for an
+ * rwlock
+ */
+ if (atomic_add_unless(&cg->refcount, -1, 1))
+ return;
+ write_lock(&css_set_lock);
+ if (!atomic_dec_and_test(&cg->refcount)) {
+ write_unlock(&css_set_lock);
+ return;
+ }
unlink_css_set(cg);
+ write_unlock(&css_set_lock);
rcu_read_lock();
for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
@@ -276,32 +284,22 @@
kfree(cg);
}
-static void release_css_set(struct kref *k)
-{
- __release_css_set(k, 0);
-}
-
-static void release_css_set_taskexit(struct kref *k)
-{
- __release_css_set(k, 1);
-}
-
/*
* refcounted get/put for css_set objects
*/
static inline void get_css_set(struct css_set *cg)
{
- kref_get(&cg->ref);
+ atomic_inc(&cg->refcount);
}
static inline void put_css_set(struct css_set *cg)
{
- kref_put(&cg->ref, release_css_set);
+ __put_css_set(cg, 0);
}
static inline void put_css_set_taskexit(struct css_set *cg)
{
- kref_put(&cg->ref, release_css_set_taskexit);
+ __put_css_set(cg, 1);
}
/*
@@ -427,7 +425,7 @@
return NULL;
}
- kref_init(&res->ref);
+ atomic_set(&res->refcount, 1);
INIT_LIST_HEAD(&res->cg_links);
INIT_LIST_HEAD(&res->tasks);
INIT_HLIST_NODE(&res->hlist);
@@ -1728,7 +1726,7 @@
read_lock(&css_set_lock);
list_for_each_entry(link, &cgrp->css_sets, cgrp_link_list) {
- count += atomic_read(&link->cg->ref.refcount);
+ count += atomic_read(&link->cg->refcount);
}
read_unlock(&css_set_lock);
return count;
@@ -2495,8 +2493,7 @@
int __init cgroup_init_early(void)
{
int i;
- kref_init(&init_css_set.ref);
- kref_get(&init_css_set.ref);
+ atomic_set(&init_css_set.refcount, 1);
INIT_LIST_HEAD(&init_css_set.cg_links);
INIT_LIST_HEAD(&init_css_set.tasks);
INIT_HLIST_NODE(&init_css_set.hlist);