commit 146aa1bd0511f88ddb4e92fafa2b8aad4f2f65f3
author Lai Jiangshan <laijs@cn.fujitsu.com> Sat Oct 18 20:28:03 2008 -0700
committer Linus Torvalds <torvalds@linux-foundation.org> Mon Oct 20 08:52:38 2008 -0700
tree d7deb46b9a38f82f109b2126317899efbbce41c2
parent 248736c2a57206388c86f8cdd3392ee986e84f9f

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>

kernel/cgroup_debug.c

diff --git a/kernel/cgroup_debug.c b/kernel/cgroup_debug.c
index c3dc3ab..daca620 100644
--- a/kernel/cgroup_debug.c
+++ b/kernel/cgroup_debug.c
@@ -57,7 +57,7 @@
 	u64 count;
 
 	rcu_read_lock();
-	count = atomic_read(&current->cgroups->ref.refcount);
+	count = atomic_read(&current->cgroups->refcount);
 	rcu_read_unlock();
 	return count;
 }
@@ -90,7 +90,7 @@
 	{
 		.name = "releasable",
 		.read_u64 = releasable_read,
-	}
+	},
 };
 
 static int debug_populate(struct cgroup_subsys *ss, struct cgroup *cont)