cgroup: fix umount vs cgroup_event_remove() race
commit 5db9a4d99b0157a513944e9a44d29c9cec2e91dc
Author: Tejun Heo <tj@kernel.org>
Date: Sat Jul 7 16:08:18 2012 -0700
cgroup: fix cgroup hierarchy umount race
This commit fixed a race caused by the dput() in css_dput_fn(), but
the dput() in cgroup_event_remove() can also lead to the same BUG().
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 0224f6b..7db2940 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3822,6 +3822,23 @@
}
/*
+ * When dput() is called asynchronously, if umount has been done and
+ * then deactivate_super() in cgroup_free_fn() kills the superblock,
+ * there's a small window that vfs will see the root dentry with non-zero
+ * refcnt and trigger BUG().
+ *
+ * That's why we hold a reference before dput() and drop it right after.
+ */
+static void cgroup_dput(struct cgroup *cgrp)
+{
+ struct super_block *sb = cgrp->root->sb;
+
+ atomic_inc(&sb->s_active);
+ dput(cgrp->dentry);
+ deactivate_super(sb);
+}
+
+/*
* Unregister event and free resources.
*
* Gets called from workqueue.
@@ -3841,7 +3858,7 @@
eventfd_ctx_put(event->eventfd);
kfree(event);
- dput(cgrp->dentry);
+ cgroup_dput(cgrp);
}
/*
@@ -4129,12 +4146,8 @@
{
struct cgroup_subsys_state *css =
container_of(work, struct cgroup_subsys_state, dput_work);
- struct dentry *dentry = css->cgroup->dentry;
- struct super_block *sb = dentry->d_sb;
- atomic_inc(&sb->s_active);
- dput(dentry);
- deactivate_super(sb);
+ cgroup_dput(css->cgroup);
}
static void css_release(struct percpu_ref *ref)