[73/88] cgroup: fix umount vs cgroup_event_remove() race

Steven Rostedt
2013-08-13T16:33:53Z

[-- Attachment #1: 0073-cgroup-fix-umount-vs-cgroup_event_remove-race.patch --]
[-- Type: text/plain, Size: 2210 bytes --]

3.6.11.7-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Li Zefan <lizefan@huawei.com>

[ Upstream commit 1c8158eeae0f37d0eee9f1fbe68080df6a408df2 ]

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
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
kernel/cgroup.c | 25 +++++++++++++++++++------
1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index bfc0578..9cf9102 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3613,6 +3613,23 @@ static int cgroup_write_notify_on_release(struct cgroup *cgrp,
}

/*
+ * 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.
@@ -3627,7 +3644,7 @@ static void cgroup_event_remove(struct work_struct *work)

eventfd_ctx_put(event->eventfd);
kfree(event);
- dput(cgrp->dentry);
+ cgroup_dput(cgrp);
}

/*
@@ -3878,12 +3895,8 @@ static void css_dput_fn(struct work_struct *work)
{
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 init_cgroup_css(struct cgroup_subsys_state *css,
--
1.7.10.4