linux-kernel - [ 19/72] cgroup: fix umount vs cgroup_event

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Message-Id: <20130719052601.181230149@linuxfoundation.org>
Date:	Thu, 18 Jul 2013 22:25:36 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
	Tejun Heo <tj@...nel.org>
Subject: [ 19/72] cgroup: fix umount vs cgroup_event_remove() race

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

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

From: Li Zefan <lizefan@...wei.com>

commit 1c8158eeae0f37d0eee9f1fbe68080df6a408df2 upstream.

 commit 5db9a4d99b0157a513944e9a44d29c9cec2e91dc
 Author: Tejun Heo <tj@...nel.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@...wei.com>
Signed-off-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/cgroup.c |   25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3727,6 +3727,23 @@ static int cgroup_write_notify_on_releas
 }
 
 /*
+ * 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.
@@ -3746,7 +3763,7 @@ static void cgroup_event_remove(struct w
 
 	eventfd_ctx_put(event->eventfd);
 	kfree(event);
-	dput(cgrp->dentry);
+	cgroup_dput(cgrp);
 }
 
 /*
@@ -4031,12 +4048,8 @@ static void css_dput_fn(struct work_stru
 {
 	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,


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