[PATCH 1/3] cgroup: fix umount vs cgroup_cfs_commit() race

[PATCH 1/3] cgroup: fix umount vs cgroup_cfs_commit() race

[Li Zefan]

cgroup_cfs_commit() uses dget() to keep cgroup alive after cgroup_mutex
is dropped, but dget() won't prevent cgroupfs from being umounted. When
the race happens, vfs will see some dentries with non-zero refcnt while
umount is in process.

Keep running this:
  mount -t cgroup -o blkio xxx /cgroup
  umount /cgroup

And this:
  modprobe cfq-iosched
  rmmod cfs-iosched

After a while, the BUG() in shrink_dcache_for_umount_subtree() may
be triggered:

  BUG: Dentry xxx{i=0,n=blkio.yyy} still in use (1) [umount of cgroup cgroup]

Signed-off-by: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
---
 kernel/cgroup.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 25f1054..482252a 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2788,13 +2788,17 @@ static void cgroup_cfts_commit(struct cgroup_subsys *ss,
 {
 	LIST_HEAD(pending);
 	struct cgroup *cgrp, *n;
+	struct super_block *sb = ss->root->sb;
 
 	/* %NULL @cfts indicates abort and don't bother if @ss isn't attached */
-	if (cfts && ss->root != &rootnode) {
+	if (cfts && ss->root != &rootnode &&
+	    atomic_inc_not_zero(sb->s_active)) {
 		list_for_each_entry(cgrp, &ss->root->allcg_list, allcg_node) {
 			dget(cgrp->dentry);
 			list_add_tail(&cgrp->cft_q_node, &pending);
 		}
+	} else {
+		sb = NULL;
 	}
 
 	mutex_unlock(&cgroup_mutex);
@@ -2817,6 +2821,9 @@ static void cgroup_cfts_commit(struct cgroup_subsys *ss,
 		dput(cgrp->dentry);
 	}
 
+	if (sb)
+		deactivate_super(sb);
+
 	mutex_unlock(&cgroup_cft_mutex);
 }
 
-- 
1.8.0.2

[PATCH 2/3] cgroup: fix umount vs cgroup_event_remove() race

[Li Zefan]

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

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 482252a..e2dcf08 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3812,6 +3812,23 @@ 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.
@@ -3831,7 +3848,7 @@ static void cgroup_event_remove(struct work_struct *work)
 
 	eventfd_ctx_put(event->eventfd);
 	kfree(event);
-	dput(cgrp->dentry);
+	cgroup_dput(cgrp);
 }
 
 /*
@@ -4119,12 +4136,8 @@ 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 css_release(struct percpu_ref *ref)
-- 
1.8.0.2

[PATCH 3/3] cgroup: fix memory leak in cgroup_rm_cftypes()

[Li Zefan]

The memory allocated in cgroup_add_cftypes() should be freed. The
effect of this bug is we leak a bit memory everytime we unload
cfq-iosched module if blkio cgroup is enabled.

Signed-off-by: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
---
 kernel/cgroup.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index e2dcf08..1d5b1d6 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2879,7 +2879,8 @@ int cgroup_rm_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
 
 	list_for_each_entry(set, &ss->cftsets, node) {
 		if (set->cfts == cfts) {
-			list_del_init(&set->node);
+			list_del(&set->node);
+			kfree(set);
 			cgroup_cfts_commit(ss, cfts, false);
 			return 0;
 		}
-- 
1.8.0.2

Re: [PATCH 1/3] cgroup: fix umount vs cgroup_cfs_commit() race

[Tejun Heo]

On Tue, Jun 18, 2013 at 06:40:19PM +0800, Li Zefan wrote:
> cgroup_cfs_commit() uses dget() to keep cgroup alive after cgroup_mutex
> is dropped, but dget() won't prevent cgroupfs from being umounted. When
> the race happens, vfs will see some dentries with non-zero refcnt while
> umount is in process.
> 
> Keep running this:
>   mount -t cgroup -o blkio xxx /cgroup
>   umount /cgroup
> 
> And this:
>   modprobe cfq-iosched
>   rmmod cfs-iosched
> 
> After a while, the BUG() in shrink_dcache_for_umount_subtree() may
> be triggered:
> 
>   BUG: Dentry xxx{i=0,n=blkio.yyy} still in use (1) [umount of cgroup cgroup]
> 
> Signed-off-by: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>

Applied 1-3 to cgroup/for-3.11 w/ stable cc'd on 1 and 2.

Thanks!

-- 
tejun