Merge branch 'for-3.16-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup

Pull cgroup fixes from Tejun Heo:
 "Mostly fixes for the fallouts from the recent cgroup core changes.

  The decoupled nature of cgroup dynamic hierarchy management
  (hierarchies are created dynamically on mount but may or may not be
  reused once unmounted depending on remaining usages) led to more
  ugliness being added to kernfs.

  Hopefully, this is the last of it"

* 'for-3.16-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup:
  cpuset: break kernfs active protection in cpuset_write_resmask()
  cgroup: fix a race between cgroup_mount() and cgroup_kill_sb()
  kernfs: introduce kernfs_pin_sb()
  cgroup: fix mount failure in a corner case
  cpuset,mempolicy: fix sleeping function called from invalid context
  cgroup: fix broken css_has_online_children()

1 files changed, 30 insertions, 0 deletions

diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index d171b98a6cdd..f973ae9b05f1 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -211,6 +211,36 @@ void kernfs_kill_sb(struct super_block *sb)
         kernfs_put(root_kn);
 }
 
+/**
+ * kernfs_pin_sb: try to pin the superblock associated with a kernfs_root
+ * @kernfs_root: the kernfs_root in question
+ * @ns: the namespace tag
+ *
+ * Pin the superblock so the superblock won't be destroyed in subsequent
+ * operations.  This can be used to block ->kill_sb() which may be useful
+ * for kernfs users which dynamically manage superblocks.
+ *
+ * Returns NULL if there's no superblock associated to this kernfs_root, or
+ * -EINVAL if the superblock is being freed.
+ */
+struct super_block *kernfs_pin_sb(struct kernfs_root *root, const void *ns)
+{
+        struct kernfs_super_info *info;
+        struct super_block *sb = NULL;
+
+        mutex_lock(&kernfs_mutex);
+        list_for_each_entry(info, &root->supers, node) {
+                if (info->ns == ns) {
+                        sb = info->sb;
+                        if (!atomic_inc_not_zero(&info->sb->s_active))
+                                sb = ERR_PTR(-EINVAL);
+                        break;
+                }
+        }
+        mutex_unlock(&kernfs_mutex);
+        return sb;
+}
+
 void __init kernfs_init(void)
 {
         kernfs_node_cache = kmem_cache_create("kernfs_node_cache",