kernfs: introduce kernfs_pin_sb()

kernfs_pin_sb() tries to get a refcnt of the superblock.

This will be used by cgroupfs.

v2:
- make kernfs_pin_sb() return the superblock.
- drop kernfs_drop_sb().

tj: Updated the comment a bit.

[ This is a prerequisite for a bugfix. ]
Cc: <stable@vger.kernel.org> # 3.15
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

2 files changed, 31 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",
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 17aa1cce6f8e..20f493564917 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -304,6 +304,7 @@ struct dentry *kernfs_mount_ns(struct file_system_type *fs_type, int flags,
                                struct kernfs_root *root, unsigned long magic,
                                bool *new_sb_created, const void *ns);
 void kernfs_kill_sb(struct super_block *sb);
+struct super_block *kernfs_pin_sb(struct kernfs_root *root, const void *ns);
 
 void kernfs_init(void);