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Subject[PATCH 4/5] kernfs: introduce kernfs_pin_sb() and kernfs_drop_sb()

kernfs_pin_sb() tries to get a refcnt of the superblock, while
kernfs_drop_sb() drops this refcnt.

This will be used by cgroupfs.

Signed-off-by: Li Zefan <lizefan@huawei.com>
---
fs/kernfs/mount.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
include/linux/kernfs.h | 3 +++
2 files changed, 48 insertions(+)

diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index f25a7c0..4f924e0 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -210,6 +210,51 @@ 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.
+ */
+bool kernfs_pin_sb(struct kernfs_root *root, const void *ns)
+{
+ struct kernfs_super_info *info;
+ int ret = false;
+
+ mutex_lock(&kernfs_mutex);
+ list_for_each_entry(info, &root->supers, node) {
+ if (info->ns == ns) {
+ ret = atomic_inc_not_zero(&info->sb->s_active);
+ break;
+ }
+ }
+ mutex_unlock(&kernfs_mutex);
+ return ret;
+}
+
+/**
+ * kernfs_drop_sb: drop the refcnt that we got by kernfs_pin_sb()
+ * @root: the kernfs_root in question
+ * @ns: the namespace tag
+ *
+ * This must be paired with kernfs_pin_sb(). It will require sb->u_mount
+ * if the refcnt reaches zero.
+ */
+void kernfs_drop_sb(struct kernfs_root *root, const void *ns)
+{
+ struct kernfs_super_info *info;
+
+ mutex_lock(&kernfs_mutex);
+ list_for_each_entry(info, &root->supers, node) {
+ if (info->ns == ns)
+ break;
+ }
+ mutex_unlock(&kernfs_mutex);
+ deactivate_super(info->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 589318b..1958017 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -288,6 +288,9 @@ struct dentry *kernfs_mount_ns(struct file_system_type *fs_type, int flags,
const void *ns);
void kernfs_kill_sb(struct super_block *sb);

+bool kernfs_pin_sb(struct kernfs_root *root, const void *ns);
+void kernfs_drop_sb(struct kernfs_root *root, const void *ns);
+
void kernfs_init(void);

#else /* CONFIG_KERNFS */
--
1.8.0.2


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