sysfs, kernfs: implement kernfs_create/destroy_root()

There currently is single kernfs hierarchy in the whole system which
is used for sysfs.  kernfs needs to support multiple hierarchies to
allow other users.  This patch introduces struct kernfs_root which
serves as the root of each kernfs hierarchy and implements
kernfs_create/destroy_root().

* Each kernfs_root is associated with a root sd (sysfs_dentry).  The
  root is freed when the root sd is released and kernfs_destory_root()
  simply invokes kernfs_remove() on the root sd.  sysfs_remove_one()
  is updated to handle release of the root sd.  Note that ps_iattr
  update in sysfs_remove_one() is trivially updated for readability.

* Root sd's are now dynamically allocated using sysfs_new_dirent().
  Update sysfs_alloc_ino() so that it gives out ino from 1 so that the
  root sd still gets ino 1.

* While kernfs currently only points to the root sd, it'll soon grow
  fields which are specific to each hierarchy.  As determining a given
  sd's root will be necessary, sd->s_dir.root is added.  This backlink
  fits better as a separate field in sd; however, sd->s_dir is inside
  union with space to spare, so use it to save space and provide
  kernfs_root() accessor to determine the root sd.

* As hierarchies may be destroyed now, each mount needs to hold onto
  the hierarchy it's attached to.  Update sysfs_fill_super() and
  sysfs_kill_sb() so that they get and put the kernfs_root
  respectively.

* sysfs_root is replaced with kernfs_root which is dynamically created
  by invoking kernfs_create_root() from sysfs_init().

This patch doesn't introduce any visible behavior changes.

v2: kernfs_create_root() forgot to set @sd->priv.  Fixed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 62 insertions, 9 deletions

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index a4ca4de3cb21..246740a741ef 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -211,7 +211,7 @@ static int sysfs_alloc_ino(unsigned int *pino)
 
  retry:
         spin_lock(&sysfs_ino_lock);
-        rc = ida_get_new_above(&sysfs_ino_ida, 2, &ino);
+        rc = ida_get_new_above(&sysfs_ino_ida, 1, &ino);
         spin_unlock(&sysfs_ino_lock);
 
         if (rc == -EAGAIN) {
@@ -253,9 +253,11 @@ EXPORT_SYMBOL_GPL(kernfs_get);
 void kernfs_put(struct sysfs_dirent *sd)
 {
         struct sysfs_dirent *parent_sd;
+        struct kernfs_root *root;
 
         if (!sd || !atomic_dec_and_test(&sd->s_count))
                 return;
+        root = kernfs_root(sd);
  repeat:
         /* Moving/renaming is always done while holding reference.
          * sd->s_parent won't change beneath us.
@@ -278,8 +280,13 @@ void kernfs_put(struct sysfs_dirent *sd)
         kmem_cache_free(sysfs_dir_cachep, sd);
 
         sd = parent_sd;
-        if (sd && atomic_dec_and_test(&sd->s_count))
-                goto repeat;
+        if (sd) {
+                if (atomic_dec_and_test(&sd->s_count))
+                        goto repeat;
+        } else {
+                /* just released the root sd, free @root too */
+                kfree(root);
+        }
 }
 EXPORT_SYMBOL_GPL(kernfs_put);
 
@@ -493,13 +500,15 @@ static void sysfs_remove_one(struct sysfs_addrm_cxt *acxt,
         if (sd->s_flags & SYSFS_FLAG_REMOVED)
                 return;
 
-        sysfs_unlink_sibling(sd);
+        if (sd->s_parent) {
+                sysfs_unlink_sibling(sd);
 
-        /* Update timestamps on the parent */
-        ps_iattr = sd->s_parent->s_iattr;
-        if (ps_iattr) {
-                struct iattr *ps_iattrs = &ps_iattr->ia_iattr;
-                ps_iattrs->ia_ctime = ps_iattrs->ia_mtime = CURRENT_TIME;
+                /* Update timestamps on the parent */
+                ps_iattr = sd->s_parent->s_iattr;
+                if (ps_iattr) {
+                        ps_iattr->ia_iattr.ia_ctime = CURRENT_TIME;
+                        ps_iattr->ia_iattr.ia_mtime = CURRENT_TIME;
+                }
         }
 
         sd->s_flags |= SYSFS_FLAG_REMOVED;
@@ -604,6 +613,49 @@ struct sysfs_dirent *kernfs_find_and_get_ns(struct sysfs_dirent *parent,
 EXPORT_SYMBOL_GPL(kernfs_find_and_get_ns);
 
 /**
+ * kernfs_create_root - create a new kernfs hierarchy
+ * @priv: opaque data associated with the new directory
+ *
+ * Returns the root of the new hierarchy on success, ERR_PTR() value on
+ * failure.
+ */
+struct kernfs_root *kernfs_create_root(void *priv)
+{
+        struct kernfs_root *root;
+        struct sysfs_dirent *sd;
+
+        root = kzalloc(sizeof(*root), GFP_KERNEL);
+        if (!root)
+                return ERR_PTR(-ENOMEM);
+
+        sd = sysfs_new_dirent("", S_IFDIR | S_IRUGO | S_IXUGO, SYSFS_DIR);
+        if (!sd) {
+                kfree(root);
+                return ERR_PTR(-ENOMEM);
+        }
+
+        sd->s_flags &= ~SYSFS_FLAG_REMOVED;
+        sd->priv = priv;
+        sd->s_dir.root = root;
+
+        root->sd = sd;
+
+        return root;
+}
+
+/**
+ * kernfs_destroy_root - destroy a kernfs hierarchy
+ * @root: root of the hierarchy to destroy
+ *
+ * Destroy the hierarchy anchored at @root by removing all existing
+ * directories and destroying @root.
+ */
+void kernfs_destroy_root(struct kernfs_root *root)
+{
+        kernfs_remove(root->sd);        /* will also free @root */
+}
+
+/**
  * kernfs_create_dir_ns - create a directory
  * @parent: parent in which to create a new directory
  * @name: name of the new directory
@@ -626,6 +678,7 @@ struct sysfs_dirent *kernfs_create_dir_ns(struct sysfs_dirent *parent,
         if (!sd)
                 return ERR_PTR(-ENOMEM);
 
+        sd->s_dir.root = parent->s_dir.root;
         sd->s_ns = ns;
         sd->priv = priv;