+/**
+ * nameidata_drop_rcu - drop this nameidata out of rcu-walk
+ * @nd: nameidata pathwalk data to drop
+ * Returns: 0 on success, -ECHILD on failure
+ *
+ * Path walking has 2 modes, rcu-walk and ref-walk (see
+ * Documentation/filesystems/path-lookup.txt). __drop_rcu* functions attempt
+ * to drop out of rcu-walk mode and take normal reference counts on dentries
+ * and vfsmounts to transition to rcu-walk mode. __drop_rcu* functions take
+ * refcounts at the last known good point before rcu-walk got stuck, so
+ * ref-walk may continue from there. If this is not successful (eg. a seqcount
+ * has changed), then failure is returned and path walk restarts from the
+ * beginning in ref-walk mode.
+ *
+ * nameidata_drop_rcu attempts to drop the current nd->path and nd->root into
+ * ref-walk. Must be called from rcu-walk context.
+ */
+static int nameidata_drop_rcu(struct nameidata *nd)
+{
+ struct fs_struct *fs = current->fs;
+ struct dentry *dentry = nd->path.dentry;
+
+ BUG_ON(!(nd->flags & LOOKUP_RCU));
+ if (nd->root.mnt) {
+ spin_lock(&fs->lock);
+ if (nd->root.mnt != fs->root.mnt ||
+ nd->root.dentry != fs->root.dentry)
+ goto err_root;
+ }
+ spin_lock(&dentry->d_lock);
+ if (!__d_rcu_to_refcount(dentry, nd->seq))
+ goto err;
+ BUG_ON(nd->inode != dentry->d_inode);
+ spin_unlock(&dentry->d_lock);
+ if (nd->root.mnt) {
+ path_get(&nd->root);
+ spin_unlock(&fs->lock);
+ }
+ mntget(nd->path.mnt);
+
+ rcu_read_unlock();
+ br_read_unlock(vfsmount_lock);
+ nd->flags &= ~LOOKUP_RCU;
+ return 0;
+err:
+ spin_unlock(&dentry->d_lock);
+err_root:
+ if (nd->root.mnt)
+ spin_unlock(&fs->lock);
+ return -ECHILD;
+}
+
+/* Try to drop out of rcu-walk mode if we were in it, otherwise do nothing. */
+static inline int nameidata_drop_rcu_maybe(struct nameidata *nd)
+{
+ if (nd->flags & LOOKUP_RCU)
+ return nameidata_drop_rcu(nd);
+ return 0;
+}
+
+/**
+ * nameidata_dentry_drop_rcu - drop nameidata and dentry out of rcu-walk
+ * @nd: nameidata pathwalk data to drop
+ * @dentry: dentry to drop
+ * Returns: 0 on success, -ECHILD on failure
+ *
+ * nameidata_dentry_drop_rcu attempts to drop the current nd->path and nd->root,
+ * and dentry into ref-walk. @dentry must be a path found by a do_lookup call on
+ * @nd. Must be called from rcu-walk context.
+ */
+static int nameidata_dentry_drop_rcu(struct nameidata *nd, struct dentry *dentry)
+{
+ struct fs_struct *fs = current->fs;
+ struct dentry *parent = nd->path.dentry;
+
+ /*
+ * It can be possible to revalidate the dentry that we started
+ * the path walk with. force_reval_path may also revalidate the
+ * dentry already committed to the nameidata.
+ */
+ if (unlikely(parent == dentry))
+ return nameidata_drop_rcu(nd);
+
+ BUG_ON(!(nd->flags & LOOKUP_RCU));
+ if (nd->root.mnt) {
+ spin_lock(&fs->lock);
+ if (nd->root.mnt != fs->root.mnt ||
+ nd->root.dentry != fs->root.dentry)
+ goto err_root;
+ }
+ spin_lock(&parent->d_lock);
+ spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
+ if (!__d_rcu_to_refcount(dentry, nd->seq))
+ goto err;
+ /*
+ * If the sequence check on the child dentry passed, then the child has
+ * not been removed from its parent. This means the parent dentry must
+ * be valid and able to take a reference at this point.
+ */
+ BUG_ON(!IS_ROOT(dentry) && dentry->d_parent != parent);
+ BUG_ON(!parent->d_count);
+ parent->d_count++;
+ spin_unlock(&dentry->d_lock);
+ spin_unlock(&parent->d_lock);
+ if (nd->root.mnt) {
+ path_get(&nd->root);
+ spin_unlock(&fs->lock);
+ }
+ mntget(nd->path.mnt);
+
+ rcu_read_unlock();
+ br_read_unlock(vfsmount_lock);
+ nd->flags &= ~LOOKUP_RCU;
+ return 0;
+err:
+ spin_unlock(&dentry->d_lock);
+ spin_unlock(&parent->d_lock);
+err_root:
+ if (nd->root.mnt)
+ spin_unlock(&fs->lock);
+ return -ECHILD;
+}
+
+/* Try to drop out of rcu-walk mode if we were in it, otherwise do nothing. */
+static inline int nameidata_dentry_drop_rcu_maybe(struct nameidata *nd, struct dentry *dentry)
+{
+ if (nd->flags & LOOKUP_RCU)
+ return nameidata_dentry_drop_rcu(nd, dentry);
+ return 0;
+}
+
+/**
+ * nameidata_drop_rcu_last - drop nameidata ending path walk out of rcu-walk
+ * @nd: nameidata pathwalk data to drop
+ * Returns: 0 on success, -ECHILD on failure
+ *
+ * nameidata_drop_rcu_last attempts to drop the current nd->path into ref-walk.
+ * nd->path should be the final element of the lookup, so nd->root is discarded.
+ * Must be called from rcu-walk context.
+ */
+static int nameidata_drop_rcu_last(struct nameidata *nd)
+{
+ struct dentry *dentry = nd->path.dentry;
+
+ BUG_ON(!(nd->flags & LOOKUP_RCU));
+ nd->flags &= ~LOOKUP_RCU;
+ nd->root.mnt = NULL;
+ spin_lock(&dentry->d_lock);
+ if (!__d_rcu_to_refcount(dentry, nd->seq))
+ goto err_unlock;
+ BUG_ON(nd->inode != dentry->d_inode);
+ spin_unlock(&dentry->d_lock);
+
+ mntget(nd->path.mnt);
+
+ rcu_read_unlock();
+ br_read_unlock(vfsmount_lock);
+
+ return 0;
+
+err_unlock:
+ spin_unlock(&dentry->d_lock);
+ rcu_read_unlock();
+ br_read_unlock(vfsmount_lock);
+ return -ECHILD;
+}
+
+/* Try to drop out of rcu-walk mode if we were in it, otherwise do nothing. */
+static inline int nameidata_drop_rcu_last_maybe(struct nameidata *nd)
+{
+ if (likely(nd->flags & LOOKUP_RCU))
+ return nameidata_drop_rcu_last(nd);
+ return 0;
+}
+