exportfs: more detailed comment for path_reconnect

/*

 * Make sure target_dir is fully connected to the dentry tree.

 *

 * It may already be, as the flag isn't always updated when connection happens.

 * On successful return, DCACHE_DISCONNECTED will be cleared on

 * target_dir, and target_dir->d_parent->...->d_parent will reach the

 * root of the filesystem.

 *

 * Whenever DCACHE_DISCONNECTED is unset, target_dir is fully connected.

 * But the converse is not true: target_dir may have DCACHE_DISCONNECTED

 * set but already be connected.  In that case we'll verify the

 * connection to root and then clear the flag.

 *

 * Note that target_dir could be removed by a concurrent operation.  In

 * that case reconnect_path may still succeed with target_dir fully

 * connected, but further operations using the filehandle will fail when

 * necessary (due to S_DEAD being set on the directory).

 */

static int

reconnect_path(struct vfsmount *mnt, struct dentry *target_dir, char *nbuf)