+ * DIO_LOCKING (simple locking for regular files)
+ * For writes we are called under i_mutex and return with i_mutex held, even
+ * though it is internally dropped.
+ * For reads, i_mutex is not held on entry, but it is taken and dropped before
+ * returning.
+ *
+ * DIO_OWN_LOCKING (filesystem provides synchronisation and handling of
+ * uninitialised data, allowing parallel direct readers and writers)
+ * For writes we are called without i_mutex, return without it, never touch it.
+ * For reads we are called under i_mutex and return with i_mutex held, even
+ * though it may be internally dropped.
+ *
+ * Additional i_alloc_sem locking requirements described inline below.