- * So the first lock acquired will have a lockdep subclass of 4, the
- * second lock will have a lockdep subclass of 5, and so on. It is
- * the responsibility of the class builder to shift this to the correct
- * portion of the lock_mode lockdep mask.
+ * However, MAX_LOCKDEP_SUBCLASSES == 8, which means we are greatly
+ * limited to the subclasses we can represent via nesting. We need at least
+ * 5 inodes nest depth for the ILOCK through rename, and we also have to support
+ * XFS_ILOCK_PARENT, which gives 6 subclasses. Then we have XFS_ILOCK_RTBITMAP
+ * and XFS_ILOCK_RTSUM, which are another 2 unique subclasses, so that's all
+ * 8 subclasses supported by lockdep.
+ *
+ * This also means we have to number the sub-classes in the lowest bits of
+ * the mask we keep, and we have to ensure we never exceed 3 bits of lockdep
+ * mask and we can't use bit-masking to build the subclasses. What a mess.
+ *
+ * Bit layout:
+ *
+ * Bit Lock Region
+ * 16-19 XFS_IOLOCK_SHIFT dependencies
+ * 20-23 XFS_MMAPLOCK_SHIFT dependencies
+ * 24-31 XFS_ILOCK_SHIFT dependencies
+ *
+ * IOLOCK values
+ *
+ * 0-3 subclass value
+ * 4-7 PARENT subclass values
+ *
+ * MMAPLOCK values
+ *
+ * 0-3 subclass value
+ * 4-7 unused
+ *
+ * ILOCK values
+ * 0-4 subclass values
+ * 5 PARENT subclass (not nestable)
+ * 6 RTBITMAP subclass (not nestable)
+ * 7 RTSUM subclass (not nestable)
+ *