- Document more of the vnode locking strategy.

svn path=/head/; revision=120742

sys/sys/vnode.h

@@ -95,8 +95,17 @@ struct vpollinfo {
  *	u - Only a reference to the vnode is needed to read.
  *	v - vnode lock
  *
+ * Vnodes may be found on many lists.  The general way to deal with operating
+ * on a vnode that is on a list is:
+ *	1) Lock the list and find the vnode.
+ *	2) Lock interlock so that the vnode does not go away.
+ *	3) Unlock the list to avoid lock order reversals.
+ *	4) vget with LK_INTERLOCK and check for ENOENT, or
+ *	5) Check for XLOCK if the vnode lock is not required.
+ *	6) Perform your operation, then vput().
+ *
  * XXX Not all fields are locked yet and some fields that are marked are not
- * locked consistently.  This is a work in progress.
+ * locked consistently.  This is a work in progress.  Requires Giant!
  */
 
 struct vnode {