nfsd: improve stateid access bitmask documentation

The use of the bitmaps is confusing.  Add a cross-reference to make it
easier to find the existing comment.  Add an updated reference with URL
to make it quicker to look up.  And a bit more editorializing about the
value of this.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4state.c

@@ -360,11 +360,13 @@ static const struct nfsd4_callback_ops nfsd4_cb_notify_lock_ops = {
  * st_{access,deny}_bmap field of the stateid, in order to track not
  * only what share bits are currently in force, but also what
  * combinations of share bits previous opens have used.  This allows us
- * to enforce the recommendation of rfc 3530 14.2.19 that the server
- * return an error if the client attempt to downgrade to a combination
- * of share bits not explicable by closing some of its previous opens.
+ * to enforce the recommendation in
+ * https://datatracker.ietf.org/doc/html/rfc7530#section-16.19.4 that
+ * the server return an error if the client attempt to downgrade to a
+ * combination of share bits not explicable by closing some of its
+ * previous opens.
  *
- * XXX: This enforcement is actually incomplete, since we don't keep
+ * This enforcement is arguably incomplete, since we don't keep
  * track of access/deny bit combinations; so, e.g., we allow:
  *
  *	OPEN allow read, deny write
@@ -372,6 +374,10 @@ static const struct nfsd4_callback_ops nfsd4_cb_notify_lock_ops = {
  *	DOWNGRADE allow read, deny none
  *
  * which we should reject.
+ *
+ * But you could also argue that our current code is already overkill,
+ * since it only exists to return NFS4ERR_INVAL on incorrect client
+ * behavior.
  */
 static unsigned int
 bmap_to_share_mode(unsigned long bmap)

fs/nfsd/state.h

@@ -568,6 +568,10 @@ struct nfs4_ol_stateid {
 	struct list_head		st_locks;
 	struct nfs4_stateowner		*st_stateowner;
 	struct nfs4_clnt_odstate	*st_clnt_odstate;
+/*
+ * These bitmasks use 3 separate bits for READ, ALLOW, and BOTH; see the
+ * comment above bmap_to_share_mode() for explanation:
+ */
 	unsigned char			st_access_bmap;
 	unsigned char			st_deny_bmap;
 	struct nfs4_ol_stateid		*st_openstp;