srcu: Avoid srcutorture security-based pointer obfuscation

Because pointer output is now obfuscated, and because what you really
want to know is whether or not the callback lists are empty, this commit
replaces the srcu_data structure's head callback pointer printout with
a single character that is "." is the callback list is empty or "C"
otherwise.

This is the only remaining user of rcu_segcblist_head(), so this
commit also removes this function's definition.  It also turns out that
rcu_segcblist_tail() no longer has any callers, so this commit removes
that function's definition while in the area.  They were both marked
"Interim", and their end has come.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/rcu_segcblist.h

@@ -76,27 +76,6 @@ static inline bool rcu_segcblist_restempty(struct rcu_segcblist *rsclp, int seg)
 	return !*rsclp->tails[seg];
 }
 
-/*
- * Interim function to return rcu_segcblist head pointer.  Longer term, the
- * rcu_segcblist will be used more pervasively, removing the need for this
- * function.
- */
-static inline struct rcu_head *rcu_segcblist_head(struct rcu_segcblist *rsclp)
-{
-	return rsclp->head;
-}
-
-/*
- * Interim function to return rcu_segcblist head pointer.  Longer term, the
- * rcu_segcblist will be used more pervasively, removing the need for this
- * function.
- */
-static inline struct rcu_head **rcu_segcblist_tail(struct rcu_segcblist *rsclp)
-{
-	WARN_ON_ONCE(rcu_segcblist_empty(rsclp));
-	return rsclp->tails[RCU_NEXT_TAIL];
-}
-
 void rcu_segcblist_init(struct rcu_segcblist *rsclp);
 void rcu_segcblist_disable(struct rcu_segcblist *rsclp);
 bool rcu_segcblist_ready_cbs(struct rcu_segcblist *rsclp);

kernel/rcu/srcutree.c

@@ -1279,8 +1279,9 @@ void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf)
 
 		c0 = l0 - u0;
 		c1 = l1 - u1;
-		pr_cont(" %d(%ld,%ld %1p)",
-			cpu, c0, c1, rcu_segcblist_head(&sdp->srcu_cblist));
+		pr_cont(" %d(%ld,%ld %c)",
+			cpu, c0, c1,
+			"C."[rcu_segcblist_empty(&sdp->srcu_cblist)]);
 		s0 += c0;
 		s1 += c1;
 	}