rcu/nocb: Use separate flag to indicate disabled ->cblist

NULLing the RCU_NEXT_TAIL pointer was a clever way to save a byte, but
forward-progress considerations would require that this pointer be both
NULL and non-NULL, which, absent a quantum-computer port of the Linux
kernel, simply won't happen.  This commit therefore creates as separate
->enabled flag to replace the current NULL checks.

[ paulmck: Add include files per 0day test robot and -next. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

include/linux/rcu_segcblist.h

@@ -14,6 +14,9 @@
 #ifndef __INCLUDE_LINUX_RCU_SEGCBLIST_H
 #define __INCLUDE_LINUX_RCU_SEGCBLIST_H
 
+#include <linux/types.h>
+#include <linux/atomic.h>
+
 /* Simple unsegmented callback lists. */
 struct rcu_cblist {
 	struct rcu_head *head;
@@ -67,6 +70,7 @@ struct rcu_segcblist {
 	unsigned long gp_seq[RCU_CBLIST_NSEGS];
 	long len;
 	long len_lazy;
+	u8 enabled;
 };
 
 #define RCU_SEGCBLIST_INITIALIZER(n) \

kernel/rcu/rcu_segcblist.c

@@ -58,6 +58,7 @@ void rcu_segcblist_init(struct rcu_segcblist *rsclp)
 		rsclp->tails[i] = &rsclp->head;
 	rsclp->len = 0;
 	rsclp->len_lazy = 0;
+	rsclp->enabled = 1;
 }
 
 /*
@@ -69,7 +70,7 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
 	WARN_ON_ONCE(!rcu_segcblist_empty(rsclp));
 	WARN_ON_ONCE(rcu_segcblist_n_cbs(rsclp));
 	WARN_ON_ONCE(rcu_segcblist_n_lazy_cbs(rsclp));
-	rsclp->tails[RCU_NEXT_TAIL] = NULL;
+	rsclp->enabled = 0;
 }
 
 /*

kernel/rcu/rcu_segcblist.h

@@ -63,7 +63,7 @@ static inline long rcu_segcblist_n_nonlazy_cbs(struct rcu_segcblist *rsclp)
  */
 static inline bool rcu_segcblist_is_enabled(struct rcu_segcblist *rsclp)
 {
-	return !!rsclp->tails[RCU_NEXT_TAIL];
+	return rsclp->enabled;
 }
 
 /*

kernel/rcu/tree_plugin.h

@@ -2189,8 +2189,8 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
 				rcu_segcblist_n_cbs(&rdp->cblist));
 		atomic_long_set(&rdp->nocb_q_count_lazy,
 				rcu_segcblist_n_lazy_cbs(&rdp->cblist));
-		rcu_segcblist_init(&rdp->cblist);
 	}
+	rcu_segcblist_init(&rdp->cblist);
 	rcu_segcblist_disable(&rdp->cblist);
 	return true;
 }