rcu: Add functions to compare grace-period state values

This commit adds same_state_synchronize_rcu() and
same_state_synchronize_rcu_full() functions to compare grace-period state
values, for example, those obtained from get_state_synchronize_rcu()
and get_state_synchronize_rcu_full().  These functions allow small
structures to omit these state values by placing them in list headers for
lists containing structures with the same token value.  Presumably the
per-structure list pointers are the same ones used to link the structures
into whatever reader-accessible data structure was used.

This commit also adds both NUM_ACTIVE_RCU_POLL_OLDSTATE and
NUM_ACTIVE_RCU_POLL_FULL_OLDSTATE, which define the maximum number of
distinct unsigned long values and rcu_gp_oldstate values, respectively,
corresponding to not-yet-completed grace periods.  These values can be
used to size arrays of the list headers described above.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/rcupdate.h

@@ -47,6 +47,27 @@ struct rcu_gp_oldstate;
 unsigned long get_completed_synchronize_rcu(void);
 void get_completed_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp);
 
+// Maximum number of unsigned long values corresponding to
+// not-yet-completed RCU grace periods.
+#define NUM_ACTIVE_RCU_POLL_OLDSTATE 2
+
+/**
+ * same_state_synchronize_rcu - Are two old-state values identical?
+ * @oldstate1: First old-state value.
+ * @oldstate2: Second old-state value.
+ *
+ * The two old-state values must have been obtained from either
+ * get_state_synchronize_rcu(), start_poll_synchronize_rcu(), or
+ * get_completed_synchronize_rcu().  Returns @true if the two values are
+ * identical and @false otherwise.  This allows structures whose lifetimes
+ * are tracked by old-state values to push these values to a list header,
+ * allowing those structures to be slightly smaller.
+ */
+static inline bool same_state_synchronize_rcu(unsigned long oldstate1, unsigned long oldstate2)
+{
+	return oldstate1 == oldstate2;
+}
+
 #ifdef CONFIG_PREEMPT_RCU
 
 void __rcu_read_lock(void);

include/linux/rcutiny.h

@@ -18,6 +18,20 @@ struct rcu_gp_oldstate {
 	unsigned long rgos_norm;
 };
 
+// Maximum number of rcu_gp_oldstate values corresponding to
+// not-yet-completed RCU grace periods.
+#define NUM_ACTIVE_RCU_POLL_FULL_OLDSTATE 2
+
+/*
+ * Are the two oldstate values the same?  See the Tree RCU version for
+ * docbook header.
+ */
+static inline bool same_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp1,
+						   struct rcu_gp_oldstate *rgosp2)
+{
+	return rgosp1->rgos_norm == rgosp2->rgos_norm;
+}
+
 unsigned long get_state_synchronize_rcu(void);
 
 static inline void get_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp)

include/linux/rcutree.h

@@ -46,6 +46,34 @@ struct rcu_gp_oldstate {
 	unsigned long rgos_exp;
 };
 
+// Maximum number of rcu_gp_oldstate values corresponding to
+// not-yet-completed RCU grace periods.
+#define NUM_ACTIVE_RCU_POLL_FULL_OLDSTATE 4
+
+/**
+ * same_state_synchronize_rcu_full - Are two old-state values identical?
+ * @rgosp1: First old-state value.
+ * @rgosp2: Second old-state value.
+ *
+ * The two old-state values must have been obtained from either
+ * get_state_synchronize_rcu_full(), start_poll_synchronize_rcu_full(),
+ * or get_completed_synchronize_rcu_full().  Returns @true if the two
+ * values are identical and @false otherwise.  This allows structures
+ * whose lifetimes are tracked by old-state values to push these values
+ * to a list header, allowing those structures to be slightly smaller.
+ *
+ * Note that equality is judged on a bitwise basis, so that an
+ * @rcu_gp_oldstate structure with an already-completed state in one field
+ * will compare not-equal to a structure with an already-completed state
+ * in the other field.  After all, the @rcu_gp_oldstate structure is opaque
+ * so how did such a situation come to pass in the first place?
+ */
+static inline bool same_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp1,
+						   struct rcu_gp_oldstate *rgosp2)
+{
+	return rgosp1->rgos_norm == rgosp2->rgos_norm && rgosp1->rgos_exp == rgosp2->rgos_exp;
+}
+
 unsigned long start_poll_synchronize_rcu_expedited(void);
 void start_poll_synchronize_rcu_expedited_full(struct rcu_gp_oldstate *rgosp);
 void cond_synchronize_rcu_expedited(unsigned long oldstate);