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>

1 files changed, 14 insertions, 0 deletions

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 4405e9112cee..768196a5f39d 100644
--- a/include/linux/rcutiny.h
+++ b/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)