rcu: Add full-sized polling for start_poll_expedited()

The start_poll_synchronize_rcu_expedited() API compresses the combined
expedited and normal grace-period states into a single unsigned long,
which conserves storage, but can miss grace periods in certain cases
involving overlapping normal and expedited grace periods.  Missing the
occasional grace period is usually not a problem, but there are use
cases that care about each and every grace period.

This commit therefore adds yet another member of the
full-state RCU grace-period polling API, which is the
start_poll_synchronize_rcu_expedited_full() function.  This uses up to
three times the storage (rcu_gp_oldstate structure instead of unsigned
long), but is guaranteed not to miss grace periods.

[ paulmck: Apply feedback from kernel test robot and Julia Lawall. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index be667583a554..18128ee0d36c 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -1028,6 +1028,24 @@ unsigned long start_poll_synchronize_rcu_expedited(void)
 EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_expedited);
 
 /**
+ * start_poll_synchronize_rcu_expedited_full - Take a full snapshot and start expedited grace period
+ * @rgosp: Place to put snapshot of grace-period state
+ *
+ * Places the normal and expedited grace-period states in rgosp.  This
+ * state value can be passed to a later call to cond_synchronize_rcu_full()
+ * or poll_state_synchronize_rcu_full() to determine whether or not a
+ * grace period (whether normal or expedited) has elapsed in the meantime.
+ * If the needed expedited grace period is not already slated to start,
+ * initiates that grace period.
+ */
+void start_poll_synchronize_rcu_expedited_full(struct rcu_gp_oldstate *rgosp)
+{
+	get_state_synchronize_rcu_full(rgosp);
+	(void)start_poll_synchronize_rcu_expedited();
+}
+EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_expedited_full);
+
+/**
  * cond_synchronize_rcu_expedited - Conditionally wait for an expedited RCU grace period
  *
  * @oldstate: value from get_state_synchronize_rcu(), start_poll_synchronize_rcu(), or start_poll_synchronize_rcu_expedited()