rcu: Provide polling interfaces for Tree RCU grace periods

There is a need for a non-blocking polling interface for RCU grace
periods, so this commit supplies start_poll_synchronize_rcu() and
poll_state_synchronize_rcu() for this purpose.  Note that the existing
get_state_synchronize_rcu() may be used if future grace periods are
inevitable (perhaps due to a later call_rcu() invocation).  The new
start_poll_synchronize_rcu() is to be used if future grace periods
might not otherwise happen.  Finally, poll_state_synchronize_rcu()
provides a lockless check for a grace period having elapsed since
the corresponding call to either of the get_state_synchronize_rcu()
or start_poll_synchronize_rcu().

As with get_state_synchronize_rcu(), the return value from either
get_state_synchronize_rcu() or start_poll_synchronize_rcu() is passed in
to a later call to either poll_state_synchronize_rcu() or the existing
(might_sleep) cond_synchronize_rcu().

[ paulmck: Remove redundant smp_mb() per Frederic Weisbecker feedback. ]
[ Update poll_state_synchronize_rcu() docbook per Frederic Weisbecker feedback. ]
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index df578b73960f..b89b54130f49 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -41,6 +41,8 @@ void rcu_momentary_dyntick_idle(void);
 void kfree_rcu_scheduler_running(void);
 bool rcu_gp_might_be_stalled(void);
 unsigned long get_state_synchronize_rcu(void);
+unsigned long start_poll_synchronize_rcu(void);
+bool poll_state_synchronize_rcu(unsigned long oldstate);
 void cond_synchronize_rcu(unsigned long oldstate);
 
 void rcu_idle_enter(void);