rcu: Immediately boost preempted readers for strict grace periods

The intent of the CONFIG_RCU_STRICT_GRACE_PERIOD Konfig option is to
cause normal grace periods to complete quickly in order to better catch
errors resulting from improperly leaking pointers from RCU read-side
critical sections.  However, kernels built with this option enabled still
wait for some hundreds of milliseconds before boosting RCU readers that
have been preempted within their current critical section.  The value
of this delay is set by the CONFIG_RCU_BOOST_DELAY Kconfig option,
which defaults to 500 milliseconds.

This commit therefore causes kernels build with strict grace periods
to ignore CONFIG_RCU_BOOST_DELAY.  This causes rcu_initiate_boost()
to start boosting immediately after all CPUs on a given leaf rcu_node
structure have passed through their quiescent states.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_plugin.h

@@ -1159,7 +1159,8 @@ static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
 	    (rnp->gp_tasks != NULL &&
 	     rnp->boost_tasks == NULL &&
 	     rnp->qsmask == 0 &&
-	     (!time_after(rnp->boost_time, jiffies) || rcu_state.cbovld))) {
+	     (!time_after(rnp->boost_time, jiffies) || rcu_state.cbovld ||
+	      IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)))) {
 		if (rnp->exp_tasks == NULL)
 			WRITE_ONCE(rnp->boost_tasks, rnp->gp_tasks);
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);