rcu: Make cond_resched_rcu_qs() supply RCU-sched expedited QS

Although cond_resched_rcu_qs() supplies quiescent states to all flavors
of normal RCU grace periods, it does nothing for expedited RCU-sched
grace periods.  This commit therefore adds a check for a need for a
quiescent state from the current CPU by an expedited RCU-sched grace
period, and invokes rcu_sched_qs() to supply that quiescent state if so.

Note that the check is racy in that we might be migrated to some other
CPU just after checking the per-CPU variable.  This is OK because the
act of migration will do a context switch, which will supply the needed
quiescent state.  The only downside is that we might do an unnecessary
call to rcu_sched_qs(), but the probability is low and the overhead
is small.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -370,6 +370,21 @@ void rcu_all_qs(void)
 		rcu_momentary_dyntick_idle();
 		local_irq_restore(flags);
 	}
+	if (unlikely(raw_cpu_read(rcu_sched_data.cpu_no_qs.b.exp))) {
+		/*
+		 * Yes, we just checked a per-CPU variable with preemption
+		 * enabled, so we might be migrated to some other CPU at
+		 * this point.  That is OK because in that case, the
+		 * migration will supply the needed quiescent state.
+		 * We might end up needlessly disabling preemption and
+		 * invoking rcu_sched_qs() on the destination CPU, but
+		 * the probability and cost are both quite low, so this
+		 * should not be a problem in practice.
+		 */
+		preempt_disable();
+		rcu_sched_qs();
+		preempt_enable();
+	}
 	this_cpu_inc(rcu_qs_ctr);
 	barrier(); /* Avoid RCU read-side critical sections leaking up. */
 }