rcu: Avoid RCU-preempt expedited grace-period botch

Because rcu_read_unlock_special() samples rcu_preempted_readers_exp(rnp)
after dropping rnp->lock, the following sequence of events is possible:

1.	Task A exits its RCU read-side critical section, and removes
	itself from the ->blkd_tasks list, releases rnp->lock, and is
	then preempted.  Task B remains on the ->blkd_tasks list, and
	blocks the current expedited grace period.

2.	Task B exits from its RCU read-side critical section and removes
	itself from the ->blkd_tasks list.  Because it is the last task
	blocking the current expedited grace period, it ends that
	expedited grace period.

3.	Task A resumes, and samples rcu_preempted_readers_exp(rnp) which
	of course indicates that nothing is blocking the nonexistent
	expedited grace period. Task A is again preempted.

4.	Some other CPU starts an expedited grace period.  There are several
	tasks blocking this expedited grace period queued on the
	same rcu_node structure that Task A was using in step 1 above.

5.	Task A examines its state and incorrectly concludes that it was
	the last task blocking the expedited grace period on the current
	rcu_node structure.  It therefore reports completion up the
	rcu_node tree.

6.	The expedited grace period can then incorrectly complete before
	the tasks blocked on this same rcu_node structure exit their
	RCU read-side critical sections.  Arbitrarily bad things happen.

This commit therefore takes a snapshot of rcu_preempted_readers_exp(rnp)
prior to dropping the lock, so that only the last task thinks that it is
the last task, thus avoiding the failure scenario laid out above.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcutree_plugin.h

@@ -312,6 +312,7 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
 {
 	int empty;
 	int empty_exp;
+	int empty_exp_now;
 	unsigned long flags;
 	struct list_head *np;
 #ifdef CONFIG_RCU_BOOST
@@ -382,8 +383,10 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
 		/*
 		 * If this was the last task on the current list, and if
 		 * we aren't waiting on any CPUs, report the quiescent state.
-		 * Note that rcu_report_unblock_qs_rnp() releases rnp->lock.
+		 * Note that rcu_report_unblock_qs_rnp() releases rnp->lock,
+		 * so we must take a snapshot of the expedited state.
 		 */
+		empty_exp_now = !rcu_preempted_readers_exp(rnp);
 		if (!empty && !rcu_preempt_blocked_readers_cgp(rnp)) {
 			trace_rcu_quiescent_state_report("preempt_rcu",
 							 rnp->gpnum,
@@ -406,7 +409,7 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
 		 * If this was the last task on the expedited lists,
 		 * then we need to report up the rcu_node hierarchy.
 		 */
-		if (!empty_exp && !rcu_preempted_readers_exp(rnp))
+		if (!empty_exp && empty_exp_now)
 			rcu_report_exp_rnp(&rcu_preempt_state, rnp);
 	} else {
 		local_irq_restore(flags);