rcu: Add assertions verifying blocked-tasks list

This commit adds assertions verifying the consistency of the rcu_node
structure's ->blkd_tasks list and its ->gp_tasks, ->exp_tasks, and
->boost_tasks pointers.  In particular, the ->blkd_tasks lists must be
empty except for leaf rcu_node structures.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -2410,6 +2410,8 @@ rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
 			return;
 		}
 		WARN_ON_ONCE(oldmask); /* Any child must be all zeroed! */
+		WARN_ON_ONCE(rnp->level != rcu_num_lvls - 1 &&
+			     rcu_preempt_blocked_readers_cgp(rnp));
 		rnp->qsmask &= ~mask;
 		trace_rcu_quiescent_state_report(rsp->name, rnp->gpnum,
 						 mask, rnp->qsmask, rnp->level,

kernel/rcu/tree_plugin.h

@@ -180,6 +180,8 @@ static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
 	struct task_struct *t = current;
 
 	lockdep_assert_held(&rnp->lock);
+	WARN_ON_ONCE(rdp->mynode != rnp);
+	WARN_ON_ONCE(rnp->level != rcu_num_lvls - 1);
 
 	/*
 	 * Decide where to queue the newly blocked task.  In theory,
@@ -261,6 +263,10 @@ static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
 		rnp->gp_tasks = &t->rcu_node_entry;
 	if (!rnp->exp_tasks && (blkd_state & RCU_EXP_BLKD))
 		rnp->exp_tasks = &t->rcu_node_entry;
+	WARN_ON_ONCE(!(blkd_state & RCU_GP_BLKD) !=
+		     !(rnp->qsmask & rdp->grpmask));
+	WARN_ON_ONCE(!(blkd_state & RCU_EXP_BLKD) !=
+		     !(rnp->expmask & rdp->grpmask));
 	raw_spin_unlock_rcu_node(rnp); /* interrupts remain disabled. */
 
 	/*
@@ -482,6 +488,7 @@ void rcu_read_unlock_special(struct task_struct *t)
 		rnp = t->rcu_blocked_node;
 		raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
 		WARN_ON_ONCE(rnp != t->rcu_blocked_node);
+		WARN_ON_ONCE(rnp->level != rcu_num_lvls - 1);
 		empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);
 		empty_exp = sync_rcu_preempt_exp_done(rnp);
 		smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
@@ -495,10 +502,10 @@ void rcu_read_unlock_special(struct task_struct *t)
 		if (&t->rcu_node_entry == rnp->exp_tasks)
 			rnp->exp_tasks = np;
 		if (IS_ENABLED(CONFIG_RCU_BOOST)) {
-			if (&t->rcu_node_entry == rnp->boost_tasks)
-				rnp->boost_tasks = np;
 			/* Snapshot ->boost_mtx ownership w/rnp->lock held. */
 			drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx) == t;
+			if (&t->rcu_node_entry == rnp->boost_tasks)
+				rnp->boost_tasks = np;
 		}
 
 		/*