rcu: Fix thinko, actually initialize full tree

Commit de078d8 ("rcu: Need to update rnp->gpnum if preemptable RCU
is to be reliable") repeatedly and incorrectly initializes the root
rcu_node structure's ->gpnum field rather than initializing the
->gpnum field of each node in the tree.  Fix this.  Also add an
additional consistency check to catch this in the future.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
LKML-Reference: <125329262011-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/rcutree.c

@@ -601,8 +601,6 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 {
 	struct rcu_data *rdp = rsp->rda[smp_processor_id()];
 	struct rcu_node *rnp = rcu_get_root(rsp);
-	struct rcu_node *rnp_cur;
-	struct rcu_node *rnp_end;
 
 	if (!cpu_needs_another_gp(rsp, rdp)) {
 		spin_unlock_irqrestore(&rnp->lock, flags);
@@ -659,13 +657,12 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 	 * one corresponding to this CPU, due to the fact that we have
 	 * irqs disabled.
 	 */
-	rnp_end = &rsp->node[NUM_RCU_NODES];
-	for (rnp_cur = &rsp->node[0]; rnp_cur < rnp_end; rnp_cur++) {
-		spin_lock(&rnp_cur->lock);	/* irqs already disabled. */
+	for (rnp = &rsp->node[0]; rnp < &rsp->node[NUM_RCU_NODES]; rnp++) {
+		spin_lock(&rnp->lock);	/* irqs already disabled. */
 		rcu_preempt_check_blocked_tasks(rnp);
-		rnp_cur->qsmask = rnp_cur->qsmaskinit;
+		rnp->qsmask = rnp->qsmaskinit;
 		rnp->gpnum = rsp->gpnum;
-		spin_unlock(&rnp_cur->lock);	/* irqs already disabled. */
+		spin_unlock(&rnp->lock);	/* irqs already disabled. */
 	}
 
 	rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state now OK. */

kernel/rcutree_plugin.h

@@ -476,10 +476,12 @@ static void rcu_print_task_stall(struct rcu_node *rnp)
 
 /*
  * Because there is no preemptable RCU, there can be no readers blocked,
- * so there is no need to check for blocked tasks.
+ * so there is no need to check for blocked tasks.  So check only for
+ * bogus qsmask values.
  */
 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 {
+	WARN_ON_ONCE(rnp->qsmask);
 }
 
 /*