[PATCHv4] rcu: tree: correctly handle sparse possible cpus

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0801601..88d3f95 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1287,9 +1287,9 @@  static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
 	rcu_for_each_leaf_node(rsp, rnp) {
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 		if (rnp->qsmask != 0) {
-			for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
-				if (rnp->qsmask & (1UL << cpu))
-					dump_cpu_task(rnp->grplo + cpu);
+			for_each_leaf_node_possible_cpu(rnp, cpu)
+				if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
+					dump_cpu_task(cpu);
 		}
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	}
@@ -1361,10 +1361,9 @@  static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 		ndetected += rcu_print_task_stall(rnp);
 		if (rnp->qsmask != 0) {
-			for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
-				if (rnp->qsmask & (1UL << cpu)) {
-					print_cpu_stall_info(rsp,
-							     rnp->grplo + cpu);
+			for_each_leaf_node_possible_cpu(rnp, cpu)
+				if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
+					print_cpu_stall_info(rsp, cpu);
 					ndetected++;
 				}
 		}
@@ -2885,7 +2884,6 @@  static void force_qs_rnp(struct rcu_state *rsp,
 				  unsigned long *maxj),
 			 bool *isidle, unsigned long *maxj)
 {
-	unsigned long bit;
 	int cpu;
 	unsigned long flags;
 	unsigned long mask;
@@ -2920,9 +2918,8 @@  static void force_qs_rnp(struct rcu_state *rsp,
 				continue;
 			}
 		}
-		cpu = rnp->grplo;
-		bit = 1;
-		for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {
+		for_each_leaf_node_possible_cpu(rnp, cpu) {
+			unsigned long bit = leaf_node_cpu_bit(rnp, cpu);
 			if ((rnp->qsmask & bit) != 0) {
 				if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
 					mask |= bit;
@@ -3751,7 +3748,7 @@  rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp)
 
 	/* Set up local state, ensuring consistent view of global state. */
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
-	rdp->grpmask = 1UL << (cpu - rdp->mynode->grplo);
+	rdp->grpmask = leaf_node_cpu_bit(rdp->mynode, cpu);
 	rdp->dynticks = &per_cpu(rcu_dynticks, cpu);
 	WARN_ON_ONCE(rdp->dynticks->dynticks_nesting != DYNTICK_TASK_EXIT_IDLE);
 	WARN_ON_ONCE(atomic_read(&rdp->dynticks->dynticks) != 1);
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index e3959f5..f714f87 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -254,6 +254,13 @@  struct rcu_node {
 } ____cacheline_internodealigned_in_smp;
 
 /*
+ * Bitmasks in an rcu_node cover the interval [grplo, grphi] of CPU IDs, and
+ * are indexed relative to this interval rather than the global CPU ID space.
+ * This generates the bit for a CPU in node-local masks.
+ */
+#define leaf_node_cpu_bit(rnp, cpu) (1UL << ((cpu) - (rnp)->grplo))
+
+/*
  * Do a full breadth-first scan of the rcu_node structures for the
  * specified rcu_state structure.
  */
@@ -281,6 +288,14 @@  struct rcu_node {
 	     (rnp) < &(rsp)->node[rcu_num_nodes]; (rnp)++)
 
 /*
+ * Iterate over all possible CPUs in a leaf RCU node.
+ */
+#define for_each_leaf_node_possible_cpu(rnp, cpu) \
+	for ((cpu) = cpumask_next(rnp->grplo - 1, cpu_possible_mask); \
+	     cpu <= rnp->grphi; \
+	     cpu = cpumask_next((cpu), cpu_possible_mask))
+
+/*
  * Union to allow "aggregate OR" operation on the need for a quiescent
  * state by the normal and expedited grace periods.
  */
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 00a02a2..d400434 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -344,7 +344,6 @@  static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 {
 	int cpu;
 	unsigned long flags;
-	unsigned long mask;
 	unsigned long mask_ofl_test;
 	unsigned long mask_ofl_ipi;
 	int ret;
@@ -356,7 +355,7 @@  static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 
 		/* Each pass checks a CPU for identity, offline, and idle. */
 		mask_ofl_test = 0;
-		for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++) {
+		for_each_leaf_node_possible_cpu(rnp, cpu) {
 			struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
 			struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
 
@@ -376,8 +375,8 @@  static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 
 		/* IPI the remaining CPUs for expedited quiescent state. */
-		mask = 1;
-		for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
+		for_each_leaf_node_possible_cpu(rnp, cpu) {
+			unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
 			if (!(mask_ofl_ipi & mask))
 				continue;
 retry_ipi:
@@ -440,10 +439,10 @@  static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 		ndetected = 0;
 		rcu_for_each_leaf_node(rsp, rnp) {
 			ndetected += rcu_print_task_exp_stall(rnp);
-			mask = 1;
-			for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
+			for_each_leaf_node_possible_cpu(rnp, cpu) {
 				struct rcu_data *rdp;
 
+				mask = leaf_node_cpu_bit(rnp, cpu);
 				if (!(rnp->expmask & mask))
 					continue;
 				ndetected++;
@@ -453,7 +452,6 @@  static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 					"o."[!!(rdp->grpmask & rnp->expmaskinit)],
 					"N."[!!(rdp->grpmask & rnp->expmaskinitnext)]);
 			}
-			mask <<= 1;
 		}
 		pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
 			jiffies - jiffies_start, rsp->expedited_sequence,
@@ -473,8 +471,8 @@  static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 			pr_cont("\n");
 		}
 		rcu_for_each_leaf_node(rsp, rnp) {
-			mask = 1;
-			for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
+			for_each_leaf_node_possible_cpu(rnp, cpu) {
+				mask = leaf_node_cpu_bit(rnp, cpu);
 				if (!(rnp->expmask & mask))
 					continue;
 				dump_cpu_task(cpu);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 02a9197..0082fce 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1164,8 +1164,9 @@  static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 		return;
 	if (!zalloc_cpumask_var(&cm, GFP_KERNEL))
 		return;
-	for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask >>= 1)
-		if ((mask & 0x1) && cpu != outgoingcpu)
+	for_each_leaf_node_possible_cpu(rnp, cpu)
+		if ((mask & leaf_node_cpu_bit(rnp, cpu)) &&
+		    cpu != outgoingcpu)
 			cpumask_set_cpu(cpu, cm);
 	if (cpumask_weight(cm) == 0)
 		cpumask_setall(cm);