rcu: Conditionally build CPU-hotplug teardown callbacks

Among the three CPU-hotplug teardown RCU callbacks, two of them early
exit if CONFIG_HOTPLUG_CPU=n, and one is left unchanged. In any case
all of them have an implementation when CONFIG_HOTPLUG_CPU=n.

Align instead with the common way to deal with CPU-hotplug teardown
callbacks and provide a proper stub when they are not supported.

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

1 files changed, 54 insertions, 60 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2e1e7eadf2cc..f9c6b2680cbb 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4238,25 +4238,6 @@ static bool rcu_init_invoked(void)
 }
 
 /*
- * Near the end of the offline process.  Trace the fact that this CPU
- * is going offline.
- */
-int rcutree_dying_cpu(unsigned int cpu)
-{
-	bool blkd;
-	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
-	struct rcu_node *rnp = rdp->mynode;
-
-	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
-		return 0;
-
-	blkd = !!(READ_ONCE(rnp->qsmask) & rdp->grpmask);
-	trace_rcu_grace_period(rcu_state.name, READ_ONCE(rnp->gp_seq),
-			       blkd ? TPS("cpuofl-bgp") : TPS("cpuofl"));
-	return 0;
-}
-
-/*
  * All CPUs for the specified rcu_node structure have gone offline,
  * and all tasks that were preempted within an RCU read-side critical
  * section while running on one of those CPUs have since exited their RCU
@@ -4302,23 +4283,6 @@ static void rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf)
 }
 
 /*
- * The CPU has been completely removed, and some other CPU is reporting
- * this fact from process context.  Do the remainder of the cleanup.
- * There can only be one CPU hotplug operation at a time, so no need for
- * explicit locking.
- */
-int rcutree_dead_cpu(unsigned int cpu)
-{
-	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
-		return 0;
-
-	WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
-	// Stop-machine done, so allow nohz_full to disable tick.
-	tick_dep_clear(TICK_DEP_BIT_RCU);
-	return 0;
-}
-
-/*
  * Propagate ->qsinitmask bits up the rcu_node tree to account for the
  * first CPU in a given leaf rcu_node structure coming online.  The caller
  * must hold the corresponding leaf rcu_node ->lock with interrupts
@@ -4471,29 +4435,6 @@ int rcutree_online_cpu(unsigned int cpu)
 }
 
 /*
- * Near the beginning of the process.  The CPU is still very much alive
- * with pretty much all services enabled.
- */
-int rcutree_offline_cpu(unsigned int cpu)
-{
-	unsigned long flags;
-	struct rcu_data *rdp;
-	struct rcu_node *rnp;
-
-	rdp = per_cpu_ptr(&rcu_data, cpu);
-	rnp = rdp->mynode;
-	raw_spin_lock_irqsave_rcu_node(rnp, flags);
-	rnp->ffmask &= ~rdp->grpmask;
-	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
-
-	rcutree_affinity_setting(cpu, cpu);
-
-	// nohz_full CPUs need the tick for stop-machine to work quickly
-	tick_dep_set(TICK_DEP_BIT_RCU);
-	return 0;
-}
-
-/*
  * Mark the specified CPU as being online so that subsequent grace periods
  * (both expedited and normal) will wait on it.  Note that this means that
  * incoming CPUs are not allowed to use RCU read-side critical sections
@@ -4646,7 +4587,60 @@ void rcutree_migrate_callbacks(int cpu)
 		  cpu, rcu_segcblist_n_cbs(&rdp->cblist),
 		  rcu_segcblist_first_cb(&rdp->cblist));
 }
-#endif
+
+/*
+ * The CPU has been completely removed, and some other CPU is reporting
+ * this fact from process context.  Do the remainder of the cleanup.
+ * There can only be one CPU hotplug operation at a time, so no need for
+ * explicit locking.
+ */
+int rcutree_dead_cpu(unsigned int cpu)
+{
+	WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
+	// Stop-machine done, so allow nohz_full to disable tick.
+	tick_dep_clear(TICK_DEP_BIT_RCU);
+	return 0;
+}
+
+/*
+ * Near the end of the offline process.  Trace the fact that this CPU
+ * is going offline.
+ */
+int rcutree_dying_cpu(unsigned int cpu)
+{
+	bool blkd;
+	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+	struct rcu_node *rnp = rdp->mynode;
+
+	blkd = !!(READ_ONCE(rnp->qsmask) & rdp->grpmask);
+	trace_rcu_grace_period(rcu_state.name, READ_ONCE(rnp->gp_seq),
+			       blkd ? TPS("cpuofl-bgp") : TPS("cpuofl"));
+	return 0;
+}
+
+/*
+ * Near the beginning of the process.  The CPU is still very much alive
+ * with pretty much all services enabled.
+ */
+int rcutree_offline_cpu(unsigned int cpu)
+{
+	unsigned long flags;
+	struct rcu_data *rdp;
+	struct rcu_node *rnp;
+
+	rdp = per_cpu_ptr(&rcu_data, cpu);
+	rnp = rdp->mynode;
+	raw_spin_lock_irqsave_rcu_node(rnp, flags);
+	rnp->ffmask &= ~rdp->grpmask;
+	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+
+	rcutree_affinity_setting(cpu, cpu);
+
+	// nohz_full CPUs need the tick for stop-machine to work quickly
+	tick_dep_set(TICK_DEP_BIT_RCU);
+	return 0;
+}
+#endif /* #ifdef CONFIG_HOTPLUG_CPU */
 
 /*
  * On non-huge systems, use expedited RCU grace periods to make suspend