rcu: Simplify RCU_TINY RCU callback invocation

TINY_PREEMPT_RCU could use a kthread to handle RCU callback invocation,
which required an API to abstract kthread vs. softirq invocation.
Now that TINY_PREEMPT_RCU is no longer with us, this commit retires
this API in favor of direct use of the relevant softirq primitives.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

include/linux/rcupdate.h

@@ -216,6 +216,7 @@ static inline int rcu_preempt_depth(void)
 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 
 /* Internal to kernel */
+extern void rcu_init(void);
 extern void rcu_sched_qs(int cpu);
 extern void rcu_bh_qs(int cpu);
 extern void rcu_check_callbacks(int cpu, int user);

include/linux/rcutiny.h

@@ -27,10 +27,6 @@
 
 #include <linux/cache.h>
 
-static inline void rcu_init(void)
-{
-}
-
 static inline void rcu_barrier_bh(void)
 {
 	wait_rcu_gp(call_rcu_bh);

include/linux/rcutree.h

@@ -30,7 +30,6 @@
 #ifndef __LINUX_RCUTREE_H
 #define __LINUX_RCUTREE_H
 
-extern void rcu_init(void);
 extern void rcu_note_context_switch(int cpu);
 extern int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies);
 extern void rcu_cpu_stall_reset(void);

kernel/rcutiny.c

@@ -44,7 +44,6 @@
 
 /* Forward declarations for rcutiny_plugin.h. */
 struct rcu_ctrlblk;
-static void invoke_rcu_callbacks(void);
 static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp);
 static void rcu_process_callbacks(struct softirq_action *unused);
 static void __call_rcu(struct rcu_head *head,
@@ -227,7 +226,7 @@ void rcu_sched_qs(int cpu)
 	local_irq_save(flags);
 	if (rcu_qsctr_help(&rcu_sched_ctrlblk) +
 	    rcu_qsctr_help(&rcu_bh_ctrlblk))
-		invoke_rcu_callbacks();
+		raise_softirq(RCU_SOFTIRQ);
 	local_irq_restore(flags);
 }
 
@@ -240,7 +239,7 @@ void rcu_bh_qs(int cpu)
 
 	local_irq_save(flags);
 	if (rcu_qsctr_help(&rcu_bh_ctrlblk))
-		invoke_rcu_callbacks();
+		raise_softirq(RCU_SOFTIRQ);
 	local_irq_restore(flags);
 }
 
@@ -277,7 +276,7 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
 					      ACCESS_ONCE(rcp->rcucblist),
 					      need_resched(),
 					      is_idle_task(current),
-					      rcu_is_callbacks_kthread()));
+					      false));
 		return;
 	}
 
@@ -307,7 +306,7 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
 	RCU_TRACE(rcu_trace_sub_qlen(rcp, cb_count));
 	RCU_TRACE(trace_rcu_batch_end(rcp->name, cb_count, 0, need_resched(),
 				      is_idle_task(current),
-				      rcu_is_callbacks_kthread()));
+				      false));
 }
 
 static void rcu_process_callbacks(struct softirq_action *unused)
@@ -379,3 +378,8 @@ void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 	__call_rcu(head, func, &rcu_bh_ctrlblk);
 }
 EXPORT_SYMBOL_GPL(call_rcu_bh);
+
+void rcu_init(void)
+{
+	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
+}

kernel/rcutiny_plugin.h

@@ -102,39 +102,6 @@ static void check_cpu_stalls(void)
 	RCU_TRACE(check_cpu_stall_preempt());
 }
 
-/* Hold off callback invocation until early_initcall() time. */
-static int rcu_scheduler_fully_active __read_mostly;
-
-/*
- * Start up softirq processing of callbacks.
- */
-void invoke_rcu_callbacks(void)
-{
-	if (rcu_scheduler_fully_active)
-		raise_softirq(RCU_SOFTIRQ);
-}
-
-#ifdef CONFIG_RCU_TRACE
-
-/*
- * There is no callback kthread, so this thread is never it.
- */
-static bool rcu_is_callbacks_kthread(void)
-{
-	return false;
-}
-
-#endif /* #ifdef CONFIG_RCU_TRACE */
-
-static int __init rcu_scheduler_really_started(void)
-{
-	rcu_scheduler_fully_active = 1;
-	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
-	raise_softirq(RCU_SOFTIRQ);  /* Invoke any callbacks from early boot. */
-	return 0;
-}
-early_initcall(rcu_scheduler_really_started);
-
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 #include <linux/kernel_stat.h>