rcu-tasks: Refactor RCU-tasks to allow variants to be added

This commit splits out generic processing from RCU-tasks-specific
processing in order to allow additional flavors to be added.  It also
adds a def_bool TASKS_RCU_GENERIC to enable the common RCU-tasks
infrastructure code.

This is primarily, but not entirely, a code-movement commit.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/rcupdate.h

@@ -129,7 +129,7 @@ static inline void rcu_init_nohz(void) { }
  * Note a quasi-voluntary context switch for RCU-tasks's benefit.
  * This is a macro rather than an inline function to avoid #include hell.
  */
-#ifdef CONFIG_TASKS_RCU
+#ifdef CONFIG_TASKS_RCU_GENERIC
 #define rcu_tasks_qs(t) \
 	do { \
 		if (READ_ONCE((t)->rcu_tasks_holdout)) \
@@ -140,14 +140,14 @@ void call_rcu_tasks(struct rcu_head *head, rcu_callback_t func);
 void synchronize_rcu_tasks(void);
 void exit_tasks_rcu_start(void);
 void exit_tasks_rcu_finish(void);
-#else /* #ifdef CONFIG_TASKS_RCU */
+#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
 #define rcu_tasks_qs(t)	do { } while (0)
 #define rcu_note_voluntary_context_switch(t) do { } while (0)
 #define call_rcu_tasks call_rcu
 #define synchronize_rcu_tasks synchronize_rcu
 static inline void exit_tasks_rcu_start(void) { }
 static inline void exit_tasks_rcu_finish(void) { }
-#endif /* #else #ifdef CONFIG_TASKS_RCU */
+#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
 
 /**
  * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU

kernel/rcu/Kconfig

@@ -70,13 +70,19 @@ config TREE_SRCU
 	help
 	  This option selects the full-fledged version of SRCU.
 
+config TASKS_RCU_GENERIC
+	def_bool TASKS_RCU
+	select SRCU
+	help
+	  This option enables generic infrastructure code supporting
+	  task-based RCU implementations.  Not for manual selection.
+
 config TASKS_RCU
 	def_bool PREEMPTION
-	select SRCU
 	help
 	  This option enables a task-based RCU implementation that uses
 	  only voluntary context switch (not preemption!), idle, and
-	  user-mode execution as quiescent states.
+	  user-mode execution as quiescent states.  Not for manual selection.
 
 config RCU_STALL_COMMON
 	def_bool TREE_RCU

kernel/rcu/update.c

@@ -584,7 +584,11 @@ late_initcall(rcu_verify_early_boot_tests);
 void rcu_early_boot_tests(void) {}
 #endif /* CONFIG_PROVE_RCU */
 
+#ifdef CONFIG_TASKS_RCU_GENERIC
 #include "tasks.h"
+#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
+static inline void rcu_tasks_bootup_oddness(void) {}
+#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
 
 #ifndef CONFIG_TINY_RCU