rcu: Define RCU-bh update API in terms of RCU

Now that the main RCU API knows about softirq disabling and softirq's
quiescent states, the RCU-bh update code can be dispensed with.
This commit therefore removes the RCU-bh update-side implementation and
defines RCU-bh's update-side API in terms of that of either RCU-preempt or
RCU-sched, depending on the setting of the CONFIG_PREEMPT Kconfig option.

In kernels built with CONFIG_RCU_NOCB_CPU=y this has the knock-on effect
of reducing by one the number of rcuo kthreads per CPU.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/rcupdate.h

@@ -55,11 +55,15 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func);
 #define	call_rcu	call_rcu_sched
 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 
-void call_rcu_bh(struct rcu_head *head, rcu_callback_t func);
 void call_rcu_sched(struct rcu_head *head, rcu_callback_t func);
 void synchronize_sched(void);
 void rcu_barrier_tasks(void);
 
+static inline void call_rcu_bh(struct rcu_head *head, rcu_callback_t func)
+{
+	call_rcu(head, func);
+}
+
 #ifdef CONFIG_PREEMPT_RCU
 
 void __rcu_read_lock(void);
@@ -104,7 +108,6 @@ static inline int rcu_preempt_depth(void)
 void rcu_init(void);
 extern int rcu_scheduler_active __read_mostly;
 void rcu_sched_qs(void);
-void rcu_bh_qs(void);
 void rcu_check_callbacks(int user);
 void rcu_report_dead(unsigned int cpu);
 void rcutree_migrate_callbacks(int cpu);
@@ -326,8 +329,7 @@ static inline void rcu_preempt_sleep_check(void) { }
  * and rcu_assign_pointer().  Some of these could be folded into their
  * callers, but they are left separate in order to ease introduction of
  * multiple flavors of pointers to match the multiple flavors of RCU
- * (e.g., __rcu_bh, * __rcu_sched, and __srcu), should this make sense in
- * the future.
+ * (e.g., __rcu_sched, and __srcu), should this make sense in the future.
  */
 
 #ifdef __CHECKER__

include/linux/rcutiny.h

@@ -56,19 +56,23 @@ static inline void cond_synchronize_sched(unsigned long oldstate)
 	might_sleep();
 }
 
-extern void rcu_barrier_bh(void);
-extern void rcu_barrier_sched(void);
-
 static inline void synchronize_rcu_expedited(void)
 {
 	synchronize_sched();	/* Only one CPU, so pretty fast anyway!!! */
 }
 
+extern void rcu_barrier_sched(void);
+
 static inline void rcu_barrier(void)
 {
 	rcu_barrier_sched();  /* Only one CPU, so only one list of callbacks! */
 }
 
+static inline void rcu_barrier_bh(void)
+{
+	rcu_barrier();
+}
+
 static inline void synchronize_rcu_bh(void)
 {
 	synchronize_sched();

include/linux/rcutree.h

@@ -45,7 +45,11 @@ static inline void rcu_virt_note_context_switch(int cpu)
 	rcu_note_context_switch(false);
 }
 
-void synchronize_rcu_bh(void);
+static inline void synchronize_rcu_bh(void)
+{
+	synchronize_rcu();
+}
+
 void synchronize_sched_expedited(void);
 void synchronize_rcu_expedited(void);
 
@@ -69,7 +73,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func);
  */
 static inline void synchronize_rcu_bh_expedited(void)
 {
-	synchronize_sched_expedited();
+	synchronize_rcu_expedited();
 }
 
 void rcu_barrier(void);

kernel/rcu/tiny.c

@@ -51,64 +51,22 @@ static struct rcu_ctrlblk rcu_sched_ctrlblk = {
 	.curtail	= &rcu_sched_ctrlblk.rcucblist,
 };
 
-static struct rcu_ctrlblk rcu_bh_ctrlblk = {
-	.donetail	= &rcu_bh_ctrlblk.rcucblist,
-	.curtail	= &rcu_bh_ctrlblk.rcucblist,
-};
-
-void rcu_barrier_bh(void)
-{
-	wait_rcu_gp(call_rcu_bh);
-}
-EXPORT_SYMBOL(rcu_barrier_bh);
-
 void rcu_barrier_sched(void)
 {
 	wait_rcu_gp(call_rcu_sched);
 }
 EXPORT_SYMBOL(rcu_barrier_sched);
 
-/*
- * Helper function for rcu_sched_qs() and rcu_bh_qs().
- * Also irqs are disabled to avoid confusion due to interrupt handlers
- * invoking call_rcu().
- */
-static int rcu_qsctr_help(struct rcu_ctrlblk *rcp)
-{
-	if (rcp->donetail != rcp->curtail) {
-		rcp->donetail = rcp->curtail;
-		return 1;
-	}
-
-	return 0;
-}
-
-/*
- * Record an rcu quiescent state.  And an rcu_bh quiescent state while we
- * are at it, given that any rcu quiescent state is also an rcu_bh
- * quiescent state.  Use "+" instead of "||" to defeat short circuiting.
- */
+/* Record an rcu quiescent state.  */
 void rcu_sched_qs(void)
 {
 	unsigned long flags;
 
 	local_irq_save(flags);
-	if (rcu_qsctr_help(&rcu_sched_ctrlblk) +
-	    rcu_qsctr_help(&rcu_bh_ctrlblk))
-		raise_softirq(RCU_SOFTIRQ);
-	local_irq_restore(flags);
-}
-
-/*
- * Record an rcu_bh quiescent state.
- */
-void rcu_bh_qs(void)
-{
-	unsigned long flags;
-
-	local_irq_save(flags);
-	if (rcu_qsctr_help(&rcu_bh_ctrlblk))
+	if (rcu_sched_ctrlblk.donetail != rcu_sched_ctrlblk.curtail) {
+		rcu_sched_ctrlblk.donetail = rcu_sched_ctrlblk.curtail;
 		raise_softirq(RCU_SOFTIRQ);
+	}
 	local_irq_restore(flags);
 }
 
@@ -122,32 +80,27 @@ void rcu_check_callbacks(int user)
 {
 	if (user)
 		rcu_sched_qs();
-	if (user || !in_softirq())
-		rcu_bh_qs();
 }
 
-/*
- * Invoke the RCU callbacks on the specified rcu_ctrlkblk structure
- * whose grace period has elapsed.
- */
-static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
+/* Invoke the RCU callbacks whose grace period has elapsed.  */
+static __latent_entropy void rcu_process_callbacks(struct softirq_action *unused)
 {
 	struct rcu_head *next, *list;
 	unsigned long flags;
 
 	/* Move the ready-to-invoke callbacks to a local list. */
 	local_irq_save(flags);
-	if (rcp->donetail == &rcp->rcucblist) {
+	if (rcu_sched_ctrlblk.donetail == &rcu_sched_ctrlblk.rcucblist) {
 		/* No callbacks ready, so just leave. */
 		local_irq_restore(flags);
 		return;
 	}
-	list = rcp->rcucblist;
-	rcp->rcucblist = *rcp->donetail;
-	*rcp->donetail = NULL;
-	if (rcp->curtail == rcp->donetail)
-		rcp->curtail = &rcp->rcucblist;
-	rcp->donetail = &rcp->rcucblist;
+	list = rcu_sched_ctrlblk.rcucblist;
+	rcu_sched_ctrlblk.rcucblist = *rcu_sched_ctrlblk.donetail;
+	*rcu_sched_ctrlblk.donetail = NULL;
+	if (rcu_sched_ctrlblk.curtail == rcu_sched_ctrlblk.donetail)
+		rcu_sched_ctrlblk.curtail = &rcu_sched_ctrlblk.rcucblist;
+	rcu_sched_ctrlblk.donetail = &rcu_sched_ctrlblk.rcucblist;
 	local_irq_restore(flags);
 
 	/* Invoke the callbacks on the local list. */
@@ -162,19 +115,13 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
 	}
 }
 
-static __latent_entropy void rcu_process_callbacks(struct softirq_action *unused)
-{
-	__rcu_process_callbacks(&rcu_sched_ctrlblk);
-	__rcu_process_callbacks(&rcu_bh_ctrlblk);
-}
-
 /*
  * Wait for a grace period to elapse.  But it is illegal to invoke
  * synchronize_sched() from within an RCU read-side critical section.
  * Therefore, any legal call to synchronize_sched() is a quiescent
  * state, and so on a UP system, synchronize_sched() need do nothing.
- * Ditto for synchronize_rcu_bh().  (But Lai Jiangshan points out the
- * benefits of doing might_sleep() to reduce latency.)
+ * (But Lai Jiangshan points out the benefits of doing might_sleep()
+ * to reduce latency.)
  *
  * Cool, huh?  (Due to Josh Triplett.)
  */
@@ -188,11 +135,11 @@ void synchronize_sched(void)
 EXPORT_SYMBOL_GPL(synchronize_sched);
 
 /*
- * Helper function for call_rcu() and call_rcu_bh().
+ * Post an RCU callback to be invoked after the end of an RCU-sched grace
+ * period.  But since we have but one CPU, that would be after any
+ * quiescent state.
  */
-static void __call_rcu(struct rcu_head *head,
-		       rcu_callback_t func,
-		       struct rcu_ctrlblk *rcp)
+void call_rcu_sched(struct rcu_head *head, rcu_callback_t func)
 {
 	unsigned long flags;
 
@@ -201,8 +148,8 @@ static void __call_rcu(struct rcu_head *head,
 	head->next = NULL;
 
 	local_irq_save(flags);
-	*rcp->curtail = head;
-	rcp->curtail = &head->next;
+	*rcu_sched_ctrlblk.curtail = head;
+	rcu_sched_ctrlblk.curtail = &head->next;
 	local_irq_restore(flags);
 
 	if (unlikely(is_idle_task(current))) {
@@ -210,28 +157,8 @@ static void __call_rcu(struct rcu_head *head,
 		resched_cpu(0);
 	}
 }
-
-/*
- * Post an RCU callback to be invoked after the end of an RCU-sched grace
- * period.  But since we have but one CPU, that would be after any
- * quiescent state.
- */
-void call_rcu_sched(struct rcu_head *head, rcu_callback_t func)
-{
-	__call_rcu(head, func, &rcu_sched_ctrlblk);
-}
 EXPORT_SYMBOL_GPL(call_rcu_sched);
 
-/*
- * Post an RCU bottom-half callback to be invoked after any subsequent
- * quiescent state.
- */
-void call_rcu_bh(struct rcu_head *head, rcu_callback_t func)
-{
-	__call_rcu(head, func, &rcu_bh_ctrlblk);
-}
-EXPORT_SYMBOL_GPL(call_rcu_bh);
-
 void __init rcu_init(void)
 {
 	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);

kernel/rcu/tree.c

@@ -108,7 +108,6 @@ struct rcu_state sname##_state = { \
 }
 
 RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched);
-RCU_STATE_INITIALIZER(rcu_bh, 'b', call_rcu_bh);
 
 static struct rcu_state *const rcu_state_p;
 LIST_HEAD(rcu_struct_flavors);
@@ -244,17 +243,6 @@ void rcu_sched_qs(void)
 			   this_cpu_ptr(&rcu_sched_data), true);
 }
 
-void rcu_bh_qs(void)
-{
-	RCU_LOCKDEP_WARN(preemptible(), "rcu_bh_qs() invoked with preemption enabled!!!");
-	if (__this_cpu_read(rcu_bh_data.cpu_no_qs.s)) {
-		trace_rcu_grace_period(TPS("rcu_bh"),
-				       __this_cpu_read(rcu_bh_data.gp_seq),
-				       TPS("cpuqs"));
-		__this_cpu_write(rcu_bh_data.cpu_no_qs.b.norm, false);
-	}
-}
-
 void rcu_softirq_qs(void)
 {
 	rcu_sched_qs();
@@ -581,7 +569,7 @@ EXPORT_SYMBOL_GPL(rcu_sched_get_gp_seq);
  */
 unsigned long rcu_bh_get_gp_seq(void)
 {
-	return READ_ONCE(rcu_bh_state.gp_seq);
+	return READ_ONCE(rcu_state_p->gp_seq);
 }
 EXPORT_SYMBOL_GPL(rcu_bh_get_gp_seq);
 
@@ -621,7 +609,7 @@ EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);
  */
 void rcu_bh_force_quiescent_state(void)
 {
-	force_quiescent_state(&rcu_bh_state);
+	force_quiescent_state(rcu_state_p);
 }
 EXPORT_SYMBOL_GPL(rcu_bh_force_quiescent_state);
 
@@ -680,10 +668,8 @@ void rcutorture_get_gp_data(enum rcutorture_type test_type, int *flags,
 
 	switch (test_type) {
 	case RCU_FLAVOR:
-		rsp = rcu_state_p;
-		break;
 	case RCU_BH_FLAVOR:
-		rsp = &rcu_bh_state;
+		rsp = rcu_state_p;
 		break;
 	case RCU_SCHED_FLAVOR:
 		rsp = &rcu_sched_state;
@@ -2673,26 +2659,15 @@ void rcu_check_callbacks(int user)
 		 * nested interrupt.  In this case, the CPU is in
 		 * a quiescent state, so note it.
 		 *
-		 * No memory barrier is required here because both
-		 * rcu_sched_qs() and rcu_bh_qs() reference only CPU-local
-		 * variables that other CPUs neither access nor modify,
-		 * at least not while the corresponding CPU is online.
+		 * No memory barrier is required here because
+		 * rcu_sched_qs() references only CPU-local variables
+		 * that other CPUs neither access nor modify, at least
+		 * not while the corresponding CPU is online.
 		 */
 
 		rcu_sched_qs();
-		rcu_bh_qs();
 		rcu_note_voluntary_context_switch(current);
 
-	} else if (!in_softirq()) {
-
-		/*
-		 * Get here if this CPU did not take its interrupt from
-		 * softirq, in other words, if it is not interrupting
-		 * a rcu_bh read-side critical section.  This is an _bh
-		 * critical section, so note it.
-		 */
-
-		rcu_bh_qs();
 	}
 	rcu_preempt_check_callbacks();
 	if (rcu_pending())
@@ -3079,34 +3054,6 @@ void call_rcu_sched(struct rcu_head *head, rcu_callback_t func)
 }
 EXPORT_SYMBOL_GPL(call_rcu_sched);
 
-/**
- * call_rcu_bh() - Queue an RCU for invocation after a quicker grace period.
- * @head: structure to be used for queueing the RCU updates.
- * @func: actual callback function to be invoked after the grace period
- *
- * The callback function will be invoked some time after a full grace
- * period elapses, in other words after all currently executing RCU
- * read-side critical sections have completed. call_rcu_bh() assumes
- * that the read-side critical sections end on completion of a softirq
- * handler. This means that read-side critical sections in process
- * context must not be interrupted by softirqs. This interface is to be
- * used when most of the read-side critical sections are in softirq context.
- * RCU read-side critical sections are delimited by:
- *
- * - rcu_read_lock() and  rcu_read_unlock(), if in interrupt context, OR
- * - rcu_read_lock_bh() and rcu_read_unlock_bh(), if in process context.
- *
- * These may be nested.
- *
- * See the description of call_rcu() for more detailed information on
- * memory ordering guarantees.
- */
-void call_rcu_bh(struct rcu_head *head, rcu_callback_t func)
-{
-	__call_rcu(head, func, &rcu_bh_state, -1, 0);
-}
-EXPORT_SYMBOL_GPL(call_rcu_bh);
-
 /*
  * Queue an RCU callback for lazy invocation after a grace period.
  * This will likely be later named something like "call_rcu_lazy()",
@@ -3191,33 +3138,6 @@ void synchronize_sched(void)
 }
 EXPORT_SYMBOL_GPL(synchronize_sched);
 
-/**
- * synchronize_rcu_bh - wait until an rcu_bh grace period has elapsed.
- *
- * Control will return to the caller some time after a full rcu_bh grace
- * period has elapsed, in other words after all currently executing rcu_bh
- * read-side critical sections have completed.  RCU read-side critical
- * sections are delimited by rcu_read_lock_bh() and rcu_read_unlock_bh(),
- * and may be nested.
- *
- * See the description of synchronize_sched() for more detailed information
- * on memory ordering guarantees.
- */
-void synchronize_rcu_bh(void)
-{
-	RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
-			 lock_is_held(&rcu_lock_map) ||
-			 lock_is_held(&rcu_sched_lock_map),
-			 "Illegal synchronize_rcu_bh() in RCU-bh read-side critical section");
-	if (rcu_blocking_is_gp())
-		return;
-	if (rcu_gp_is_expedited())
-		synchronize_rcu_bh_expedited();
-	else
-		wait_rcu_gp(call_rcu_bh);
-}
-EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
-
 /**
  * get_state_synchronize_rcu - Snapshot current RCU state
  *
@@ -3529,7 +3449,7 @@ static void _rcu_barrier(struct rcu_state *rsp)
  */
 void rcu_barrier_bh(void)
 {
-	_rcu_barrier(&rcu_bh_state);
+	_rcu_barrier(rcu_state_p);
 }
 EXPORT_SYMBOL_GPL(rcu_barrier_bh);
 
@@ -4180,7 +4100,6 @@ void __init rcu_init(void)
 
 	rcu_bootup_announce();
 	rcu_init_geometry();
-	rcu_init_one(&rcu_bh_state);
 	rcu_init_one(&rcu_sched_state);
 	if (dump_tree)
 		rcu_dump_rcu_node_tree(&rcu_sched_state);

kernel/rcu/tree_plugin.h

@@ -1320,7 +1320,6 @@ static int rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
 static void rcu_kthread_do_work(void)
 {
 	rcu_do_batch(&rcu_sched_state, this_cpu_ptr(&rcu_sched_data));
-	rcu_do_batch(&rcu_bh_state, this_cpu_ptr(&rcu_bh_data));
 	rcu_do_batch(&rcu_preempt_state, this_cpu_ptr(&rcu_preempt_data));
 }
 

kernel/softirq.c

@@ -301,7 +301,6 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
 		pending >>= softirq_bit;
 	}
 
-	rcu_bh_qs();
 	if (__this_cpu_read(ksoftirqd) == current)
 		rcu_softirq_qs();
 	local_irq_disable();