[PATCH tip/core/rcu 09/19] rcu: Define RCU-bh update API in terms of RCU

From: Paul E. McKenney
Date: Wed Aug 29 2018 - 18:21:35 EST

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@xxxxxxxxxxxxxxxxxx>
---
include/linux/rcupdate.h | 10 ++--
include/linux/rcutiny.h | 10 +++-
include/linux/rcutree.h | 8 ++-
kernel/rcu/tiny.c | 115 +++++++--------------------------------
kernel/rcu/tree.c | 97 +++------------------------------
kernel/rcu/tree_plugin.h | 1 -
kernel/softirq.c | 1 -
7 files changed, 48 insertions(+), 194 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 75e5b393cf44..9ebfd436cec7 100644
--- a/include/linux/rcupdate.h
+++ b/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__
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index bcfbc40a7239..ac26c27ccde8 100644
--- a/include/linux/rcutiny.h
+++ b/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();
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 664b580695d6..c789c302a2c9 100644
--- a/include/linux/rcutree.h
+++ b/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);
diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
index befc9321a89c..cadcf63c4889 100644
--- a/kernel/rcu/tiny.c
+++ b/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);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9b6bd12133c4..b602d60462ba 100644
--- a/kernel/rcu/tree.c
+++ b/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;
@@ -2672,26 +2658,15 @@ 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())
@@ -3078,34 +3053,6 @@ 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()",
@@ -3190,33 +3137,6 @@ 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
*
@@ -3528,7 +3448,7 @@ 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);

@@ -4179,7 +4099,6 @@ 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);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 0d7107fb3dec..1ff742a3c8d1 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/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));
}

diff --git a/kernel/softirq.c b/kernel/softirq.c
index ebd69694144a..7a0720a20003 100644
--- a/kernel/softirq.c
+++ b/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();
--
2.17.1