[PATCH v4 tip/core/rcu 09/38] rcu-tasks: Refactor RCU-tasks to allow variants to be added

paulmck
2020-04-15T18:24:14Z

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 | 6 +-
kernel/rcu/Kconfig | 10 +-
kernel/rcu/tasks.h | 491 +++++++++++++++++++++++++----------------------
kernel/rcu/update.c | 4 +
4 files changed, 272 insertions(+), 239 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 2678a37..5523145 100644
--- a/include/linux/rcupdate.h
+++ b/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
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 1cc940f..38475d0 100644
--- a/kernel/rcu/Kconfig
+++ b/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
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 5ccfe0d..d77921e 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -5,7 +5,13 @@
* Copyright (C) 2020 Paul E. McKenney
*/

-#ifdef CONFIG_TASKS_RCU
+
+////////////////////////////////////////////////////////////////////////
+//
+// Generic data structures.
+
+struct rcu_tasks;
+typedef void (*rcu_tasks_gp_func_t)(struct rcu_tasks *rtp);

/**
* Definition for a Tasks-RCU-like mechanism.
@@ -14,6 +20,8 @@
* @cbs_wq: Wait queue allowning new callback to get kthread's attention.
* @cbs_lock: Lock protecting callback list.
* @kthread_ptr: This flavor's grace-period/callback-invocation kthread.
+ * @gp_func: This flavor's grace-period-wait function.
+ * @call_func: This flavor's call_rcu()-equivalent function.
*/
struct rcu_tasks {
struct rcu_head *cbs_head;
@@ -21,29 +29,20 @@ struct rcu_tasks {
struct wait_queue_head cbs_wq;
raw_spinlock_t cbs_lock;
struct task_struct *kthread_ptr;
+ rcu_tasks_gp_func_t gp_func;
+ call_rcu_func_t call_func;
};

-#define DEFINE_RCU_TASKS(name) \
+#define DEFINE_RCU_TASKS(name, gp, call) \
static struct rcu_tasks name = \
{ \
.cbs_tail = &name.cbs_head, \
.cbs_wq = __WAIT_QUEUE_HEAD_INITIALIZER(name.cbs_wq), \
.cbs_lock = __RAW_SPIN_LOCK_UNLOCKED(name.cbs_lock), \
+ .gp_func = gp, \
+ .call_func = call, \
}

-/*
- * Simple variant of RCU whose quiescent states are voluntary context
- * switch, cond_resched_rcu_qs(), user-space execution, and idle.
- * As such, grace periods can take one good long time. There are no
- * read-side primitives similar to rcu_read_lock() and rcu_read_unlock()
- * because this implementation is intended to get the system into a safe
- * state for some of the manipulations involved in tracing and the like.
- * Finally, this implementation does not support high call_rcu_tasks()
- * rates from multiple CPUs. If this is required, per-CPU callback lists
- * will be needed.
- */
-DEFINE_RCU_TASKS(rcu_tasks);
-
/* Track exiting tasks in order to allow them to be waited for. */
DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);

@@ -52,29 +51,16 @@ DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);
static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
module_param(rcu_task_stall_timeout, int, 0644);

-/**
- * call_rcu_tasks() - Queue an RCU for invocation task-based grace period
- * @rhp: 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_tasks() assumes
- * that the read-side critical sections end at a voluntary context
- * switch (not a preemption!), cond_resched_rcu_qs(), entry into idle,
- * or transition to usermode execution. As such, there are no read-side
- * primitives analogous to rcu_read_lock() and rcu_read_unlock() because
- * this primitive is intended to determine that all tasks have passed
- * through a safe state, not so much for data-strcuture synchronization.
- *
- * See the description of call_rcu() for more detailed information on
- * memory ordering guarantees.
- */
-void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
+////////////////////////////////////////////////////////////////////////
+//
+// Generic code.
+
+// Enqueue a callback for the specified flavor of Tasks RCU.
+static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
+ struct rcu_tasks *rtp)
{
unsigned long flags;
bool needwake;
- struct rcu_tasks *rtp = &rcu_tasks;

rhp->next = NULL;
rhp->func = func;
@@ -87,108 +73,25 @@ void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
if (needwake && READ_ONCE(rtp->kthread_ptr))
wake_up(&rtp->cbs_wq);
}
-EXPORT_SYMBOL_GPL(call_rcu_tasks);

-/**
- * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed.
- *
- * Control will return to the caller some time after a full rcu-tasks
- * grace period has elapsed, in other words after all currently
- * executing rcu-tasks read-side critical sections have elapsed. These
- * read-side critical sections are delimited by calls to schedule(),
- * cond_resched_tasks_rcu_qs(), idle execution, userspace execution, calls
- * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
- *
- * This is a very specialized primitive, intended only for a few uses in
- * tracing and other situations requiring manipulation of function
- * preambles and profiling hooks. The synchronize_rcu_tasks() function
- * is not (yet) intended for heavy use from multiple CPUs.
- *
- * Note that this guarantee implies further memory-ordering guarantees.
- * On systems with more than one CPU, when synchronize_rcu_tasks() returns,
- * each CPU is guaranteed to have executed a full memory barrier since the
- * end of its last RCU-tasks read-side critical section whose beginning
- * preceded the call to synchronize_rcu_tasks(). In addition, each CPU
- * having an RCU-tasks read-side critical section that extends beyond
- * the return from synchronize_rcu_tasks() is guaranteed to have executed
- * a full memory barrier after the beginning of synchronize_rcu_tasks()
- * and before the beginning of that RCU-tasks read-side critical section.
- * Note that these guarantees include CPUs that are offline, idle, or
- * executing in user mode, as well as CPUs that are executing in the kernel.
- *
- * Furthermore, if CPU A invoked synchronize_rcu_tasks(), which returned
- * to its caller on CPU B, then both CPU A and CPU B are guaranteed
- * to have executed a full memory barrier during the execution of
- * synchronize_rcu_tasks() -- even if CPU A and CPU B are the same CPU
- * (but again only if the system has more than one CPU).
- */
-void synchronize_rcu_tasks(void)
+// Wait for a grace period for the specified flavor of Tasks RCU.
+static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
{
/* Complain if the scheduler has not started. */
RCU_LOCKDEP_WARN(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
"synchronize_rcu_tasks called too soon");

/* Wait for the grace period. */
- wait_rcu_gp(call_rcu_tasks);
-}
-EXPORT_SYMBOL_GPL(synchronize_rcu_tasks);
-
-/**
- * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks.
- *
- * Although the current implementation is guaranteed to wait, it is not
- * obligated to, for example, if there are no pending callbacks.
- */
-void rcu_barrier_tasks(void)
-{
- /* There is only one callback queue, so this is easy. ;-) */
- synchronize_rcu_tasks();
-}
-EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
-
-/* See if tasks are still holding out, complain if so. */
-static void check_holdout_task(struct task_struct *t,
- bool needreport, bool *firstreport)
-{
- int cpu;
-
- if (!READ_ONCE(t->rcu_tasks_holdout) ||
- t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
- !READ_ONCE(t->on_rq) ||
- (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
- !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
- WRITE_ONCE(t->rcu_tasks_holdout, false);
- list_del_init(&t->rcu_tasks_holdout_list);
- put_task_struct(t);
- return;
- }
- rcu_request_urgent_qs_task(t);
- if (!needreport)
- return;
- if (*firstreport) {
- pr_err("INFO: rcu_tasks detected stalls on tasks:\n");
- *firstreport = false;
- }
- cpu = task_cpu(t);
- pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n",
- t, ".I"[is_idle_task(t)],
- "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
- t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
- t->rcu_tasks_idle_cpu, cpu);
- sched_show_task(t);
+ wait_rcu_gp(rtp->call_func);
}

/* RCU-tasks kthread that detects grace periods and invokes callbacks. */
static int __noreturn rcu_tasks_kthread(void *arg)
{
unsigned long flags;
- struct task_struct *g, *t;
- unsigned long lastreport;
struct rcu_head *list;
struct rcu_head *next;
- LIST_HEAD(rcu_tasks_holdouts);
struct rcu_tasks *rtp = arg;
- int fract;

/* Run on housekeeping CPUs by default. Sysadm can move if desired. */
housekeeping_affine(current, HK_FLAG_RCU);
@@ -220,111 +123,8 @@ static int __noreturn rcu_tasks_kthread(void *arg)
continue;
}

- /*
- * Wait for all pre-existing t->on_rq and t->nvcsw
- * transitions to complete. Invoking synchronize_rcu()
- * suffices because all these transitions occur with
- * interrupts disabled. Without this synchronize_rcu(),
- * a read-side critical section that started before the
- * grace period might be incorrectly seen as having started
- * after the grace period.
- *
- * This synchronize_rcu() also dispenses with the
- * need for a memory barrier on the first store to
- * t->rcu_tasks_holdout, as it forces the store to happen
- * after the beginning of the grace period.
- */
- synchronize_rcu();
-
- /*
- * There were callbacks, so we need to wait for an
- * RCU-tasks grace period. Start off by scanning
- * the task list for tasks that are not already
- * voluntarily blocked. Mark these tasks and make
- * a list of them in rcu_tasks_holdouts.
- */
- rcu_read_lock();
- for_each_process_thread(g, t) {
- if (t != current && READ_ONCE(t->on_rq) &&
- !is_idle_task(t)) {
- get_task_struct(t);
- t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
- WRITE_ONCE(t->rcu_tasks_holdout, true);
- list_add(&t->rcu_tasks_holdout_list,
- &rcu_tasks_holdouts);
- }
- }
- rcu_read_unlock();
-
- /*
- * Wait for tasks that are in the process of exiting.
- * This does only part of the job, ensuring that all
- * tasks that were previously exiting reach the point
- * where they have disabled preemption, allowing the
- * later synchronize_rcu() to finish the job.
- */
- synchronize_srcu(&tasks_rcu_exit_srcu);
-
- /*
- * Each pass through the following loop scans the list
- * of holdout tasks, removing any that are no longer
- * holdouts. When the list is empty, we are done.
- */
- lastreport = jiffies;
-
- /* Start off with HZ/10 wait and slowly back off to 1 HZ wait*/
- fract = 10;
-
- for (;;) {
- bool firstreport;
- bool needreport;
- int rtst;
- struct task_struct *t1;
-
- if (list_empty(&rcu_tasks_holdouts))
- break;
-
- /* Slowly back off waiting for holdouts */
- schedule_timeout_interruptible(HZ/fract);
-
- if (fract > 1)
- fract--;
-
- rtst = READ_ONCE(rcu_task_stall_timeout);
- needreport = rtst > 0 &&
- time_after(jiffies, lastreport + rtst);
- if (needreport)
- lastreport = jiffies;
- firstreport = true;
- WARN_ON(signal_pending(current));
- list_for_each_entry_safe(t, t1, &rcu_tasks_holdouts,
- rcu_tasks_holdout_list) {
- check_holdout_task(t, needreport, &firstreport);
- cond_resched();
- }
- }
-
- /*
- * Because ->on_rq and ->nvcsw are not guaranteed
- * to have a full memory barriers prior to them in the
- * schedule() path, memory reordering on other CPUs could
- * cause their RCU-tasks read-side critical sections to
- * extend past the end of the grace period. However,
- * because these ->nvcsw updates are carried out with
- * interrupts disabled, we can use synchronize_rcu()
- * to force the needed ordering on all such CPUs.
- *
- * This synchronize_rcu() also confines all
- * ->rcu_tasks_holdout accesses to be within the grace
- * period, avoiding the need for memory barriers for
- * ->rcu_tasks_holdout accesses.
- *
- * In addition, this synchronize_rcu() waits for exiting
- * tasks to complete their final preempt_disable() region
- * of execution, cleaning up after the synchronize_srcu()
- * above.
- */
- synchronize_rcu();
+ // Wait for one grace period.
+ rtp->gp_func(rtp);

/* Invoke the callbacks. */
while (list) {
@@ -340,18 +140,16 @@ static int __noreturn rcu_tasks_kthread(void *arg)
}
}

-/* Spawn rcu_tasks_kthread() at core_initcall() time. */
-static int __init rcu_spawn_tasks_kthread(void)
+/* Spawn RCU-tasks grace-period kthread, e.g., at core_initcall() time. */
+static void __init rcu_spawn_tasks_kthread_generic(struct rcu_tasks *rtp)
{
struct task_struct *t;

- t = kthread_run(rcu_tasks_kthread, &rcu_tasks, "rcu_tasks_kthread");
+ t = kthread_run(rcu_tasks_kthread, rtp, "rcu_tasks_kthread");
if (WARN_ONCE(IS_ERR(t), "%s: Could not start Tasks-RCU grace-period kthread, OOM is now expected behavior\n", __func__))
- return 0;
+ return;
smp_mb(); /* Ensure others see full kthread. */
- return 0;
}
-core_initcall(rcu_spawn_tasks_kthread);

/* Do the srcu_read_lock() for the above synchronize_srcu(). */
void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
@@ -369,8 +167,6 @@ void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu)
preempt_enable();
}

-#endif /* #ifdef CONFIG_TASKS_RCU */
-
#ifndef CONFIG_TINY_RCU

/*
@@ -387,3 +183,230 @@ static void __init rcu_tasks_bootup_oddness(void)
}

#endif /* #ifndef CONFIG_TINY_RCU */
+
+#ifdef CONFIG_TASKS_RCU
+
+////////////////////////////////////////////////////////////////////////
+//
+// Simple variant of RCU whose quiescent states are voluntary context
+// switch, cond_resched_rcu_qs(), user-space execution, and idle.
+// As such, grace periods can take one good long time. There are no
+// read-side primitives similar to rcu_read_lock() and rcu_read_unlock()
+// because this implementation is intended to get the system into a safe
+// state for some of the manipulations involved in tracing and the like.
+// Finally, this implementation does not support high call_rcu_tasks()
+// rates from multiple CPUs. If this is required, per-CPU callback lists
+// will be needed.
+
+/* See if tasks are still holding out, complain if so. */
+static void check_holdout_task(struct task_struct *t,
+ bool needreport, bool *firstreport)
+{
+ int cpu;
+
+ if (!READ_ONCE(t->rcu_tasks_holdout) ||
+ t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
+ !READ_ONCE(t->on_rq) ||
+ (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
+ !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
+ WRITE_ONCE(t->rcu_tasks_holdout, false);
+ list_del_init(&t->rcu_tasks_holdout_list);
+ put_task_struct(t);
+ return;
+ }
+ rcu_request_urgent_qs_task(t);
+ if (!needreport)
+ return;
+ if (*firstreport) {
+ pr_err("INFO: rcu_tasks detected stalls on tasks:\n");
+ *firstreport = false;
+ }
+ cpu = task_cpu(t);
+ pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n",
+ t, ".I"[is_idle_task(t)],
+ "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
+ t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
+ t->rcu_tasks_idle_cpu, cpu);
+ sched_show_task(t);
+}
+
+/* Wait for one RCU-tasks grace period. */
+static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
+{
+ struct task_struct *g, *t;
+ unsigned long lastreport;
+ LIST_HEAD(rcu_tasks_holdouts);
+ int fract;
+
+ /*
+ * Wait for all pre-existing t->on_rq and t->nvcsw transitions
+ * to complete. Invoking synchronize_rcu() suffices because all
+ * these transitions occur with interrupts disabled. Without this
+ * synchronize_rcu(), a read-side critical section that started
+ * before the grace period might be incorrectly seen as having
+ * started after the grace period.
+ *
+ * This synchronize_rcu() also dispenses with the need for a
+ * memory barrier on the first store to t->rcu_tasks_holdout,
+ * as it forces the store to happen after the beginning of the
+ * grace period.
+ */
+ synchronize_rcu();
+
+ /*
+ * There were callbacks, so we need to wait for an RCU-tasks
+ * grace period. Start off by scanning the task list for tasks
+ * that are not already voluntarily blocked. Mark these tasks
+ * and make a list of them in rcu_tasks_holdouts.
+ */
+ rcu_read_lock();
+ for_each_process_thread(g, t) {
+ if (t != current && READ_ONCE(t->on_rq) && !is_idle_task(t)) {
+ get_task_struct(t);
+ t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
+ WRITE_ONCE(t->rcu_tasks_holdout, true);
+ list_add(&t->rcu_tasks_holdout_list,
+ &rcu_tasks_holdouts);
+ }
+ }
+ rcu_read_unlock();
+
+ /*
+ * Wait for tasks that are in the process of exiting. This
+ * does only part of the job, ensuring that all tasks that were
+ * previously exiting reach the point where they have disabled
+ * preemption, allowing the later synchronize_rcu() to finish
+ * the job.
+ */
+ synchronize_srcu(&tasks_rcu_exit_srcu);
+
+ /*
+ * Each pass through the following loop scans the list of holdout
+ * tasks, removing any that are no longer holdouts. When the list
+ * is empty, we are done.
+ */
+ lastreport = jiffies;
+
+ /* Start off with HZ/10 wait and slowly back off to 1 HZ wait. */
+ fract = 10;
+
+ for (;;) {
+ bool firstreport;
+ bool needreport;
+ int rtst;
+ struct task_struct *t1;
+
+ if (list_empty(&rcu_tasks_holdouts))
+ break;
+
+ /* Slowly back off waiting for holdouts */
+ schedule_timeout_interruptible(HZ/fract);
+
+ if (fract > 1)
+ fract--;
+
+ rtst = READ_ONCE(rcu_task_stall_timeout);
+ needreport = rtst > 0 && time_after(jiffies, lastreport + rtst);
+ if (needreport)
+ lastreport = jiffies;
+ firstreport = true;
+ WARN_ON(signal_pending(current));
+ list_for_each_entry_safe(t, t1, &rcu_tasks_holdouts,
+ rcu_tasks_holdout_list) {
+ check_holdout_task(t, needreport, &firstreport);
+ cond_resched();
+ }
+ }
+
+ /*
+ * Because ->on_rq and ->nvcsw are not guaranteed to have a full
+ * memory barriers prior to them in the schedule() path, memory
+ * reordering on other CPUs could cause their RCU-tasks read-side
+ * critical sections to extend past the end of the grace period.
+ * However, because these ->nvcsw updates are carried out with
+ * interrupts disabled, we can use synchronize_rcu() to force the
+ * needed ordering on all such CPUs.
+ *
+ * This synchronize_rcu() also confines all ->rcu_tasks_holdout
+ * accesses to be within the grace period, avoiding the need for
+ * memory barriers for ->rcu_tasks_holdout accesses.
+ *
+ * In addition, this synchronize_rcu() waits for exiting tasks
+ * to complete their final preempt_disable() region of execution,
+ * cleaning up after the synchronize_srcu() above.
+ */
+ synchronize_rcu();
+}
+
+void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func);
+DEFINE_RCU_TASKS(rcu_tasks, rcu_tasks_wait_gp, call_rcu_tasks);
+
+/**
+ * call_rcu_tasks() - Queue an RCU for invocation task-based grace period
+ * @rhp: 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_tasks() assumes
+ * that the read-side critical sections end at a voluntary context
+ * switch (not a preemption!), cond_resched_rcu_qs(), entry into idle,
+ * or transition to usermode execution. As such, there are no read-side
+ * primitives analogous to rcu_read_lock() and rcu_read_unlock() because
+ * this primitive is intended to determine that all tasks have passed
+ * through a safe state, not so much for data-strcuture synchronization.
+ *
+ * See the description of call_rcu() for more detailed information on
+ * memory ordering guarantees.
+ */
+void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
+{
+ call_rcu_tasks_generic(rhp, func, &rcu_tasks);
+}
+EXPORT_SYMBOL_GPL(call_rcu_tasks);
+
+/**
+ * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed.
+ *
+ * Control will return to the caller some time after a full rcu-tasks
+ * grace period has elapsed, in other words after all currently
+ * executing rcu-tasks read-side critical sections have elapsed. These
+ * read-side critical sections are delimited by calls to schedule(),
+ * cond_resched_tasks_rcu_qs(), idle execution, userspace execution, calls
+ * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
+ *
+ * This is a very specialized primitive, intended only for a few uses in
+ * tracing and other situations requiring manipulation of function
+ * preambles and profiling hooks. The synchronize_rcu_tasks() function
+ * is not (yet) intended for heavy use from multiple CPUs.
+ *
+ * See the description of synchronize_rcu() for more detailed information
+ * on memory ordering guarantees.
+ */
+void synchronize_rcu_tasks(void)
+{
+ synchronize_rcu_tasks_generic(&rcu_tasks);
+}
+EXPORT_SYMBOL_GPL(synchronize_rcu_tasks);
+
+/**
+ * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks.
+ *
+ * Although the current implementation is guaranteed to wait, it is not
+ * obligated to, for example, if there are no pending callbacks.
+ */
+void rcu_barrier_tasks(void)
+{
+ /* There is only one callback queue, so this is easy. ;-) */
+ synchronize_rcu_tasks();
+}
+EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
+
+static int __init rcu_spawn_tasks_kthread(void)
+{
+ rcu_spawn_tasks_kthread_generic(&rcu_tasks);
+ return 0;
+}
+core_initcall(rcu_spawn_tasks_kthread);
+
+#endif /* #ifdef CONFIG_TASKS_RCU */
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index c579934..30dce20 100644
--- a/kernel/rcu/update.c
+++ b/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

--
2.9.5