[PATCH RFC v2 tip/core/rcu 04/22] rcu-tasks: Move Tasks RCU to its own file

paulmck
2020-03-19T00:11:53Z

This code-movement-only commit is in preparation for adding an additional
flavor of Tasks RCU, which relies on workqueues to detect grace periods.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tasks.h | 370 ++++++++++++++++++++++++++++++++++++++++++++++++++++
kernel/rcu/update.c | 366 +--------------------------------------------------
2 files changed, 372 insertions(+), 364 deletions(-)
create mode 100644 kernel/rcu/tasks.h

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
new file mode 100644
index 0000000..be8d179
--- /dev/null
+++ b/kernel/rcu/tasks.h
@@ -0,0 +1,370 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Task-based RCU implementations.
+ *
+ * Copyright (C) 2020 Paul E. McKenney
+ */
+
+#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.
+ */
+
+/* Global list of callbacks and associated lock. */
+static struct rcu_head *rcu_tasks_cbs_head;
+static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
+static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq);
+static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock);
+
+/* Track exiting tasks in order to allow them to be waited for. */
+DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);
+
+/* Control stall timeouts. Disable with <= 0, otherwise jiffies till stall. */
+#define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10)
+static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
+module_param(rcu_task_stall_timeout, int, 0644);
+
+static struct task_struct *rcu_tasks_kthread_ptr;
+
+/**
+ * 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)
+{
+ unsigned long flags;
+ bool needwake;
+
+ rhp->next = NULL;
+ rhp->func = func;
+ raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
+ needwake = !rcu_tasks_cbs_head;
+ WRITE_ONCE(*rcu_tasks_cbs_tail, rhp);
+ rcu_tasks_cbs_tail = &rhp->next;
+ raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
+ /* We can't create the thread unless interrupts are enabled. */
+ if (needwake && READ_ONCE(rcu_tasks_kthread_ptr))
+ wake_up(&rcu_tasks_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)
+{
+ /* 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);
+}
+
+/* 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);
+ int fract;
+
+ /* Run on housekeeping CPUs by default. Sysadm can move if desired. */
+ housekeeping_affine(current, HK_FLAG_RCU);
+
+ /*
+ * Each pass through the following loop makes one check for
+ * newly arrived callbacks, and, if there are some, waits for
+ * one RCU-tasks grace period and then invokes the callbacks.
+ * This loop is terminated by the system going down. ;-)
+ */
+ for (;;) {
+
+ /* Pick up any new callbacks. */
+ raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
+ list = rcu_tasks_cbs_head;
+ rcu_tasks_cbs_head = NULL;
+ rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
+ raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
+
+ /* If there were none, wait a bit and start over. */
+ if (!list) {
+ wait_event_interruptible(rcu_tasks_cbs_wq,
+ READ_ONCE(rcu_tasks_cbs_head));
+ if (!rcu_tasks_cbs_head) {
+ WARN_ON(signal_pending(current));
+ schedule_timeout_interruptible(HZ/10);
+ }
+ 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
+ * ->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();
+
+ /* Invoke the callbacks. */
+ while (list) {
+ next = list->next;
+ local_bh_disable();
+ list->func(list);
+ local_bh_enable();
+ list = next;
+ cond_resched();
+ }
+ /* Paranoid sleep to keep this from entering a tight loop */
+ schedule_timeout_uninterruptible(HZ/10);
+ }
+}
+
+/* Spawn rcu_tasks_kthread() at core_initcall() time. */
+static int __init rcu_spawn_tasks_kthread(void)
+{
+ struct task_struct *t;
+
+ t = kthread_run(rcu_tasks_kthread, NULL, "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;
+ smp_mb(); /* Ensure others see full kthread. */
+ WRITE_ONCE(rcu_tasks_kthread_ptr, t);
+ 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)
+{
+ preempt_disable();
+ current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
+ preempt_enable();
+}
+
+/* Do the srcu_read_unlock() for the above synchronize_srcu(). */
+void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu)
+{
+ preempt_disable();
+ __srcu_read_unlock(&tasks_rcu_exit_srcu, current->rcu_tasks_idx);
+ preempt_enable();
+}
+
+#endif /* #ifdef CONFIG_TASKS_RCU */
+
+#ifndef CONFIG_TINY_RCU
+
+/*
+ * Print any non-default Tasks RCU settings.
+ */
+static void __init rcu_tasks_bootup_oddness(void)
+{
+#ifdef CONFIG_TASKS_RCU
+ if (rcu_task_stall_timeout != RCU_TASK_STALL_TIMEOUT)
+ pr_info("\tTasks-RCU CPU stall warnings timeout set to %d (rcu_task_stall_timeout).\n", rcu_task_stall_timeout);
+ else
+ pr_info("\tTasks RCU enabled.\n");
+#endif /* #ifdef CONFIG_TASKS_RCU */
+}
+
+#endif /* #ifndef CONFIG_TINY_RCU */
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index dd837da..0fb2a9e 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -489,370 +489,6 @@ int rcu_cpu_stall_suppress_at_boot __read_mostly; // !0 = suppress boot stalls.
EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress_at_boot);
module_param(rcu_cpu_stall_suppress_at_boot, int, 0444);

-#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.
- */
-
-/* Global list of callbacks and associated lock. */
-static struct rcu_head *rcu_tasks_cbs_head;
-static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
-static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq);
-static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock);
-
-/* Track exiting tasks in order to allow them to be waited for. */
-DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);
-
-/* Control stall timeouts. Disable with <= 0, otherwise jiffies till stall. */
-#define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10)
-static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
-module_param(rcu_task_stall_timeout, int, 0644);
-
-static struct task_struct *rcu_tasks_kthread_ptr;
-
-/**
- * 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)
-{
- unsigned long flags;
- bool needwake;
-
- rhp->next = NULL;
- rhp->func = func;
- raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
- needwake = !rcu_tasks_cbs_head;
- WRITE_ONCE(*rcu_tasks_cbs_tail, rhp);
- rcu_tasks_cbs_tail = &rhp->next;
- raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
- /* We can't create the thread unless interrupts are enabled. */
- if (needwake && READ_ONCE(rcu_tasks_kthread_ptr))
- wake_up(&rcu_tasks_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)
-{
- /* 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);
-}
-
-/* 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);
- int fract;
-
- /* Run on housekeeping CPUs by default. Sysadm can move if desired. */
- housekeeping_affine(current, HK_FLAG_RCU);
-
- /*
- * Each pass through the following loop makes one check for
- * newly arrived callbacks, and, if there are some, waits for
- * one RCU-tasks grace period and then invokes the callbacks.
- * This loop is terminated by the system going down. ;-)
- */
- for (;;) {
-
- /* Pick up any new callbacks. */
- raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
- list = rcu_tasks_cbs_head;
- rcu_tasks_cbs_head = NULL;
- rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
- raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
-
- /* If there were none, wait a bit and start over. */
- if (!list) {
- wait_event_interruptible(rcu_tasks_cbs_wq,
- READ_ONCE(rcu_tasks_cbs_head));
- if (!rcu_tasks_cbs_head) {
- WARN_ON(signal_pending(current));
- schedule_timeout_interruptible(HZ/10);
- }
- 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
- * ->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();
-
- /* Invoke the callbacks. */
- while (list) {
- next = list->next;
- local_bh_disable();
- list->func(list);
- local_bh_enable();
- list = next;
- cond_resched();
- }
- /* Paranoid sleep to keep this from entering a tight loop */
- schedule_timeout_uninterruptible(HZ/10);
- }
-}
-
-/* Spawn rcu_tasks_kthread() at core_initcall() time. */
-static int __init rcu_spawn_tasks_kthread(void)
-{
- struct task_struct *t;
-
- t = kthread_run(rcu_tasks_kthread, NULL, "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;
- smp_mb(); /* Ensure others see full kthread. */
- WRITE_ONCE(rcu_tasks_kthread_ptr, t);
- 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)
-{
- preempt_disable();
- current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
- preempt_enable();
-}
-
-/* Do the srcu_read_unlock() for the above synchronize_srcu(). */
-void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu)
-{
- preempt_disable();
- __srcu_read_unlock(&tasks_rcu_exit_srcu, current->rcu_tasks_idx);
- preempt_enable();
-}
-
-#endif /* #ifdef CONFIG_TASKS_RCU */
-
-#ifndef CONFIG_TINY_RCU
-
-/*
- * Print any non-default Tasks RCU settings.
- */
-static void __init rcu_tasks_bootup_oddness(void)
-{
-#ifdef CONFIG_TASKS_RCU
- if (rcu_task_stall_timeout != RCU_TASK_STALL_TIMEOUT)
- pr_info("\tTasks-RCU CPU stall warnings timeout set to %d (rcu_task_stall_timeout).\n", rcu_task_stall_timeout);
- else
- pr_info("\tTasks RCU enabled.\n");
-#endif /* #ifdef CONFIG_TASKS_RCU */
-}
-
-#endif /* #ifndef CONFIG_TINY_RCU */
-
#ifdef CONFIG_PROVE_RCU

/*
@@ -923,6 +559,8 @@ late_initcall(rcu_verify_early_boot_tests);
void rcu_early_boot_tests(void) {}
#endif /* CONFIG_PROVE_RCU */

+#include "tasks.h"
+
#ifndef CONFIG_TINY_RCU

/*
--
2.9.5