rcu-tasks: Move Tasks RCU to its own file

/* 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 */