linux/kernel/rcu/tasks.h

/* 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 */