Merge tag 'for-linus-5.9-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/rw...

[mirror_ubuntu-jammy-kernel.git] / kernel / rcu / tasks.h

/* SPDX-License-Identifier: GPL-2.0+ */
/*
 * Task-based RCU implementations.
 *
 * Copyright (C) 2020 Paul E. McKenney
 */

#ifdef CONFIG_TASKS_RCU_GENERIC

////////////////////////////////////////////////////////////////////////
//
// Generic data structures.

struct rcu_tasks;
typedef void (*rcu_tasks_gp_func_t)(struct rcu_tasks *rtp);
typedef void (*pregp_func_t)(void);
typedef void (*pertask_func_t)(struct task_struct *t, struct list_head *hop);
typedef void (*postscan_func_t)(struct list_head *hop);
typedef void (*holdouts_func_t)(struct list_head *hop, bool ndrpt, bool *frptp);
typedef void (*postgp_func_t)(struct rcu_tasks *rtp);

/**
 * Definition for a Tasks-RCU-like mechanism.
 * @cbs_head: Head of callback list.
 * @cbs_tail: Tail pointer for callback list.
 * @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.
 * @gp_state: Grace period's most recent state transition (debugging).
 * @gp_jiffies: Time of last @gp_state transition.
 * @gp_start: Most recent grace-period start in jiffies.
 * @n_gps: Number of grace periods completed since boot.
 * @n_ipis: Number of IPIs sent to encourage grace periods to end.
 * @n_ipis_fails: Number of IPI-send failures.
 * @pregp_func: This flavor's pre-grace-period function (optional).
 * @pertask_func: This flavor's per-task scan function (optional).
 * @postscan_func: This flavor's post-task scan function (optional).
 * @holdout_func: This flavor's holdout-list scan function (optional).
 * @postgp_func: This flavor's post-grace-period function (optional).
 * @call_func: This flavor's call_rcu()-equivalent function.
 * @name: This flavor's textual name.
 * @kname: This flavor's kthread name.
 */
struct rcu_tasks {
        struct rcu_head *cbs_head;
        struct rcu_head **cbs_tail;
        struct wait_queue_head cbs_wq;
        raw_spinlock_t cbs_lock;
        int gp_state;
        unsigned long gp_jiffies;
        unsigned long gp_start;
        unsigned long n_gps;
        unsigned long n_ipis;
        unsigned long n_ipis_fails;
        struct task_struct *kthread_ptr;
        rcu_tasks_gp_func_t gp_func;
        pregp_func_t pregp_func;
        pertask_func_t pertask_func;
        postscan_func_t postscan_func;
        holdouts_func_t holdouts_func;
        postgp_func_t postgp_func;
        call_rcu_func_t call_func;
        char *name;
        char *kname;
};

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

/* Track exiting tasks in order to allow them to be waited for. */
DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);

/* Avoid IPIing CPUs early in the grace period. */
#define RCU_TASK_IPI_DELAY (HZ / 2)
static int rcu_task_ipi_delay __read_mostly = RCU_TASK_IPI_DELAY;
module_param(rcu_task_ipi_delay, int, 0644);

/* 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);

/* RCU tasks grace-period state for debugging. */
#define RTGS_INIT                0
#define RTGS_WAIT_WAIT_CBS       1
#define RTGS_WAIT_GP             2
#define RTGS_PRE_WAIT_GP         3
#define RTGS_SCAN_TASKLIST       4
#define RTGS_POST_SCAN_TASKLIST  5
#define RTGS_WAIT_SCAN_HOLDOUTS  6
#define RTGS_SCAN_HOLDOUTS       7
#define RTGS_POST_GP             8
#define RTGS_WAIT_READERS        9
#define RTGS_INVOKE_CBS         10
#define RTGS_WAIT_CBS           11
#ifndef CONFIG_TINY_RCU
static const char * const rcu_tasks_gp_state_names[] = {
        "RTGS_INIT",
        "RTGS_WAIT_WAIT_CBS",
        "RTGS_WAIT_GP",
        "RTGS_PRE_WAIT_GP",
        "RTGS_SCAN_TASKLIST",
        "RTGS_POST_SCAN_TASKLIST",
        "RTGS_WAIT_SCAN_HOLDOUTS",
        "RTGS_SCAN_HOLDOUTS",
        "RTGS_POST_GP",
        "RTGS_WAIT_READERS",
        "RTGS_INVOKE_CBS",
        "RTGS_WAIT_CBS",
};
#endif /* #ifndef CONFIG_TINY_RCU */

////////////////////////////////////////////////////////////////////////
//
// Generic code.

/* Record grace-period phase and time. */
static void set_tasks_gp_state(struct rcu_tasks *rtp, int newstate)
{
        rtp->gp_state = newstate;
        rtp->gp_jiffies = jiffies;
}

#ifndef CONFIG_TINY_RCU
/* Return state name. */
static const char *tasks_gp_state_getname(struct rcu_tasks *rtp)
{
        int i = data_race(rtp->gp_state); // Let KCSAN detect update races
        int j = READ_ONCE(i); // Prevent the compiler from reading twice

        if (j >= ARRAY_SIZE(rcu_tasks_gp_state_names))
                return "???";
        return rcu_tasks_gp_state_names[j];
}
#endif /* #ifndef CONFIG_TINY_RCU */

// 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;

        rhp->next = NULL;
        rhp->func = func;
        raw_spin_lock_irqsave(&rtp->cbs_lock, flags);
        needwake = !rtp->cbs_head;
        WRITE_ONCE(*rtp->cbs_tail, rhp);
        rtp->cbs_tail = &rhp->next;
        raw_spin_unlock_irqrestore(&rtp->cbs_lock, flags);
        /* We can't create the thread unless interrupts are enabled. */
        if (needwake && READ_ONCE(rtp->kthread_ptr))
                wake_up(&rtp->cbs_wq);
}

// 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(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 rcu_head *list;
        struct rcu_head *next;
        struct rcu_tasks *rtp = arg;

        /* Run on housekeeping CPUs by default.  Sysadm can move if desired. */
        housekeeping_affine(current, HK_FLAG_RCU);
        WRITE_ONCE(rtp->kthread_ptr, current); // Let GPs start!

        /*
         * 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(&rtp->cbs_lock, flags);
                smp_mb__after_spinlock(); // Order updates vs. GP.
                list = rtp->cbs_head;
                rtp->cbs_head = NULL;
                rtp->cbs_tail = &rtp->cbs_head;
                raw_spin_unlock_irqrestore(&rtp->cbs_lock, flags);

                /* If there were none, wait a bit and start over. */
                if (!list) {
                        wait_event_interruptible(rtp->cbs_wq,
                                                 READ_ONCE(rtp->cbs_head));
                        if (!rtp->cbs_head) {
                                WARN_ON(signal_pending(current));
                                set_tasks_gp_state(rtp, RTGS_WAIT_WAIT_CBS);
                                schedule_timeout_idle(HZ/10);
                        }
                        continue;
                }

                // Wait for one grace period.
                set_tasks_gp_state(rtp, RTGS_WAIT_GP);
                rtp->gp_start = jiffies;
                rtp->gp_func(rtp);
                rtp->n_gps++;

                /* Invoke the callbacks. */
                set_tasks_gp_state(rtp, RTGS_INVOKE_CBS);
                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_idle(HZ/10);

                set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
        }
}

/* 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, rtp, "%s_kthread", rtp->kname);
        if (WARN_ONCE(IS_ERR(t), "%s: Could not start %s grace-period kthread, OOM is now expected behavior\n", __func__, rtp->name))
                return;
        smp_mb(); /* Ensure others see full kthread. */
}

#ifndef CONFIG_TINY_RCU

/*
 * Print any non-default Tasks RCU settings.
 */
static void __init rcu_tasks_bootup_oddness(void)
{
#if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_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);
#endif /* #ifdef CONFIG_TASKS_RCU */
#ifdef CONFIG_TASKS_RCU
        pr_info("\tTrampoline variant of Tasks RCU enabled.\n");
#endif /* #ifdef CONFIG_TASKS_RCU */
#ifdef CONFIG_TASKS_RUDE_RCU
        pr_info("\tRude variant of Tasks RCU enabled.\n");
#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
#ifdef CONFIG_TASKS_TRACE_RCU
        pr_info("\tTracing variant of Tasks RCU enabled.\n");
#endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
}

#endif /* #ifndef CONFIG_TINY_RCU */

#ifndef CONFIG_TINY_RCU
/* Dump out rcutorture-relevant state common to all RCU-tasks flavors. */
static void show_rcu_tasks_generic_gp_kthread(struct rcu_tasks *rtp, char *s)
{
        pr_info("%s: %s(%d) since %lu g:%lu i:%lu/%lu %c%c %s\n",
                rtp->kname,
                tasks_gp_state_getname(rtp), data_race(rtp->gp_state),
                jiffies - data_race(rtp->gp_jiffies),
                data_race(rtp->n_gps),
                data_race(rtp->n_ipis_fails), data_race(rtp->n_ipis),
                ".k"[!!data_race(rtp->kthread_ptr)],
                ".C"[!!data_race(rtp->cbs_head)],
                s);
}
#endif /* #ifndef CONFIG_TINY_RCU */

static void exit_tasks_rcu_finish_trace(struct task_struct *t);

#if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)

////////////////////////////////////////////////////////////////////////
//
// Shared code between task-list-scanning variants of Tasks RCU.

/* 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(holdouts);
        int fract;

        set_tasks_gp_state(rtp, RTGS_PRE_WAIT_GP);
        rtp->pregp_func();

        /*
         * 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 holdouts.
         */
        set_tasks_gp_state(rtp, RTGS_SCAN_TASKLIST);
        rcu_read_lock();
        for_each_process_thread(g, t)
                rtp->pertask_func(t, &holdouts);
        rcu_read_unlock();

        set_tasks_gp_state(rtp, RTGS_POST_SCAN_TASKLIST);
        rtp->postscan_func(&holdouts);

        /*
         * 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;

                if (list_empty(&holdouts))
                        break;

                /* Slowly back off waiting for holdouts */
                set_tasks_gp_state(rtp, RTGS_WAIT_SCAN_HOLDOUTS);
                schedule_timeout_idle(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));
                set_tasks_gp_state(rtp, RTGS_SCAN_HOLDOUTS);
                rtp->holdouts_func(&holdouts, needreport, &firstreport);
        }

        set_tasks_gp_state(rtp, RTGS_POST_GP);
        rtp->postgp_func(rtp);
}

#endif /* #if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_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.

/* Pre-grace-period preparation. */
static void rcu_tasks_pregp_step(void)
{
        /*
         * 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();
}

/* Per-task initial processing. */
static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
{
        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, hop);
        }
}

/* Processing between scanning taskslist and draining the holdout list. */
static void rcu_tasks_postscan(struct list_head *hop)
{
        /*
         * 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);
}

/* 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);
}

/* Scan the holdout lists for tasks no longer holding out. */
static void check_all_holdout_tasks(struct list_head *hop,
                                    bool needreport, bool *firstreport)
{
        struct task_struct *t, *t1;

        list_for_each_entry_safe(t, t1, hop, rcu_tasks_holdout_list) {
                check_holdout_task(t, needreport, firstreport);
                cond_resched();
        }
}

/* Finish off the Tasks-RCU grace period. */
static void rcu_tasks_postgp(struct rcu_tasks *rtp)
{
        /*
         * 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, "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_tasks.pregp_func = rcu_tasks_pregp_step;
        rcu_tasks.pertask_func = rcu_tasks_pertask;
        rcu_tasks.postscan_func = rcu_tasks_postscan;
        rcu_tasks.holdouts_func = check_all_holdout_tasks;
        rcu_tasks.postgp_func = rcu_tasks_postgp;
        rcu_spawn_tasks_kthread_generic(&rcu_tasks);
        return 0;
}
core_initcall(rcu_spawn_tasks_kthread);

#ifndef CONFIG_TINY_RCU
static void show_rcu_tasks_classic_gp_kthread(void)
{
        show_rcu_tasks_generic_gp_kthread(&rcu_tasks, "");
}
#endif /* #ifndef CONFIG_TINY_RCU */

/* 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)
{
        struct task_struct *t = current;

        preempt_disable();
        __srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx);
        preempt_enable();
        exit_tasks_rcu_finish_trace(t);
}

#else /* #ifdef CONFIG_TASKS_RCU */
static void show_rcu_tasks_classic_gp_kthread(void) { }
void exit_tasks_rcu_start(void) { }
void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); }
#endif /* #else #ifdef CONFIG_TASKS_RCU */

#ifdef CONFIG_TASKS_RUDE_RCU

////////////////////////////////////////////////////////////////////////
//
// "Rude" variant of Tasks RCU, inspired by Steve Rostedt's trick of
// passing an empty function to schedule_on_each_cpu().  This approach
// provides an asynchronous call_rcu_tasks_rude() API and batching
// of concurrent calls to the synchronous synchronize_rcu_rude() API.
// This sends IPIs far and wide and induces otherwise unnecessary context
// switches on all online CPUs, whether idle or not.

// Empty function to allow workqueues to force a context switch.
static void rcu_tasks_be_rude(struct work_struct *work)
{
}

// Wait for one rude RCU-tasks grace period.
static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
{
        rtp->n_ipis += cpumask_weight(cpu_online_mask);
        schedule_on_each_cpu(rcu_tasks_be_rude);
}

void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
DEFINE_RCU_TASKS(rcu_tasks_rude, rcu_tasks_rude_wait_gp, call_rcu_tasks_rude,
                 "RCU Tasks Rude");

/**
 * call_rcu_tasks_rude() - Queue a callback rude 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_rude()
 * assumes that the read-side critical sections end at context switch,
 * cond_resched_rcu_qs(), 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_rude(struct rcu_head *rhp, rcu_callback_t func)
{
        call_rcu_tasks_generic(rhp, func, &rcu_tasks_rude);
}
EXPORT_SYMBOL_GPL(call_rcu_tasks_rude);

/**
 * synchronize_rcu_tasks_rude - wait for a rude rcu-tasks grace period
 *
 * Control will return to the caller some time after a rude 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(), userspace execution, 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_rude() 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_rude(void)
{
        synchronize_rcu_tasks_generic(&rcu_tasks_rude);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_rude);

/**
 * rcu_barrier_tasks_rude - Wait for in-flight call_rcu_tasks_rude() 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_rude(void)
{
        /* There is only one callback queue, so this is easy.  ;-) */
        synchronize_rcu_tasks_rude();
}
EXPORT_SYMBOL_GPL(rcu_barrier_tasks_rude);

static int __init rcu_spawn_tasks_rude_kthread(void)
{
        rcu_spawn_tasks_kthread_generic(&rcu_tasks_rude);
        return 0;
}
core_initcall(rcu_spawn_tasks_rude_kthread);

#ifndef CONFIG_TINY_RCU
static void show_rcu_tasks_rude_gp_kthread(void)
{
        show_rcu_tasks_generic_gp_kthread(&rcu_tasks_rude, "");
}
#endif /* #ifndef CONFIG_TINY_RCU */

#else /* #ifdef CONFIG_TASKS_RUDE_RCU */
static void show_rcu_tasks_rude_gp_kthread(void) {}
#endif /* #else #ifdef CONFIG_TASKS_RUDE_RCU */

////////////////////////////////////////////////////////////////////////
//
// Tracing variant of Tasks RCU.  This variant is designed to be used
// to protect tracing hooks, including those of BPF.  This variant
// therefore:
//
// 1.   Has explicit read-side markers to allow finite grace periods
//      in the face of in-kernel loops for PREEMPT=n builds.
//
// 2.   Protects code in the idle loop, exception entry/exit, and
//      CPU-hotplug code paths, similar to the capabilities of SRCU.
//
// 3.   Avoids expensive read-side instruction, having overhead similar
//      to that of Preemptible RCU.
//
// There are of course downsides.  The grace-period code can send IPIs to
// CPUs, even when those CPUs are in the idle loop or in nohz_full userspace.
// It is necessary to scan the full tasklist, much as for Tasks RCU.  There
// is a single callback queue guarded by a single lock, again, much as for
// Tasks RCU.  If needed, these downsides can be at least partially remedied.
//
// Perhaps most important, this variant of RCU does not affect the vanilla
// flavors, rcu_preempt and rcu_sched.  The fact that RCU Tasks Trace
// readers can operate from idle, offline, and exception entry/exit in no
// way allows rcu_preempt and rcu_sched readers to also do so.

// The lockdep state must be outside of #ifdef to be useful.
#ifdef CONFIG_DEBUG_LOCK_ALLOC
static struct lock_class_key rcu_lock_trace_key;
struct lockdep_map rcu_trace_lock_map =
        STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_trace", &rcu_lock_trace_key);
EXPORT_SYMBOL_GPL(rcu_trace_lock_map);
#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

#ifdef CONFIG_TASKS_TRACE_RCU

static atomic_t trc_n_readers_need_end;         // Number of waited-for readers.
static DECLARE_WAIT_QUEUE_HEAD(trc_wait);       // List of holdout tasks.

// Record outstanding IPIs to each CPU.  No point in sending two...
static DEFINE_PER_CPU(bool, trc_ipi_to_cpu);

// The number of detections of task quiescent state relying on
// heavyweight readers executing explicit memory barriers.
unsigned long n_heavy_reader_attempts;
unsigned long n_heavy_reader_updates;
unsigned long n_heavy_reader_ofl_updates;

void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);
DEFINE_RCU_TASKS(rcu_tasks_trace, rcu_tasks_wait_gp, call_rcu_tasks_trace,
                 "RCU Tasks Trace");

/*
 * This irq_work handler allows rcu_read_unlock_trace() to be invoked
 * while the scheduler locks are held.
 */
static void rcu_read_unlock_iw(struct irq_work *iwp)
{
        wake_up(&trc_wait);
}
static DEFINE_IRQ_WORK(rcu_tasks_trace_iw, rcu_read_unlock_iw);

/* If we are the last reader, wake up the grace-period kthread. */
void rcu_read_unlock_trace_special(struct task_struct *t, int nesting)
{
        int nq = t->trc_reader_special.b.need_qs;

        if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) &&
            t->trc_reader_special.b.need_mb)
                smp_mb(); // Pairs with update-side barriers.
        // Update .need_qs before ->trc_reader_nesting for irq/NMI handlers.
        if (nq)
                WRITE_ONCE(t->trc_reader_special.b.need_qs, false);
        WRITE_ONCE(t->trc_reader_nesting, nesting);
        if (nq && atomic_dec_and_test(&trc_n_readers_need_end))
                irq_work_queue(&rcu_tasks_trace_iw);
}
EXPORT_SYMBOL_GPL(rcu_read_unlock_trace_special);

/* Add a task to the holdout list, if it is not already on the list. */
static void trc_add_holdout(struct task_struct *t, struct list_head *bhp)
{
        if (list_empty(&t->trc_holdout_list)) {
                get_task_struct(t);
                list_add(&t->trc_holdout_list, bhp);
        }
}

/* Remove a task from the holdout list, if it is in fact present. */
static void trc_del_holdout(struct task_struct *t)
{
        if (!list_empty(&t->trc_holdout_list)) {
                list_del_init(&t->trc_holdout_list);
                put_task_struct(t);
        }
}

/* IPI handler to check task state. */
static void trc_read_check_handler(void *t_in)
{
        struct task_struct *t = current;
        struct task_struct *texp = t_in;

        // If the task is no longer running on this CPU, leave.
        if (unlikely(texp != t)) {
                if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end)))
                        wake_up(&trc_wait);
                goto reset_ipi; // Already on holdout list, so will check later.
        }

        // If the task is not in a read-side critical section, and
        // if this is the last reader, awaken the grace-period kthread.
        if (likely(!t->trc_reader_nesting)) {
                if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end)))
                        wake_up(&trc_wait);
                // Mark as checked after decrement to avoid false
                // positives on the above WARN_ON_ONCE().
                WRITE_ONCE(t->trc_reader_checked, true);
                goto reset_ipi;
        }
        WRITE_ONCE(t->trc_reader_checked, true);

        // Get here if the task is in a read-side critical section.  Set
        // its state so that it will awaken the grace-period kthread upon
        // exit from that critical section.
        WARN_ON_ONCE(t->trc_reader_special.b.need_qs);
        WRITE_ONCE(t->trc_reader_special.b.need_qs, true);

reset_ipi:
        // Allow future IPIs to be sent on CPU and for task.
        // Also order this IPI handler against any later manipulations of
        // the intended task.
        smp_store_release(&per_cpu(trc_ipi_to_cpu, smp_processor_id()), false); // ^^^
        smp_store_release(&texp->trc_ipi_to_cpu, -1); // ^^^
}

/* Callback function for scheduler to check locked-down task.  */
static bool trc_inspect_reader(struct task_struct *t, void *arg)
{
        int cpu = task_cpu(t);
        bool in_qs = false;
        bool ofl = cpu_is_offline(cpu);

        if (task_curr(t)) {
                WARN_ON_ONCE(ofl && !is_idle_task(t));

                // If no chance of heavyweight readers, do it the hard way.
                if (!ofl && !IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
                        return false;

                // If heavyweight readers are enabled on the remote task,
                // we can inspect its state despite its currently running.
                // However, we cannot safely change its state.
                n_heavy_reader_attempts++;
                if (!ofl && // Check for "running" idle tasks on offline CPUs.
                    !rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
                        return false; // No quiescent state, do it the hard way.
                n_heavy_reader_updates++;
                if (ofl)
                        n_heavy_reader_ofl_updates++;
                in_qs = true;
        } else {
                in_qs = likely(!t->trc_reader_nesting);
        }

        // Mark as checked.  Because this is called from the grace-period
        // kthread, also remove the task from the holdout list.
        t->trc_reader_checked = true;
        trc_del_holdout(t);

        if (in_qs)
                return true;  // Already in quiescent state, done!!!

        // The task is in a read-side critical section, so set up its
        // state so that it will awaken the grace-period kthread upon exit
        // from that critical section.
        atomic_inc(&trc_n_readers_need_end); // One more to wait on.
        WARN_ON_ONCE(t->trc_reader_special.b.need_qs);
        WRITE_ONCE(t->trc_reader_special.b.need_qs, true);
        return true;
}

/* Attempt to extract the state for the specified task. */
static void trc_wait_for_one_reader(struct task_struct *t,
                                    struct list_head *bhp)
{
        int cpu;

        // If a previous IPI is still in flight, let it complete.
        if (smp_load_acquire(&t->trc_ipi_to_cpu) != -1) // Order IPI
                return;

        // The current task had better be in a quiescent state.
        if (t == current) {
                t->trc_reader_checked = true;
                trc_del_holdout(t);
                WARN_ON_ONCE(t->trc_reader_nesting);
                return;
        }

        // Attempt to nail down the task for inspection.
        get_task_struct(t);
        if (try_invoke_on_locked_down_task(t, trc_inspect_reader, NULL)) {
                put_task_struct(t);
                return;
        }
        put_task_struct(t);

        // If currently running, send an IPI, either way, add to list.
        trc_add_holdout(t, bhp);
        if (task_curr(t) && time_after(jiffies, rcu_tasks_trace.gp_start + rcu_task_ipi_delay)) {
                // The task is currently running, so try IPIing it.
                cpu = task_cpu(t);

                // If there is already an IPI outstanding, let it happen.
                if (per_cpu(trc_ipi_to_cpu, cpu) || t->trc_ipi_to_cpu >= 0)
                        return;

                atomic_inc(&trc_n_readers_need_end);
                per_cpu(trc_ipi_to_cpu, cpu) = true;
                t->trc_ipi_to_cpu = cpu;
                rcu_tasks_trace.n_ipis++;
                if (smp_call_function_single(cpu,
                                             trc_read_check_handler, t, 0)) {
                        // Just in case there is some other reason for
                        // failure than the target CPU being offline.
                        rcu_tasks_trace.n_ipis_fails++;
                        per_cpu(trc_ipi_to_cpu, cpu) = false;
                        t->trc_ipi_to_cpu = cpu;
                        if (atomic_dec_and_test(&trc_n_readers_need_end)) {
                                WARN_ON_ONCE(1);
                                wake_up(&trc_wait);
                        }
                }
        }
}

/* Initialize for a new RCU-tasks-trace grace period. */
static void rcu_tasks_trace_pregp_step(void)
{
        int cpu;

        // Allow for fast-acting IPIs.
        atomic_set(&trc_n_readers_need_end, 1);

        // There shouldn't be any old IPIs, but...
        for_each_possible_cpu(cpu)
                WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));

        // Disable CPU hotplug across the tasklist scan.
        // This also waits for all readers in CPU-hotplug code paths.
        cpus_read_lock();
}

/* Do first-round processing for the specified task. */
static void rcu_tasks_trace_pertask(struct task_struct *t,
                                    struct list_head *hop)
{
        WRITE_ONCE(t->trc_reader_special.b.need_qs, false);
        WRITE_ONCE(t->trc_reader_checked, false);
        t->trc_ipi_to_cpu = -1;
        trc_wait_for_one_reader(t, hop);
}

/*
 * Do intermediate processing between task and holdout scans and
 * pick up the idle tasks.
 */
static void rcu_tasks_trace_postscan(struct list_head *hop)
{
        int cpu;

        for_each_possible_cpu(cpu)
                rcu_tasks_trace_pertask(idle_task(cpu), hop);

        // Re-enable CPU hotplug now that the tasklist scan has completed.
        cpus_read_unlock();

        // Wait for late-stage exiting tasks to finish exiting.
        // These might have passed the call to exit_tasks_rcu_finish().
        synchronize_rcu();
        // Any tasks that exit after this point will set ->trc_reader_checked.
}

/* Show the state of a task stalling the current RCU tasks trace GP. */
static void show_stalled_task_trace(struct task_struct *t, bool *firstreport)
{
        int cpu;

        if (*firstreport) {
                pr_err("INFO: rcu_tasks_trace detected stalls on tasks:\n");
                *firstreport = false;
        }
        // FIXME: This should attempt to use try_invoke_on_nonrunning_task().
        cpu = task_cpu(t);
        pr_alert("P%d: %c%c%c nesting: %d%c cpu: %d\n",
                 t->pid,
                 ".I"[READ_ONCE(t->trc_ipi_to_cpu) > 0],
                 ".i"[is_idle_task(t)],
                 ".N"[cpu > 0 && tick_nohz_full_cpu(cpu)],
                 t->trc_reader_nesting,
                 " N"[!!t->trc_reader_special.b.need_qs],
                 cpu);
        sched_show_task(t);
}

/* List stalled IPIs for RCU tasks trace. */
static void show_stalled_ipi_trace(void)
{
        int cpu;

        for_each_possible_cpu(cpu)
                if (per_cpu(trc_ipi_to_cpu, cpu))
                        pr_alert("\tIPI outstanding to CPU %d\n", cpu);
}

/* Do one scan of the holdout list. */
static void check_all_holdout_tasks_trace(struct list_head *hop,
                                          bool needreport, bool *firstreport)
{
        struct task_struct *g, *t;

        // Disable CPU hotplug across the holdout list scan.
        cpus_read_lock();

        list_for_each_entry_safe(t, g, hop, trc_holdout_list) {
                // If safe and needed, try to check the current task.
                if (READ_ONCE(t->trc_ipi_to_cpu) == -1 &&
                    !READ_ONCE(t->trc_reader_checked))
                        trc_wait_for_one_reader(t, hop);

                // If check succeeded, remove this task from the list.
                if (READ_ONCE(t->trc_reader_checked))
                        trc_del_holdout(t);
                else if (needreport)
                        show_stalled_task_trace(t, firstreport);
        }

        // Re-enable CPU hotplug now that the holdout list scan has completed.
        cpus_read_unlock();

        if (needreport) {
                if (firstreport)
                        pr_err("INFO: rcu_tasks_trace detected stalls? (Late IPI?)\n");
                show_stalled_ipi_trace();
        }
}

/* Wait for grace period to complete and provide ordering. */
static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
{
        bool firstreport;
        struct task_struct *g, *t;
        LIST_HEAD(holdouts);
        long ret;

        // Remove the safety count.
        smp_mb__before_atomic();  // Order vs. earlier atomics
        atomic_dec(&trc_n_readers_need_end);
        smp_mb__after_atomic();  // Order vs. later atomics

        // Wait for readers.
        set_tasks_gp_state(rtp, RTGS_WAIT_READERS);
        for (;;) {
                ret = wait_event_idle_exclusive_timeout(
                                trc_wait,
                                atomic_read(&trc_n_readers_need_end) == 0,
                                READ_ONCE(rcu_task_stall_timeout));
                if (ret)
                        break;  // Count reached zero.
                // Stall warning time, so make a list of the offenders.
                for_each_process_thread(g, t)
                        if (READ_ONCE(t->trc_reader_special.b.need_qs))
                                trc_add_holdout(t, &holdouts);
                firstreport = true;
                list_for_each_entry_safe(t, g, &holdouts, trc_holdout_list)
                        if (READ_ONCE(t->trc_reader_special.b.need_qs)) {
                                show_stalled_task_trace(t, &firstreport);
                                trc_del_holdout(t);
                        }
                if (firstreport)
                        pr_err("INFO: rcu_tasks_trace detected stalls? (Counter/taskslist mismatch?)\n");
                show_stalled_ipi_trace();
                pr_err("\t%d holdouts\n", atomic_read(&trc_n_readers_need_end));
        }
        smp_mb(); // Caller's code must be ordered after wakeup.
                  // Pairs with pretty much every ordering primitive.
}

/* Report any needed quiescent state for this exiting task. */
static void exit_tasks_rcu_finish_trace(struct task_struct *t)
{
        WRITE_ONCE(t->trc_reader_checked, true);
        WARN_ON_ONCE(t->trc_reader_nesting);
        WRITE_ONCE(t->trc_reader_nesting, 0);
        if (WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs)))
                rcu_read_unlock_trace_special(t, 0);
}

/**
 * call_rcu_tasks_trace() - Queue a callback trace 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_trace()
 * assumes that the read-side critical sections end at context switch,
 * cond_resched_rcu_qs(), 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_trace(struct rcu_head *rhp, rcu_callback_t func)
{
        call_rcu_tasks_generic(rhp, func, &rcu_tasks_trace);
}
EXPORT_SYMBOL_GPL(call_rcu_tasks_trace);

/**
 * synchronize_rcu_tasks_trace - wait for a trace rcu-tasks grace period
 *
 * Control will return to the caller some time after a trace 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 rcu_read_lock_trace()
 * and rcu_read_unlock_trace().
 *
 * 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_trace() 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_trace(void)
{
        RCU_LOCKDEP_WARN(lock_is_held(&rcu_trace_lock_map), "Illegal synchronize_rcu_tasks_trace() in RCU Tasks Trace read-side critical section");
        synchronize_rcu_tasks_generic(&rcu_tasks_trace);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_trace);

/**
 * rcu_barrier_tasks_trace - Wait for in-flight call_rcu_tasks_trace() 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_trace(void)
{
        /* There is only one callback queue, so this is easy.  ;-) */
        synchronize_rcu_tasks_trace();
}
EXPORT_SYMBOL_GPL(rcu_barrier_tasks_trace);

static int __init rcu_spawn_tasks_trace_kthread(void)
{
        rcu_tasks_trace.pregp_func = rcu_tasks_trace_pregp_step;
        rcu_tasks_trace.pertask_func = rcu_tasks_trace_pertask;
        rcu_tasks_trace.postscan_func = rcu_tasks_trace_postscan;
        rcu_tasks_trace.holdouts_func = check_all_holdout_tasks_trace;
        rcu_tasks_trace.postgp_func = rcu_tasks_trace_postgp;
        rcu_spawn_tasks_kthread_generic(&rcu_tasks_trace);
        return 0;
}
core_initcall(rcu_spawn_tasks_trace_kthread);

#ifndef CONFIG_TINY_RCU
static void show_rcu_tasks_trace_gp_kthread(void)
{
        char buf[64];

        sprintf(buf, "N%d h:%lu/%lu/%lu", atomic_read(&trc_n_readers_need_end),
                data_race(n_heavy_reader_ofl_updates),
                data_race(n_heavy_reader_updates),
                data_race(n_heavy_reader_attempts));
        show_rcu_tasks_generic_gp_kthread(&rcu_tasks_trace, buf);
}
#endif /* #ifndef CONFIG_TINY_RCU */

#else /* #ifdef CONFIG_TASKS_TRACE_RCU */
static void exit_tasks_rcu_finish_trace(struct task_struct *t) { }
static inline void show_rcu_tasks_trace_gp_kthread(void) {}
#endif /* #else #ifdef CONFIG_TASKS_TRACE_RCU */

#ifndef CONFIG_TINY_RCU
void show_rcu_tasks_gp_kthreads(void)
{
        show_rcu_tasks_classic_gp_kthread();
        show_rcu_tasks_rude_gp_kthread();
        show_rcu_tasks_trace_gp_kthread();
}
#endif /* #ifndef CONFIG_TINY_RCU */

#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
static inline void rcu_tasks_bootup_oddness(void) {}
void show_rcu_tasks_gp_kthreads(void) {}
#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */