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

#ifdef CONFIG_TASKS_RCU_GENERIC
#include "rcu_segcblist.h"

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

struct rcu_tasks;
typedef void (*rcu_tasks_gp_func_t)(struct rcu_tasks *rtp);
typedef void (*pregp_func_t)(struct list_head *hop);
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);

/**
 * struct rcu_tasks_percpu - Per-CPU component of definition for a Tasks-RCU-like mechanism.
 * @cblist: Callback list.
 * @lock: Lock protecting per-CPU callback list.
 * @rtp_jiffies: Jiffies counter value for statistics.
 * @lazy_timer: Timer to unlazify callbacks.
 * @urgent_gp: Number of additional non-lazy grace periods.
 * @rtp_n_lock_retries: Rough lock-contention statistic.
 * @rtp_work: Work queue for invoking callbacks.
 * @rtp_irq_work: IRQ work queue for deferred wakeups.
 * @barrier_q_head: RCU callback for barrier operation.
 * @rtp_blkd_tasks: List of tasks blocked as readers.
 * @rtp_exit_list: List of tasks in the latter portion of do_exit().
 * @cpu: CPU number corresponding to this entry.
 * @index: Index of this CPU in rtpcp_array of the rcu_tasks structure.
 * @rtpp: Pointer to the rcu_tasks structure.
 */
struct rcu_tasks_percpu {
        struct rcu_segcblist cblist;
        raw_spinlock_t __private lock;
        unsigned long rtp_jiffies;
        unsigned long rtp_n_lock_retries;
        struct timer_list lazy_timer;
        unsigned int urgent_gp;
        struct work_struct rtp_work;
        struct irq_work rtp_irq_work;
        struct rcu_head barrier_q_head;
        struct list_head rtp_blkd_tasks;
        struct list_head rtp_exit_list;
        int cpu;
        int index;
        struct rcu_tasks *rtpp;
};

/**
 * struct rcu_tasks - Definition for a Tasks-RCU-like mechanism.
 * @cbs_wait: RCU wait allowing a new callback to get kthread's attention.
 * @cbs_gbl_lock: Lock protecting callback list.
 * @tasks_gp_mutex: Mutex protecting grace period, needed during mid-boot dead zone.
 * @gp_func: This flavor's grace-period-wait function.
 * @gp_state: Grace period's most recent state transition (debugging).
 * @gp_sleep: Per-grace-period sleep to prevent CPU-bound looping.
 * @init_fract: Initial backoff sleep interval.
 * @gp_jiffies: Time of last @gp_state transition.
 * @gp_start: Most recent grace-period start in jiffies.
 * @tasks_gp_seq: Number of grace periods completed since boot in upper bits.
 * @n_ipis: Number of IPIs sent to encourage grace periods to end.
 * @n_ipis_fails: Number of IPI-send failures.
 * @kthread_ptr: This flavor's grace-period/callback-invocation kthread.
 * @lazy_jiffies: Number of jiffies to allow callbacks to be lazy.
 * @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).
 * @holdouts_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.
 * @wait_state: Task state for synchronous grace-period waits (default TASK_UNINTERRUPTIBLE).
 * @rtpcpu: This flavor's rcu_tasks_percpu structure.
 * @rtpcp_array: Array of pointers to rcu_tasks_percpu structure of CPUs in cpu_possible_mask.
 * @percpu_enqueue_shift: Shift down CPU ID this much when enqueuing callbacks.
 * @percpu_enqueue_lim: Number of per-CPU callback queues in use for enqueuing.
 * @percpu_dequeue_lim: Number of per-CPU callback queues in use for dequeuing.
 * @percpu_dequeue_gpseq: RCU grace-period number to propagate enqueue limit to dequeuers.
 * @barrier_q_mutex: Serialize barrier operations.
 * @barrier_q_count: Number of queues being waited on.
 * @barrier_q_completion: Barrier wait/wakeup mechanism.
 * @barrier_q_seq: Sequence number for barrier operations.
 * @barrier_q_start: Most recent barrier start in jiffies.
 * @name: This flavor's textual name.
 * @kname: This flavor's kthread name.
 */
struct rcu_tasks {
        struct rcuwait cbs_wait;
        raw_spinlock_t cbs_gbl_lock;
        struct mutex tasks_gp_mutex;
        int gp_state;
        int gp_sleep;
        int init_fract;
        unsigned long gp_jiffies;
        unsigned long gp_start;
        unsigned long tasks_gp_seq;
        unsigned long n_ipis;
        unsigned long n_ipis_fails;
        struct task_struct *kthread_ptr;
        unsigned long lazy_jiffies;
        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;
        unsigned int wait_state;
        struct rcu_tasks_percpu __percpu *rtpcpu;
        struct rcu_tasks_percpu **rtpcp_array;
        int percpu_enqueue_shift;
        int percpu_enqueue_lim;
        int percpu_dequeue_lim;
        unsigned long percpu_dequeue_gpseq;
        struct mutex barrier_q_mutex;
        atomic_t barrier_q_count;
        struct completion barrier_q_completion;
        unsigned long barrier_q_seq;
        unsigned long barrier_q_start;
        char *name;
        char *kname;
};

static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp);

#define DEFINE_RCU_TASKS(rt_name, gp, call, n)                                          \
static DEFINE_PER_CPU(struct rcu_tasks_percpu, rt_name ## __percpu) = {                 \
        .lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name ## __percpu.cbs_pcpu_lock),            \
        .rtp_irq_work = IRQ_WORK_INIT_HARD(call_rcu_tasks_iw_wakeup),                   \
};                                                                                      \
static struct rcu_tasks rt_name =                                                       \
{                                                                                       \
        .cbs_wait = __RCUWAIT_INITIALIZER(rt_name.wait),                                \
        .cbs_gbl_lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name.cbs_gbl_lock),                 \
        .tasks_gp_mutex = __MUTEX_INITIALIZER(rt_name.tasks_gp_mutex),                  \
        .gp_func = gp,                                                                  \
        .call_func = call,                                                              \
        .wait_state = TASK_UNINTERRUPTIBLE,                                             \
        .rtpcpu = &rt_name ## __percpu,                                                 \
        .lazy_jiffies = DIV_ROUND_UP(HZ, 4),                                            \
        .name = n,                                                                      \
        .percpu_enqueue_shift = order_base_2(CONFIG_NR_CPUS),                           \
        .percpu_enqueue_lim = 1,                                                        \
        .percpu_dequeue_lim = 1,                                                        \
        .barrier_q_mutex = __MUTEX_INITIALIZER(rt_name.barrier_q_mutex),                \
        .barrier_q_seq = (0UL - 50UL) << RCU_SEQ_CTR_SHIFT,                             \
        .kname = #rt_name,                                                              \
}

#ifdef CONFIG_TASKS_RCU

/* Report delay of scan exiting tasklist in rcu_tasks_postscan(). */
static void tasks_rcu_exit_srcu_stall(struct timer_list *unused);
static DEFINE_TIMER(tasks_rcu_exit_srcu_stall_timer, tasks_rcu_exit_srcu_stall);
#endif

/* Control stall timeouts.  Disable with <= 0, otherwise jiffies till stall. */
#define RCU_TASK_BOOT_STALL_TIMEOUT (HZ * 30)
#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);
#define RCU_TASK_STALL_INFO (HZ * 10)
static int rcu_task_stall_info __read_mostly = RCU_TASK_STALL_INFO;
module_param(rcu_task_stall_info, int, 0644);
static int rcu_task_stall_info_mult __read_mostly = 3;
module_param(rcu_task_stall_info_mult, int, 0444);

static int rcu_task_enqueue_lim __read_mostly = -1;
module_param(rcu_task_enqueue_lim, int, 0444);

static bool rcu_task_cb_adjust;
static int rcu_task_contend_lim __read_mostly = 100;
module_param(rcu_task_contend_lim, int, 0444);
static int rcu_task_collapse_lim __read_mostly = 10;
module_param(rcu_task_collapse_lim, int, 0444);
static int rcu_task_lazy_lim __read_mostly = 32;
module_param(rcu_task_lazy_lim, int, 0444);

static int rcu_task_cpu_ids;

/* 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.

static void rcu_tasks_invoke_cbs_wq(struct work_struct *wp);

/* 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 */

// Initialize per-CPU callback lists for the specified flavor of
// Tasks RCU.  Do not enqueue callbacks before this function is invoked.
static void cblist_init_generic(struct rcu_tasks *rtp)
{
        int cpu;
        int lim;
        int shift;
        int maxcpu;
        int index = 0;

        if (rcu_task_enqueue_lim < 0) {
                rcu_task_enqueue_lim = 1;
                rcu_task_cb_adjust = true;
        } else if (rcu_task_enqueue_lim == 0) {
                rcu_task_enqueue_lim = 1;
        }
        lim = rcu_task_enqueue_lim;

        rtp->rtpcp_array = kzalloc_objs(struct rcu_tasks_percpu *,
                                        num_possible_cpus());
        BUG_ON(!rtp->rtpcp_array);

        for_each_possible_cpu(cpu) {
                struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);

                WARN_ON_ONCE(!rtpcp);
                if (cpu)
                        raw_spin_lock_init(&ACCESS_PRIVATE(rtpcp, lock));
                if (rcu_segcblist_empty(&rtpcp->cblist))
                        rcu_segcblist_init(&rtpcp->cblist);
                INIT_WORK(&rtpcp->rtp_work, rcu_tasks_invoke_cbs_wq);
                rtpcp->cpu = cpu;
                rtpcp->rtpp = rtp;
                rtpcp->index = index;
                rtp->rtpcp_array[index] = rtpcp;
                index++;
                if (!rtpcp->rtp_blkd_tasks.next)
                        INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
                if (!rtpcp->rtp_exit_list.next)
                        INIT_LIST_HEAD(&rtpcp->rtp_exit_list);
                rtpcp->barrier_q_head.next = &rtpcp->barrier_q_head;
                maxcpu = cpu;
        }

        rcu_task_cpu_ids = maxcpu + 1;
        if (lim > rcu_task_cpu_ids)
                lim = rcu_task_cpu_ids;
        shift = ilog2(rcu_task_cpu_ids / lim);
        if (((rcu_task_cpu_ids - 1) >> shift) >= lim)
                shift++;
        WRITE_ONCE(rtp->percpu_enqueue_shift, shift);
        WRITE_ONCE(rtp->percpu_dequeue_lim, lim);
        smp_store_release(&rtp->percpu_enqueue_lim, lim);

        pr_info("%s: Setting shift to %d and lim to %d rcu_task_cb_adjust=%d rcu_task_cpu_ids=%d.\n",
                        rtp->name, data_race(rtp->percpu_enqueue_shift), data_race(rtp->percpu_enqueue_lim),
                        rcu_task_cb_adjust, rcu_task_cpu_ids);
}

// Compute wakeup time for lazy callback timer.
static unsigned long rcu_tasks_lazy_time(struct rcu_tasks *rtp)
{
        return jiffies + rtp->lazy_jiffies;
}

// Timer handler that unlazifies lazy callbacks.
static void call_rcu_tasks_generic_timer(struct timer_list *tlp)
{
        unsigned long flags;
        bool needwake = false;
        struct rcu_tasks *rtp;
        struct rcu_tasks_percpu *rtpcp = timer_container_of(rtpcp, tlp,
                                                            lazy_timer);

        rtp = rtpcp->rtpp;
        raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
        if (!rcu_segcblist_empty(&rtpcp->cblist) && rtp->lazy_jiffies) {
                if (!rtpcp->urgent_gp)
                        rtpcp->urgent_gp = 1;
                needwake = true;
                mod_timer(&rtpcp->lazy_timer, rcu_tasks_lazy_time(rtp));
        }
        raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
        if (needwake)
                rcuwait_wake_up(&rtp->cbs_wait);
}

// IRQ-work handler that does deferred wakeup for call_rcu_tasks_generic().
static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp)
{
        struct rcu_tasks *rtp;
        struct rcu_tasks_percpu *rtpcp = container_of(iwp, struct rcu_tasks_percpu, rtp_irq_work);

        rtp = rtpcp->rtpp;
        rcuwait_wake_up(&rtp->cbs_wait);
}

// 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)
{
        int chosen_cpu;
        unsigned long flags;
        bool havekthread = smp_load_acquire(&rtp->kthread_ptr);
        int ideal_cpu;
        unsigned long j;
        bool needadjust = false;
        bool needwake;
        struct rcu_tasks_percpu *rtpcp;

        rhp->next = NULL;
        rhp->func = func;
        local_irq_save(flags);
        rcu_read_lock();
        ideal_cpu = smp_processor_id() >> READ_ONCE(rtp->percpu_enqueue_shift);
        chosen_cpu = cpumask_next(ideal_cpu - 1, cpu_possible_mask);
        WARN_ON_ONCE(chosen_cpu >= rcu_task_cpu_ids);
        rtpcp = per_cpu_ptr(rtp->rtpcpu, chosen_cpu);
        if (!raw_spin_trylock_rcu_node(rtpcp)) { // irqs already disabled.
                raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
                j = jiffies;
                if (rtpcp->rtp_jiffies != j) {
                        rtpcp->rtp_jiffies = j;
                        rtpcp->rtp_n_lock_retries = 0;
                }
                if (rcu_task_cb_adjust && ++rtpcp->rtp_n_lock_retries > rcu_task_contend_lim &&
                    READ_ONCE(rtp->percpu_enqueue_lim) != rcu_task_cpu_ids)
                        needadjust = true;  // Defer adjustment to avoid deadlock.
        }
        // Queuing callbacks before initialization not yet supported.
        if (WARN_ON_ONCE(!rcu_segcblist_is_enabled(&rtpcp->cblist)))
                rcu_segcblist_init(&rtpcp->cblist);
        needwake = (func == wakeme_after_rcu) ||
                   (rcu_segcblist_n_cbs(&rtpcp->cblist) == rcu_task_lazy_lim);
        if (havekthread && !needwake && !timer_pending(&rtpcp->lazy_timer)) {
                if (rtp->lazy_jiffies)
                        mod_timer(&rtpcp->lazy_timer, rcu_tasks_lazy_time(rtp));
                else
                        needwake = rcu_segcblist_empty(&rtpcp->cblist);
        }
        if (needwake)
                rtpcp->urgent_gp = 3;
        rcu_segcblist_enqueue(&rtpcp->cblist, rhp);
        raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
        if (unlikely(needadjust)) {
                raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
                if (rtp->percpu_enqueue_lim != rcu_task_cpu_ids) {
                        WRITE_ONCE(rtp->percpu_enqueue_shift, 0);
                        WRITE_ONCE(rtp->percpu_dequeue_lim, rcu_task_cpu_ids);
                        smp_store_release(&rtp->percpu_enqueue_lim, rcu_task_cpu_ids);
                        pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name);
                }
                raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
        }
        rcu_read_unlock();
        /* We can't create the thread unless interrupts are enabled. */
        if (needwake && READ_ONCE(rtp->kthread_ptr))
                irq_work_queue(&rtpcp->rtp_irq_work);
}

// RCU callback function for rcu_barrier_tasks_generic().
static void rcu_barrier_tasks_generic_cb(struct rcu_head *rhp)
{
        struct rcu_tasks *rtp;
        struct rcu_tasks_percpu *rtpcp;

        rhp->next = rhp; // Mark the callback as having been invoked.
        rtpcp = container_of(rhp, struct rcu_tasks_percpu, barrier_q_head);
        rtp = rtpcp->rtpp;
        if (atomic_dec_and_test(&rtp->barrier_q_count))
                complete(&rtp->barrier_q_completion);
}

// Wait for all in-flight callbacks for the specified RCU Tasks flavor.
// Operates in a manner similar to rcu_barrier().
static void __maybe_unused rcu_barrier_tasks_generic(struct rcu_tasks *rtp)
{
        int cpu;
        unsigned long flags;
        struct rcu_tasks_percpu *rtpcp;
        unsigned long s = rcu_seq_snap(&rtp->barrier_q_seq);

        mutex_lock(&rtp->barrier_q_mutex);
        if (rcu_seq_done(&rtp->barrier_q_seq, s)) {
                smp_mb();
                mutex_unlock(&rtp->barrier_q_mutex);
                return;
        }
        rtp->barrier_q_start = jiffies;
        rcu_seq_start(&rtp->barrier_q_seq);
        init_completion(&rtp->barrier_q_completion);
        atomic_set(&rtp->barrier_q_count, 2);
        for_each_possible_cpu(cpu) {
                if (cpu >= smp_load_acquire(&rtp->percpu_dequeue_lim))
                        break;
                rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
                rtpcp->barrier_q_head.func = rcu_barrier_tasks_generic_cb;
                raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
                if (rcu_segcblist_entrain(&rtpcp->cblist, &rtpcp->barrier_q_head))
                        atomic_inc(&rtp->barrier_q_count);
                raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
        }
        if (atomic_sub_and_test(2, &rtp->barrier_q_count))
                complete(&rtp->barrier_q_completion);
        wait_for_completion(&rtp->barrier_q_completion);
        rcu_seq_end(&rtp->barrier_q_seq);
        mutex_unlock(&rtp->barrier_q_mutex);
}

// Advance callbacks and indicate whether either a grace period or
// callback invocation is needed.
static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
{
        int cpu;
        int dequeue_limit;
        unsigned long flags;
        bool gpdone = poll_state_synchronize_rcu(rtp->percpu_dequeue_gpseq);
        long n;
        long ncbs = 0;
        long ncbsnz = 0;
        int needgpcb = 0;

        dequeue_limit = smp_load_acquire(&rtp->percpu_dequeue_lim);
        for (cpu = 0; cpu < dequeue_limit; cpu++) {
                if (!cpu_possible(cpu))
                        continue;
                struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);

                /* Advance and accelerate any new callbacks. */
                if (!rcu_segcblist_n_cbs(&rtpcp->cblist))
                        continue;
                raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
                // Should we shrink down to a single callback queue?
                n = rcu_segcblist_n_cbs(&rtpcp->cblist);
                if (n) {
                        ncbs += n;
                        if (cpu > 0)
                                ncbsnz += n;
                }
                rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
                (void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
                if (rtpcp->urgent_gp > 0 && rcu_segcblist_pend_cbs(&rtpcp->cblist)) {
                        if (rtp->lazy_jiffies)
                                rtpcp->urgent_gp--;
                        needgpcb |= 0x3;
                } else if (rcu_segcblist_empty(&rtpcp->cblist)) {
                        rtpcp->urgent_gp = 0;
                }
                if (rcu_segcblist_ready_cbs(&rtpcp->cblist))
                        needgpcb |= 0x1;
                raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
        }

        // Shrink down to a single callback queue if appropriate.
        // This is done in two stages: (1) If there are no more than
        // rcu_task_collapse_lim callbacks on CPU 0 and none on any other
        // CPU, limit enqueueing to CPU 0.  (2) After an RCU grace period,
        // if there has not been an increase in callbacks, limit dequeuing
        // to CPU 0.  Note the matching RCU read-side critical section in
        // call_rcu_tasks_generic().
        if (rcu_task_cb_adjust && ncbs <= rcu_task_collapse_lim) {
                raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
                if (rtp->percpu_enqueue_lim > 1) {
                        WRITE_ONCE(rtp->percpu_enqueue_shift, order_base_2(rcu_task_cpu_ids));
                        smp_store_release(&rtp->percpu_enqueue_lim, 1);
                        rtp->percpu_dequeue_gpseq = get_state_synchronize_rcu();
                        gpdone = false;
                        pr_info("Starting switch %s to CPU-0 callback queuing.\n", rtp->name);
                }
                raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
        }
        if (rcu_task_cb_adjust && !ncbsnz && gpdone) {
                raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
                if (rtp->percpu_enqueue_lim < rtp->percpu_dequeue_lim) {
                        WRITE_ONCE(rtp->percpu_dequeue_lim, 1);
                        pr_info("Completing switch %s to CPU-0 callback queuing.\n", rtp->name);
                }
                if (rtp->percpu_dequeue_lim == 1) {
                        for (cpu = rtp->percpu_dequeue_lim; cpu < rcu_task_cpu_ids; cpu++) {
                                if (!cpu_possible(cpu))
                                        continue;
                                struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);

                                WARN_ON_ONCE(rcu_segcblist_n_cbs(&rtpcp->cblist));
                        }
                }
                raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
        }

        return needgpcb;
}

// Advance callbacks and invoke any that are ready.
static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu *rtpcp)
{
        int cpuwq;
        unsigned long flags;
        int len;
        int index;
        struct rcu_head *rhp;
        struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
        struct rcu_tasks_percpu *rtpcp_next;

        index = rtpcp->index * 2 + 1;
        if (index < num_possible_cpus()) {
                rtpcp_next = rtp->rtpcp_array[index];
                if (rtpcp_next->cpu < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
                        cpuwq = rcu_cpu_beenfullyonline(rtpcp_next->cpu) ? rtpcp_next->cpu : WORK_CPU_UNBOUND;
                        queue_work_on(cpuwq, system_percpu_wq, &rtpcp_next->rtp_work);
                        index++;
                        if (index < num_possible_cpus()) {
                                rtpcp_next = rtp->rtpcp_array[index];
                                if (rtpcp_next->cpu < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
                                        cpuwq = rcu_cpu_beenfullyonline(rtpcp_next->cpu) ? rtpcp_next->cpu : WORK_CPU_UNBOUND;
                                        queue_work_on(cpuwq, system_percpu_wq, &rtpcp_next->rtp_work);
                                }
                        }
                }
        }

        if (rcu_segcblist_empty(&rtpcp->cblist))
                return;
        raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
        rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
        rcu_segcblist_extract_done_cbs(&rtpcp->cblist, &rcl);
        raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
        len = rcl.len;
        for (rhp = rcu_cblist_dequeue(&rcl); rhp; rhp = rcu_cblist_dequeue(&rcl)) {
                debug_rcu_head_callback(rhp);
                local_bh_disable();
                rhp->func(rhp);
                local_bh_enable();
                cond_resched();
        }
        raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
        rcu_segcblist_add_len(&rtpcp->cblist, -len);
        (void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
        raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
}

// Workqueue flood to advance callbacks and invoke any that are ready.
static void rcu_tasks_invoke_cbs_wq(struct work_struct *wp)
{
        struct rcu_tasks *rtp;
        struct rcu_tasks_percpu *rtpcp = container_of(wp, struct rcu_tasks_percpu, rtp_work);

        rtp = rtpcp->rtpp;
        rcu_tasks_invoke_cbs(rtp, rtpcp);
}

// Wait for one grace period.
static void rcu_tasks_one_gp(struct rcu_tasks *rtp, bool midboot)
{
        int needgpcb;

        mutex_lock(&rtp->tasks_gp_mutex);

        // If there were none, wait a bit and start over.
        if (unlikely(midboot)) {
                needgpcb = 0x2;
        } else {
                mutex_unlock(&rtp->tasks_gp_mutex);
                set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
                rcuwait_wait_event(&rtp->cbs_wait,
                                   (needgpcb = rcu_tasks_need_gpcb(rtp)),
                                   TASK_IDLE);
                mutex_lock(&rtp->tasks_gp_mutex);
        }

        if (needgpcb & 0x2) {
                // Wait for one grace period.
                set_tasks_gp_state(rtp, RTGS_WAIT_GP);
                rtp->gp_start = jiffies;
                rcu_seq_start(&rtp->tasks_gp_seq);
                rtp->gp_func(rtp);
                rcu_seq_end(&rtp->tasks_gp_seq);
        }

        // Invoke callbacks.
        set_tasks_gp_state(rtp, RTGS_INVOKE_CBS);
        rcu_tasks_invoke_cbs(rtp, per_cpu_ptr(rtp->rtpcpu, 0));
        mutex_unlock(&rtp->tasks_gp_mutex);
}

// RCU-tasks kthread that detects grace periods and invokes callbacks.
static int __noreturn rcu_tasks_kthread(void *arg)
{
        int cpu;
        struct rcu_tasks *rtp = arg;

        for_each_possible_cpu(cpu) {
                struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);

                timer_setup(&rtpcp->lazy_timer, call_rcu_tasks_generic_timer, 0);
                rtpcp->urgent_gp = 1;
        }

        /* Run on housekeeping CPUs by default.  Sysadm can move if desired. */
        housekeeping_affine(current, HK_TYPE_RCU);
        smp_store_release(&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 (;;) {
                // Wait for one grace period and invoke any callbacks
                // that are ready.
                rcu_tasks_one_gp(rtp, false);

                // Paranoid sleep to keep this from entering a tight loop.
                schedule_timeout_idle(rtp->gp_sleep);
        }
}

// 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.  */
        if (WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
                         "synchronize_%s() called too soon", rtp->name))
                return;

        // If the grace-period kthread is running, use it.
        if (READ_ONCE(rtp->kthread_ptr)) {
                wait_rcu_gp_state(rtp->wait_state, rtp->call_func);
                return;
        }
        rcu_tasks_one_gp(rtp, true);
}

/* Spawn RCU-tasks grace-period kthread. */
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)
        int rtsimc;

        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);
        rtsimc = clamp(rcu_task_stall_info_mult, 1, 10);
        if (rtsimc != rcu_task_stall_info_mult) {
                pr_info("\tTasks-RCU CPU stall info multiplier clamped to %d (rcu_task_stall_info_mult).\n", rtsimc);
                rcu_task_stall_info_mult = rtsimc;
        }
#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 */
}

/* 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)
{
        int cpu;
        bool havecbs = false;
        bool haveurgent = false;
        bool haveurgentcbs = false;

        for_each_possible_cpu(cpu) {
                struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);

                if (!data_race(rcu_segcblist_empty(&rtpcp->cblist)))
                        havecbs = true;
                if (data_race(rtpcp->urgent_gp))
                        haveurgent = true;
                if (!data_race(rcu_segcblist_empty(&rtpcp->cblist)) && data_race(rtpcp->urgent_gp))
                        haveurgentcbs = true;
                if (havecbs && haveurgent && haveurgentcbs)
                        break;
        }
        pr_info("%s: %s(%d) since %lu g:%lu i:%lu/%lu %c%c%c%c l:%lu %s\n",
                rtp->kname,
                tasks_gp_state_getname(rtp), data_race(rtp->gp_state),
                jiffies - data_race(rtp->gp_jiffies),
                data_race(rcu_seq_current(&rtp->tasks_gp_seq)),
                data_race(rtp->n_ipis_fails), data_race(rtp->n_ipis),
                ".k"[!!data_race(rtp->kthread_ptr)],
                ".C"[havecbs],
                ".u"[haveurgent],
                ".U"[haveurgentcbs],
                rtp->lazy_jiffies,
                s);
}

/* Dump out more rcutorture-relevant state common to all RCU-tasks flavors. */
static void rcu_tasks_torture_stats_print_generic(struct rcu_tasks *rtp, char *tt,
                                                  char *tf, char *tst)
{
        cpumask_var_t cm;
        int cpu;
        bool gotcb = false;
        unsigned long j = jiffies;

        pr_alert("%s%s Tasks%s RCU g%ld gp_start %lu gp_jiffies %lu gp_state %d (%s).\n",
                 tt, tf, tst, data_race(rtp->tasks_gp_seq),
                 j - data_race(rtp->gp_start), j - data_race(rtp->gp_jiffies),
                 data_race(rtp->gp_state), tasks_gp_state_getname(rtp));
        pr_alert("\tEnqueue shift %d limit %d Dequeue limit %d gpseq %lu.\n",
                 data_race(rtp->percpu_enqueue_shift),
                 data_race(rtp->percpu_enqueue_lim),
                 data_race(rtp->percpu_dequeue_lim),
                 data_race(rtp->percpu_dequeue_gpseq));
        (void)zalloc_cpumask_var(&cm, GFP_KERNEL);
        pr_alert("\tCallback counts:");
        for_each_possible_cpu(cpu) {
                long n;
                struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);

                if (cpumask_available(cm) && !rcu_barrier_cb_is_done(&rtpcp->barrier_q_head))
                        cpumask_set_cpu(cpu, cm);
                n = rcu_segcblist_n_cbs(&rtpcp->cblist);
                if (!n)
                        continue;
                pr_cont(" %d:%ld", cpu, n);
                gotcb = true;
        }
        if (gotcb)
                pr_cont(".\n");
        else
                pr_cont(" (none).\n");
        pr_alert("\tBarrier seq %lu start %lu count %d holdout CPUs ",
                 data_race(rtp->barrier_q_seq), j - data_race(rtp->barrier_q_start),
                 atomic_read(&rtp->barrier_q_count));
        if (cpumask_available(cm) && !cpumask_empty(cm))
                pr_cont(" %*pbl.\n", cpumask_pr_args(cm));
        else
                pr_cont("(none).\n");
        free_cpumask_var(cm);
}

#endif // #ifndef CONFIG_TINY_RCU

#if defined(CONFIG_TASKS_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;
        int fract;
        LIST_HEAD(holdouts);
        unsigned long j;
        unsigned long lastinfo;
        unsigned long lastreport;
        bool reported = false;
        int rtsi;
        struct task_struct *t;

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

        /*
         * 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);
        if (rtp->pertask_func) {
                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;
        lastinfo = lastreport;
        rtsi = READ_ONCE(rcu_task_stall_info);

        // Start off with initial wait and slowly back off to 1 HZ wait.
        fract = rtp->init_fract;

        while (!list_empty(&holdouts)) {
                ktime_t exp;
                bool firstreport;
                bool needreport;
                int rtst;

                // Slowly back off waiting for holdouts
                set_tasks_gp_state(rtp, RTGS_WAIT_SCAN_HOLDOUTS);
                if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
                        schedule_timeout_idle(fract);
                } else {
                        exp = jiffies_to_nsecs(fract);
                        __set_current_state(TASK_IDLE);
                        schedule_hrtimeout_range(&exp, jiffies_to_nsecs(HZ / 2), HRTIMER_MODE_REL_HARD);
                }

                if (fract < HZ)
                        fract++;

                rtst = READ_ONCE(rcu_task_stall_timeout);
                needreport = rtst > 0 && time_after(jiffies, lastreport + rtst);
                if (needreport) {
                        lastreport = jiffies;
                        reported = true;
                }
                firstreport = true;
                WARN_ON(signal_pending(current));
                set_tasks_gp_state(rtp, RTGS_SCAN_HOLDOUTS);
                rtp->holdouts_func(&holdouts, needreport, &firstreport);

                // Print pre-stall informational messages if needed.
                j = jiffies;
                if (rtsi > 0 && !reported && time_after(j, lastinfo + rtsi)) {
                        lastinfo = j;
                        rtsi = rtsi * rcu_task_stall_info_mult;
                        pr_info("%s: %s grace period number %lu (since boot) is %lu jiffies old.\n",
                                __func__, rtp->kname, rtp->tasks_gp_seq, j - rtp->gp_start);
                }
        }

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

#endif /* #if defined(CONFIG_TASKS_RCU) */

#ifdef CONFIG_TASKS_RCU

////////////////////////////////////////////////////////////////////////
//
// Simple variant of RCU whose quiescent states are voluntary context
// switch, cond_resched_tasks_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.
//
// The implementation uses rcu_tasks_wait_gp(), which relies on function
// pointers in the rcu_tasks structure.  The rcu_spawn_tasks_kthread()
// function sets these function pointers up so that rcu_tasks_wait_gp()
// invokes these functions in this order:
//
// rcu_tasks_pregp_step():
//      Invokes synchronize_rcu() in order to wait for all in-flight
//      t->on_rq and t->nvcsw transitions to complete.  This works because
//      all such transitions are carried out with interrupts disabled.
// rcu_tasks_pertask(), invoked on every non-idle task:
//      For every runnable non-idle task other than the current one, use
//      get_task_struct() to pin down that task, snapshot that task's
//      number of voluntary context switches, and add that task to the
//      holdout list.
// rcu_tasks_postscan():
//      Gather per-CPU lists of tasks in do_exit() to ensure that all
//      tasks that were in the process of exiting (and which thus might
//      not know to synchronize with this RCU Tasks grace period) have
//      completed exiting.  The synchronize_rcu() in rcu_tasks_postgp()
//      will take care of any tasks stuck in the non-preemptible region
//      of do_exit() following its call to exit_tasks_rcu_finish().
// check_all_holdout_tasks(), repeatedly until holdout list is empty:
//      Scans the holdout list, attempting to identify a quiescent state
//      for each task on the list.  If there is a quiescent state, the
//      corresponding task is removed from the holdout list.
// rcu_tasks_postgp():
//      Invokes synchronize_rcu() in order to ensure that all prior
//      t->on_rq and t->nvcsw transitions are seen by all CPUs and tasks
//      to have happened before the end of this RCU Tasks grace period.
//      Again, this works because all such transitions are carried out
//      with interrupts disabled.
//
// For each exiting task, the exit_tasks_rcu_start() and
// exit_tasks_rcu_finish() functions add and remove, respectively, the
// current task to a per-CPU list of tasks that rcu_tasks_postscan() must
// wait on.  This is necessary because rcu_tasks_postscan() must wait on
// tasks that have already been removed from the global list of tasks.
//
// Pre-grace-period update-side code is ordered before the grace
// via the raw_spin_lock.*rcu_node().  Pre-grace-period read-side code
// is ordered before the grace period via synchronize_rcu() call in
// rcu_tasks_pregp_step() and by the scheduler's locks and interrupt
// disabling.

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

/* Check for quiescent states since the pregp's synchronize_rcu() */
static bool rcu_tasks_is_holdout(struct task_struct *t)
{
        int cpu;

        /* Has the task been seen voluntarily sleeping? */
        if (!READ_ONCE(t->on_rq))
                return false;

        /*
         * t->on_rq && !t->se.sched_delayed *could* be considered sleeping but
         * since it is a spurious state (it will transition into the
         * traditional blocked state or get woken up without outside
         * dependencies), not considering it such should only affect timing.
         *
         * Be conservative for now and not include it.
         */

        /*
         * Idle tasks (or idle injection) within the idle loop are RCU-tasks
         * quiescent states. But CPU boot code performed by the idle task
         * isn't a quiescent state.
         */
        if (is_idle_task(t))
                return false;

        cpu = task_cpu(t);

        /* Idle tasks on offline CPUs are RCU-tasks quiescent states. */
        if (t == idle_task(cpu) && !rcu_cpu_online(cpu))
                return false;

        return true;
}

/* Per-task initial processing. */
static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
{
        if (t != current && rcu_tasks_is_holdout(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);
        }
}

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");

/* Processing between scanning taskslist and draining the holdout list. */
static void rcu_tasks_postscan(struct list_head *hop)
{
        int cpu;
        int rtsi = READ_ONCE(rcu_task_stall_info);

        if (!IS_ENABLED(CONFIG_TINY_RCU)) {
                tasks_rcu_exit_srcu_stall_timer.expires = jiffies + rtsi;
                add_timer(&tasks_rcu_exit_srcu_stall_timer);
        }

        /*
         * Exiting tasks may escape the tasklist scan. Those are vulnerable
         * until their final schedule() with TASK_DEAD state. To cope with
         * this, divide the fragile exit path part in two intersecting
         * read side critical sections:
         *
         * 1) A task_struct list addition before calling exit_notify(),
         *    which may remove the task from the tasklist, with the
         *    removal after the final preempt_disable() call in do_exit().
         *
         * 2) An _RCU_ read side starting with the final preempt_disable()
         *    call in do_exit() and ending with the final call to schedule()
         *    with TASK_DEAD state.
         *
         * This handles the part 1). And postgp will handle part 2) with a
         * call to synchronize_rcu().
         */

        for_each_possible_cpu(cpu) {
                unsigned long j = jiffies + 1;
                struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, cpu);
                struct task_struct *t;
                struct task_struct *t1;
                struct list_head tmp;

                raw_spin_lock_irq_rcu_node(rtpcp);
                list_for_each_entry_safe(t, t1, &rtpcp->rtp_exit_list, rcu_tasks_exit_list) {
                        if (list_empty(&t->rcu_tasks_holdout_list))
                                rcu_tasks_pertask(t, hop);

                        // RT kernels need frequent pauses, otherwise
                        // pause at least once per pair of jiffies.
                        if (!IS_ENABLED(CONFIG_PREEMPT_RT) && time_before(jiffies, j))
                                continue;

                        // Keep our place in the list while pausing.
                        // Nothing else traverses this list, so adding a
                        // bare list_head is OK.
                        list_add(&tmp, &t->rcu_tasks_exit_list);
                        raw_spin_unlock_irq_rcu_node(rtpcp);
                        cond_resched(); // For CONFIG_PREEMPT=n kernels
                        raw_spin_lock_irq_rcu_node(rtpcp);
                        t1 = list_entry(tmp.next, struct task_struct, rcu_tasks_exit_list);
                        list_del(&tmp);
                        j = jiffies + 1;
                }
                raw_spin_unlock_irq_rcu_node(rtpcp);
        }

        if (!IS_ENABLED(CONFIG_TINY_RCU))
                timer_delete_sync(&tasks_rcu_exit_srcu_stall_timer);
}

/* 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) ||
            !rcu_tasks_is_holdout(t) ||
            (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
             !is_idle_task(t) && READ_ONCE(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,
                 data_race(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,
         * enforcing the whole region before tasklist removal until
         * the final schedule() with TASK_DEAD state to be an RCU TASKS
         * read side critical section.
         */
        synchronize_rcu();
}

static void tasks_rcu_exit_srcu_stall(struct timer_list *unused)
{
#ifndef CONFIG_TINY_RCU
        int rtsi;

        rtsi = READ_ONCE(rcu_task_stall_info);
        pr_info("%s: %s grace period number %lu (since boot) gp_state: %s is %lu jiffies old.\n",
                __func__, rcu_tasks.kname, rcu_tasks.tasks_gp_seq,
                tasks_gp_state_getname(&rcu_tasks), jiffies - rcu_tasks.gp_jiffies);
        pr_info("Please check any exiting tasks stuck between calls to exit_tasks_rcu_start() and exit_tasks_rcu_finish()\n");
        tasks_rcu_exit_srcu_stall_timer.expires = jiffies + rtsi;
        add_timer(&tasks_rcu_exit_srcu_stall_timer);
#endif // #ifndef CONFIG_TINY_RCU
}

/**
 * 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_tasks_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-structure 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)
{
        rcu_barrier_tasks_generic(&rcu_tasks);
}
EXPORT_SYMBOL_GPL(rcu_barrier_tasks);

static int rcu_tasks_lazy_ms = -1;
module_param(rcu_tasks_lazy_ms, int, 0444);

static int __init rcu_spawn_tasks_kthread(void)
{
        rcu_tasks.gp_sleep = HZ / 10;
        rcu_tasks.init_fract = HZ / 10;
        if (rcu_tasks_lazy_ms >= 0)
                rcu_tasks.lazy_jiffies = msecs_to_jiffies(rcu_tasks_lazy_ms);
        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_tasks.wait_state = TASK_IDLE;
        rcu_spawn_tasks_kthread_generic(&rcu_tasks);
        return 0;
}

#if !defined(CONFIG_TINY_RCU)
void show_rcu_tasks_classic_gp_kthread(void)
{
        show_rcu_tasks_generic_gp_kthread(&rcu_tasks, "");
}
EXPORT_SYMBOL_GPL(show_rcu_tasks_classic_gp_kthread);

void rcu_tasks_torture_stats_print(char *tt, char *tf)
{
        rcu_tasks_torture_stats_print_generic(&rcu_tasks, tt, tf, "");
}
EXPORT_SYMBOL_GPL(rcu_tasks_torture_stats_print);
#endif // !defined(CONFIG_TINY_RCU)

struct task_struct *get_rcu_tasks_gp_kthread(void)
{
        return rcu_tasks.kthread_ptr;
}
EXPORT_SYMBOL_GPL(get_rcu_tasks_gp_kthread);

void rcu_tasks_get_gp_data(int *flags, unsigned long *gp_seq)
{
        *flags = 0;
        *gp_seq = rcu_seq_current(&rcu_tasks.tasks_gp_seq);
}
EXPORT_SYMBOL_GPL(rcu_tasks_get_gp_data);

/*
 * Protect against tasklist scan blind spot while the task is exiting and
 * may be removed from the tasklist.  Do this by adding the task to yet
 * another list.
 *
 * Note that the task will remove itself from this list, so there is no
 * need for get_task_struct(), except in the case where rcu_tasks_pertask()
 * adds it to the holdout list, in which case rcu_tasks_pertask() supplies
 * the needed get_task_struct().
 */
void exit_tasks_rcu_start(void)
{
        unsigned long flags;
        struct rcu_tasks_percpu *rtpcp;
        struct task_struct *t = current;

        WARN_ON_ONCE(!list_empty(&t->rcu_tasks_exit_list));
        preempt_disable();
        rtpcp = this_cpu_ptr(rcu_tasks.rtpcpu);
        t->rcu_tasks_exit_cpu = smp_processor_id();
        raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
        WARN_ON_ONCE(!rtpcp->rtp_exit_list.next);
        list_add(&t->rcu_tasks_exit_list, &rtpcp->rtp_exit_list);
        raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
        preempt_enable();
}

/*
 * Remove the task from the "yet another list" because do_exit() is now
 * non-preemptible, allowing synchronize_rcu() to wait beyond this point.
 */
void exit_tasks_rcu_finish(void)
{
        unsigned long flags;
        struct rcu_tasks_percpu *rtpcp;
        struct task_struct *t = current;

        WARN_ON_ONCE(list_empty(&t->rcu_tasks_exit_list));
        rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, t->rcu_tasks_exit_cpu);
        raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
        list_del_init(&t->rcu_tasks_exit_list);
        raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
}

#else /* #ifdef CONFIG_TASKS_RCU */
void exit_tasks_rcu_start(void) { }
void exit_tasks_rcu_finish(void) { }
#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 batching of concurrent calls to the synchronous
// synchronize_rcu_tasks_rude() API.  This invokes schedule_on_each_cpu()
// in order to send IPIs far and wide and induces otherwise unnecessary
// context switches on all online CPUs, whether idle or not.
//
// Callback handling is provided by the rcu_tasks_kthread() function.
//
// Ordering is provided by the scheduler's context-switch code.

// 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);
}

static 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_tasks_rcu_qs(), or transition to usermode execution (as
 * usermode execution is schedulable). 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-structure synchronization.
 *
 * See the description of call_rcu() for more detailed information on
 * memory ordering guarantees.
 *
 * This is no longer exported, and is instead reserved for use by
 * synchronize_rcu_tasks_rude().
 */
static void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func)
{
        call_rcu_tasks_generic(rhp, func, &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 (which is a schedulable
 * context), 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)
{
        if (!IS_ENABLED(CONFIG_ARCH_WANTS_NO_INSTR) || IS_ENABLED(CONFIG_FORCE_TASKS_RUDE_RCU))
                synchronize_rcu_tasks_generic(&rcu_tasks_rude);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_rude);

static int __init rcu_spawn_tasks_rude_kthread(void)
{
        rcu_tasks_rude.gp_sleep = HZ / 10;
        rcu_spawn_tasks_kthread_generic(&rcu_tasks_rude);
        return 0;
}

#if !defined(CONFIG_TINY_RCU)
void show_rcu_tasks_rude_gp_kthread(void)
{
        show_rcu_tasks_generic_gp_kthread(&rcu_tasks_rude, "");
}
EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread);

void rcu_tasks_rude_torture_stats_print(char *tt, char *tf)
{
        rcu_tasks_torture_stats_print_generic(&rcu_tasks_rude, tt, tf, "");
}
EXPORT_SYMBOL_GPL(rcu_tasks_rude_torture_stats_print);
#endif // !defined(CONFIG_TINY_RCU)

struct task_struct *get_rcu_tasks_rude_gp_kthread(void)
{
        return rcu_tasks_rude.kthread_ptr;
}
EXPORT_SYMBOL_GPL(get_rcu_tasks_rude_gp_kthread);

void rcu_tasks_rude_get_gp_data(int *flags, unsigned long *gp_seq)
{
        *flags = 0;
        *gp_seq = rcu_seq_current(&rcu_tasks_rude.tasks_gp_seq);
}
EXPORT_SYMBOL_GPL(rcu_tasks_rude_get_gp_data);

#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */

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

#ifdef CONFIG_PROVE_RCU
struct rcu_tasks_test_desc {
        struct rcu_head rh;
        const char *name;
        bool notrun;
        unsigned long runstart;
};

static struct rcu_tasks_test_desc tests[] = {
        {
                .name = "call_rcu_tasks()",
                /* If not defined, the test is skipped. */
                .notrun = IS_ENABLED(CONFIG_TASKS_RCU),
        },
        {
                .name = "call_rcu_tasks_trace()",
                /* If not defined, the test is skipped. */
                .notrun = IS_ENABLED(CONFIG_TASKS_TRACE_RCU)
        }
};

#if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)
static void test_rcu_tasks_callback(struct rcu_head *rhp)
{
        struct rcu_tasks_test_desc *rttd =
                container_of(rhp, struct rcu_tasks_test_desc, rh);

        pr_info("Callback from %s invoked.\n", rttd->name);

        rttd->notrun = false;
}
#endif // #if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)

static void rcu_tasks_initiate_self_tests(void)
{
#ifdef CONFIG_TASKS_RCU
        pr_info("Running RCU Tasks wait API self tests\n");
        tests[0].runstart = jiffies;
        synchronize_rcu_tasks();
        call_rcu_tasks(&tests[0].rh, test_rcu_tasks_callback);
#endif

#ifdef CONFIG_TASKS_RUDE_RCU
        pr_info("Running RCU Tasks Rude wait API self tests\n");
        synchronize_rcu_tasks_rude();
#endif

#ifdef CONFIG_TASKS_TRACE_RCU
        pr_info("Running RCU Tasks Trace wait API self tests\n");
        tests[1].runstart = jiffies;
        synchronize_rcu_tasks_trace();
        call_rcu_tasks_trace(&tests[1].rh, test_rcu_tasks_callback);
#endif
}

/*
 * Return:  0 - test passed
 *          1 - test failed, but have not timed out yet
 *         -1 - test failed and timed out
 */
static int rcu_tasks_verify_self_tests(void)
{
        int ret = 0;
        int i;
        unsigned long bst = rcu_task_stall_timeout;

        if (bst <= 0 || bst > RCU_TASK_BOOT_STALL_TIMEOUT)
                bst = RCU_TASK_BOOT_STALL_TIMEOUT;
        for (i = 0; i < ARRAY_SIZE(tests); i++) {
                while (tests[i].notrun) {               // still hanging.
                        if (time_after(jiffies, tests[i].runstart + bst)) {
                                pr_err("%s has failed boot-time tests.\n", tests[i].name);
                                ret = -1;
                                break;
                        }
                        ret = 1;
                        break;
                }
        }
        WARN_ON(ret < 0);

        return ret;
}

/*
 * Repeat the rcu_tasks_verify_self_tests() call once every second until the
 * test passes or has timed out.
 */
static struct delayed_work rcu_tasks_verify_work;
static void rcu_tasks_verify_work_fn(struct work_struct *work __maybe_unused)
{
        int ret = rcu_tasks_verify_self_tests();

        if (ret <= 0)
                return;

        /* Test fails but not timed out yet, reschedule another check */
        schedule_delayed_work(&rcu_tasks_verify_work, HZ);
}

static int rcu_tasks_verify_schedule_work(void)
{
        INIT_DELAYED_WORK(&rcu_tasks_verify_work, rcu_tasks_verify_work_fn);
        rcu_tasks_verify_work_fn(NULL);
        return 0;
}
late_initcall(rcu_tasks_verify_schedule_work);
#else /* #ifdef CONFIG_PROVE_RCU */
static void rcu_tasks_initiate_self_tests(void) { }
#endif /* #else #ifdef CONFIG_PROVE_RCU */

void __init tasks_cblist_init_generic(void)
{
        lockdep_assert_irqs_disabled();
        WARN_ON(num_online_cpus() > 1);

#ifdef CONFIG_TASKS_RCU
        cblist_init_generic(&rcu_tasks);
#endif

#ifdef CONFIG_TASKS_RUDE_RCU
        cblist_init_generic(&rcu_tasks_rude);
#endif
}

static int __init rcu_init_tasks_generic(void)
{
#ifdef CONFIG_TASKS_RCU
        rcu_spawn_tasks_kthread();
#endif

#ifdef CONFIG_TASKS_RUDE_RCU
        rcu_spawn_tasks_rude_kthread();
#endif

        // Run the self-tests.
        rcu_tasks_initiate_self_tests();

        return 0;
}
core_initcall(rcu_init_tasks_generic);

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

#ifdef CONFIG_TASKS_TRACE_RCU

////////////////////////////////////////////////////////////////////////
//
// Tracing variant of Tasks RCU.  This variant is designed to be used
// to protect tracing hooks, including those of BPF.  This variant
// is implemented via a straightforward mapping onto SRCU-fast.

DEFINE_SRCU_FAST(rcu_tasks_trace_srcu_struct);
EXPORT_SYMBOL_GPL(rcu_tasks_trace_srcu_struct);

#endif /* #else #ifdef CONFIG_TASKS_TRACE_RCU */