/* SPDX-License-Identifier: GPL-2.0+ */
/*
 * Read-Copy Update mechanism for mutual exclusion (tree-based version)
 * Internal non-public definitions that provide either classic
 * or preemptible semantics.
 *
 * Copyright Red Hat, 2009
 * Copyright IBM Corporation, 2009
 *
 * Author: Ingo Molnar <mingo@elte.hu>
 *         Paul E. McKenney <paulmck@linux.ibm.com>
 */

#include "../locking/rtmutex_common.h"

static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
{
        /*
         * In order to read the offloaded state of an rdp in a safe
         * and stable way and prevent from its value to be changed
         * under us, we must either hold the barrier mutex, the cpu
         * hotplug lock (read or write) or the nocb lock. Local
         * non-preemptible reads are also safe. NOCB kthreads and
         * timers have their own means of synchronization against the
         * offloaded state updaters.
         */
        RCU_NOCB_LOCKDEP_WARN(
                !(lockdep_is_held(&rcu_state.barrier_mutex) ||
                  (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) ||
                  lockdep_is_held(&rdp->nocb_lock) ||
                  lockdep_is_held(&rcu_state.nocb_mutex) ||
                  ((!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) || softirq_count()) &&
                   rdp == this_cpu_ptr(&rcu_data)) ||
                  rcu_current_is_nocb_kthread(rdp)),
                "Unsafe read of RCU_NOCB offloaded state"
        );

        return rcu_segcblist_is_offloaded(&rdp->cblist);
}

/*
 * Check the RCU kernel configuration parameters and print informative
 * messages about anything out of the ordinary.
 */
static void __init rcu_bootup_announce_oddness(void)
{
        if (IS_ENABLED(CONFIG_RCU_TRACE))
                pr_info("\tRCU event tracing is enabled.\n");
        if ((IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 64) ||
            (!IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 32))
                pr_info("\tCONFIG_RCU_FANOUT set to non-default value of %d.\n",
                        RCU_FANOUT);
        if (rcu_fanout_exact)
                pr_info("\tHierarchical RCU autobalancing is disabled.\n");
        if (IS_ENABLED(CONFIG_PROVE_RCU))
                pr_info("\tRCU lockdep checking is enabled.\n");
        if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
                pr_info("\tRCU strict (and thus non-scalable) grace periods are enabled.\n");
        if (RCU_NUM_LVLS >= 4)
                pr_info("\tFour(or more)-level hierarchy is enabled.\n");
        if (RCU_FANOUT_LEAF != 16)
                pr_info("\tBuild-time adjustment of leaf fanout to %d.\n",
                        RCU_FANOUT_LEAF);
        if (rcu_fanout_leaf != RCU_FANOUT_LEAF)
                pr_info("\tBoot-time adjustment of leaf fanout to %d.\n",
                        rcu_fanout_leaf);
        if (nr_cpu_ids != NR_CPUS)
                pr_info("\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%u.\n", NR_CPUS, nr_cpu_ids);
#ifdef CONFIG_RCU_BOOST
        pr_info("\tRCU priority boosting: priority %d delay %d ms.\n",
                kthread_prio, CONFIG_RCU_BOOST_DELAY);
#endif
        if (blimit != DEFAULT_RCU_BLIMIT)
                pr_info("\tBoot-time adjustment of callback invocation limit to %ld.\n", blimit);
        if (qhimark != DEFAULT_RCU_QHIMARK)
                pr_info("\tBoot-time adjustment of callback high-water mark to %ld.\n", qhimark);
        if (qlowmark != DEFAULT_RCU_QLOMARK)
                pr_info("\tBoot-time adjustment of callback low-water mark to %ld.\n", qlowmark);
        if (qovld != DEFAULT_RCU_QOVLD)
                pr_info("\tBoot-time adjustment of callback overload level to %ld.\n", qovld);
        if (jiffies_till_first_fqs != ULONG_MAX)
                pr_info("\tBoot-time adjustment of first FQS scan delay to %ld jiffies.\n", jiffies_till_first_fqs);
        if (jiffies_till_next_fqs != ULONG_MAX)
                pr_info("\tBoot-time adjustment of subsequent FQS scan delay to %ld jiffies.\n", jiffies_till_next_fqs);
        if (jiffies_till_sched_qs != ULONG_MAX)
                pr_info("\tBoot-time adjustment of scheduler-enlistment delay to %ld jiffies.\n", jiffies_till_sched_qs);
        if (rcu_kick_kthreads)
                pr_info("\tKick kthreads if too-long grace period.\n");
        if (IS_ENABLED(CONFIG_DEBUG_OBJECTS_RCU_HEAD))
                pr_info("\tRCU callback double-/use-after-free debug is enabled.\n");
        if (gp_preinit_delay)
                pr_info("\tRCU debug GP pre-init slowdown %d jiffies.\n", gp_preinit_delay);
        if (gp_init_delay)
                pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_init_delay);
        if (gp_cleanup_delay)
                pr_info("\tRCU debug GP cleanup slowdown %d jiffies.\n", gp_cleanup_delay);
        if (nohz_full_patience_delay < 0) {
                pr_info("\tRCU NOCB CPU patience negative (%d), resetting to zero.\n", nohz_full_patience_delay);
                nohz_full_patience_delay = 0;
        } else if (nohz_full_patience_delay > 5 * MSEC_PER_SEC) {
                pr_info("\tRCU NOCB CPU patience too large (%d), resetting to %ld.\n", nohz_full_patience_delay, 5 * MSEC_PER_SEC);
                nohz_full_patience_delay = 5 * MSEC_PER_SEC;
        } else if (nohz_full_patience_delay) {
                pr_info("\tRCU NOCB CPU patience set to %d milliseconds.\n", nohz_full_patience_delay);
        }
        nohz_full_patience_delay_jiffies = msecs_to_jiffies(nohz_full_patience_delay);
        if (!use_softirq)
                pr_info("\tRCU_SOFTIRQ processing moved to rcuc kthreads.\n");
        if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG))
                pr_info("\tRCU debug extended QS entry/exit.\n");
        rcupdate_announce_bootup_oddness();
}

#ifdef CONFIG_PREEMPT_RCU

static void rcu_report_exp_rnp(struct rcu_node *rnp, bool wake);
static void rcu_read_unlock_special(struct task_struct *t);

/*
 * Tell them what RCU they are running.
 */
static void __init rcu_bootup_announce(void)
{
        pr_info("Preemptible hierarchical RCU implementation.\n");
        rcu_bootup_announce_oddness();
}

/* Flags for rcu_preempt_ctxt_queue() decision table. */
#define RCU_GP_TASKS    0x8
#define RCU_EXP_TASKS   0x4
#define RCU_GP_BLKD     0x2
#define RCU_EXP_BLKD    0x1

/*
 * Queues a task preempted within an RCU-preempt read-side critical
 * section into the appropriate location within the ->blkd_tasks list,
 * depending on the states of any ongoing normal and expedited grace
 * periods.  The ->gp_tasks pointer indicates which element the normal
 * grace period is waiting on (NULL if none), and the ->exp_tasks pointer
 * indicates which element the expedited grace period is waiting on (again,
 * NULL if none).  If a grace period is waiting on a given element in the
 * ->blkd_tasks list, it also waits on all subsequent elements.  Thus,
 * adding a task to the tail of the list blocks any grace period that is
 * already waiting on one of the elements.  In contrast, adding a task
 * to the head of the list won't block any grace period that is already
 * waiting on one of the elements.
 *
 * This queuing is imprecise, and can sometimes make an ongoing grace
 * period wait for a task that is not strictly speaking blocking it.
 * Given the choice, we needlessly block a normal grace period rather than
 * blocking an expedited grace period.
 *
 * Note that an endless sequence of expedited grace periods still cannot
 * indefinitely postpone a normal grace period.  Eventually, all of the
 * fixed number of preempted tasks blocking the normal grace period that are
 * not also blocking the expedited grace period will resume and complete
 * their RCU read-side critical sections.  At that point, the ->gp_tasks
 * pointer will equal the ->exp_tasks pointer, at which point the end of
 * the corresponding expedited grace period will also be the end of the
 * normal grace period.
 */
static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
        __releases(rnp->lock) /* But leaves rrupts disabled. */
{
        int blkd_state = (rnp->gp_tasks ? RCU_GP_TASKS : 0) +
                         (rnp->exp_tasks ? RCU_EXP_TASKS : 0) +
                         (rnp->qsmask & rdp->grpmask ? RCU_GP_BLKD : 0) +
                         (rnp->expmask & rdp->grpmask ? RCU_EXP_BLKD : 0);
        struct task_struct *t = current;

        raw_lockdep_assert_held_rcu_node(rnp);
        WARN_ON_ONCE(rdp->mynode != rnp);
        WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
        /* RCU better not be waiting on newly onlined CPUs! */
        WARN_ON_ONCE(rnp->qsmaskinitnext & ~rnp->qsmaskinit & rnp->qsmask &
                     rdp->grpmask);

        /*
         * Decide where to queue the newly blocked task.  In theory,
         * this could be an if-statement.  In practice, when I tried
         * that, it was quite messy.
         */
        switch (blkd_state) {
        case 0:
        case                RCU_EXP_TASKS:
        case                RCU_EXP_TASKS | RCU_GP_BLKD:
        case RCU_GP_TASKS:
        case RCU_GP_TASKS | RCU_EXP_TASKS:

                /*
                 * Blocking neither GP, or first task blocking the normal
                 * GP but not blocking the already-waiting expedited GP.
                 * Queue at the head of the list to avoid unnecessarily
                 * blocking the already-waiting GPs.
                 */
                list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
                break;

        case                                              RCU_EXP_BLKD:
        case                                RCU_GP_BLKD:
        case                                RCU_GP_BLKD | RCU_EXP_BLKD:
        case RCU_GP_TASKS |                               RCU_EXP_BLKD:
        case RCU_GP_TASKS |                 RCU_GP_BLKD | RCU_EXP_BLKD:
        case RCU_GP_TASKS | RCU_EXP_TASKS | RCU_GP_BLKD | RCU_EXP_BLKD:

                /*
                 * First task arriving that blocks either GP, or first task
                 * arriving that blocks the expedited GP (with the normal
                 * GP already waiting), or a task arriving that blocks
                 * both GPs with both GPs already waiting.  Queue at the
                 * tail of the list to avoid any GP waiting on any of the
                 * already queued tasks that are not blocking it.
                 */
                list_add_tail(&t->rcu_node_entry, &rnp->blkd_tasks);
                break;

        case                RCU_EXP_TASKS |               RCU_EXP_BLKD:
        case                RCU_EXP_TASKS | RCU_GP_BLKD | RCU_EXP_BLKD:
        case RCU_GP_TASKS | RCU_EXP_TASKS |               RCU_EXP_BLKD:

                /*
                 * Second or subsequent task blocking the expedited GP.
                 * The task either does not block the normal GP, or is the
                 * first task blocking the normal GP.  Queue just after
                 * the first task blocking the expedited GP.
                 */
                list_add(&t->rcu_node_entry, rnp->exp_tasks);
                break;

        case RCU_GP_TASKS |                 RCU_GP_BLKD:
        case RCU_GP_TASKS | RCU_EXP_TASKS | RCU_GP_BLKD:

                /*
                 * Second or subsequent task blocking the normal GP.
                 * The task does not block the expedited GP. Queue just
                 * after the first task blocking the normal GP.
                 */
                list_add(&t->rcu_node_entry, rnp->gp_tasks);
                break;

        default:

                /* Yet another exercise in excessive paranoia. */
                WARN_ON_ONCE(1);
                break;
        }

        /*
         * We have now queued the task.  If it was the first one to
         * block either grace period, update the ->gp_tasks and/or
         * ->exp_tasks pointers, respectively, to reference the newly
         * blocked tasks.
         */
        if (!rnp->gp_tasks && (blkd_state & RCU_GP_BLKD)) {
                WRITE_ONCE(rnp->gp_tasks, &t->rcu_node_entry);
                WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq);
        }
        if (!rnp->exp_tasks && (blkd_state & RCU_EXP_BLKD))
                WRITE_ONCE(rnp->exp_tasks, &t->rcu_node_entry);
        WARN_ON_ONCE(!(blkd_state & RCU_GP_BLKD) !=
                     !(rnp->qsmask & rdp->grpmask));
        WARN_ON_ONCE(!(blkd_state & RCU_EXP_BLKD) !=
                     !(rnp->expmask & rdp->grpmask));
        raw_spin_unlock_rcu_node(rnp); /* interrupts remain disabled. */

        /*
         * Report the quiescent state for the expedited GP.  This expedited
         * GP should not be able to end until we report, so there should be
         * no need to check for a subsequent expedited GP.  (Though we are
         * still in a quiescent state in any case.)
         *
         * Interrupts are disabled, so ->cpu_no_qs.b.exp cannot change.
         */
        if (blkd_state & RCU_EXP_BLKD && rdp->cpu_no_qs.b.exp)
                rcu_report_exp_rdp(rdp);
        else
                WARN_ON_ONCE(rdp->cpu_no_qs.b.exp);
        ASSERT_EXCLUSIVE_WRITER_SCOPED(rdp->cpu_no_qs.b.exp);
}

/*
 * Record a preemptible-RCU quiescent state for the specified CPU.
 * Note that this does not necessarily mean that the task currently running
 * on the CPU is in a quiescent state:  Instead, it means that the current
 * grace period need not wait on any RCU read-side critical section that
 * starts later on this CPU.  It also means that if the current task is
 * in an RCU read-side critical section, it has already added itself to
 * some leaf rcu_node structure's ->blkd_tasks list.  In addition to the
 * current task, there might be any number of other tasks blocked while
 * in an RCU read-side critical section.
 *
 * Unlike non-preemptible-RCU, quiescent state reports for expedited
 * grace periods are handled separately via deferred quiescent states
 * and context switch events.
 *
 * Callers to this function must disable preemption.
 */
static void rcu_qs(void)
{
        RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!\n");
        if (__this_cpu_read(rcu_data.cpu_no_qs.b.norm)) {
                trace_rcu_grace_period(TPS("rcu_preempt"),
                                       __this_cpu_read(rcu_data.gp_seq),
                                       TPS("cpuqs"));
                __this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
                barrier(); /* Coordinate with rcu_flavor_sched_clock_irq(). */
                WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, false);
        }
}

/*
 * We have entered the scheduler, and the current task might soon be
 * context-switched away from.  If this task is in an RCU read-side
 * critical section, we will no longer be able to rely on the CPU to
 * record that fact, so we enqueue the task on the blkd_tasks list.
 * The task will dequeue itself when it exits the outermost enclosing
 * RCU read-side critical section.  Therefore, the current grace period
 * cannot be permitted to complete until the blkd_tasks list entries
 * predating the current grace period drain, in other words, until
 * rnp->gp_tasks becomes NULL.
 *
 * Caller must disable interrupts.
 */
void rcu_note_context_switch(bool preempt)
{
        struct task_struct *t = current;
        struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
        struct rcu_node *rnp;

        trace_rcu_utilization(TPS("Start context switch"));
        lockdep_assert_irqs_disabled();
        WARN_ONCE(!preempt && rcu_preempt_depth() > 0, "Voluntary context switch within RCU read-side critical section!");
        if (rcu_preempt_depth() > 0 &&
            !t->rcu_read_unlock_special.b.blocked) {

                /* Possibly blocking in an RCU read-side critical section. */
                rnp = rdp->mynode;
                raw_spin_lock_rcu_node(rnp);
                t->rcu_read_unlock_special.b.blocked = true;
                t->rcu_blocked_node = rnp;

                /*
                 * Verify the CPU's sanity, trace the preemption, and
                 * then queue the task as required based on the states
                 * of any ongoing and expedited grace periods.
                 */
                WARN_ON_ONCE(!rcu_rdp_cpu_online(rdp));
                WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
                trace_rcu_preempt_task(rcu_state.name,
                                       t->pid,
                                       (rnp->qsmask & rdp->grpmask)
                                       ? rnp->gp_seq
                                       : rcu_seq_snap(&rnp->gp_seq));
                rcu_preempt_ctxt_queue(rnp, rdp);
        } else {
                rcu_preempt_deferred_qs(t);
        }

        /*
         * Either we were not in an RCU read-side critical section to
         * begin with, or we have now recorded that critical section
         * globally.  Either way, we can now note a quiescent state
         * for this CPU.  Again, if we were in an RCU read-side critical
         * section, and if that critical section was blocking the current
         * grace period, then the fact that the task has been enqueued
         * means that we continue to block the current grace period.
         */
        rcu_qs();
        if (rdp->cpu_no_qs.b.exp)
                rcu_report_exp_rdp(rdp);
        rcu_tasks_qs(current, preempt);
        trace_rcu_utilization(TPS("End context switch"));
}
EXPORT_SYMBOL_GPL(rcu_note_context_switch);

/*
 * Check for preempted RCU readers blocking the current grace period
 * for the specified rcu_node structure.  If the caller needs a reliable
 * answer, it must hold the rcu_node's ->lock.
 */
static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
{
        return READ_ONCE(rnp->gp_tasks) != NULL;
}

/* limit value for ->rcu_read_lock_nesting. */
#define RCU_NEST_PMAX (INT_MAX / 2)

static void rcu_preempt_read_enter(void)
{
        WRITE_ONCE(current->rcu_read_lock_nesting, READ_ONCE(current->rcu_read_lock_nesting) + 1);
}

static int rcu_preempt_read_exit(void)
{
        int ret = READ_ONCE(current->rcu_read_lock_nesting) - 1;

        WRITE_ONCE(current->rcu_read_lock_nesting, ret);
        return ret;
}

static void rcu_preempt_depth_set(int val)
{
        WRITE_ONCE(current->rcu_read_lock_nesting, val);
}

/*
 * Preemptible RCU implementation for rcu_read_lock().
 * Just increment ->rcu_read_lock_nesting, shared state will be updated
 * if we block.
 */
void __rcu_read_lock(void)
{
        rcu_preempt_read_enter();
        if (IS_ENABLED(CONFIG_PROVE_LOCKING))
                WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX);
        if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
                WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);
        barrier();  /* critical section after entry code. */
}
EXPORT_SYMBOL_GPL(__rcu_read_lock);

/*
 * Preemptible RCU implementation for rcu_read_unlock().
 * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
 * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
 * invoke rcu_read_unlock_special() to clean up after a context switch
 * in an RCU read-side critical section and other special cases.
 */
void __rcu_read_unlock(void)
{
        struct task_struct *t = current;

        barrier();  // critical section before exit code.
        if (rcu_preempt_read_exit() == 0) {
                barrier();  // critical-section exit before .s check.
                if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
                        rcu_read_unlock_special(t);
        }
        if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
                int rrln = rcu_preempt_depth();

                WARN_ON_ONCE(rrln < 0 || rrln > RCU_NEST_PMAX);
        }
}
EXPORT_SYMBOL_GPL(__rcu_read_unlock);

/*
 * Advance a ->blkd_tasks-list pointer to the next entry, instead
 * returning NULL if at the end of the list.
 */
static struct list_head *rcu_next_node_entry(struct task_struct *t,
                                             struct rcu_node *rnp)
{
        struct list_head *np;

        np = t->rcu_node_entry.next;
        if (np == &rnp->blkd_tasks)
                np = NULL;
        return np;
}

/*
 * Return true if the specified rcu_node structure has tasks that were
 * preempted within an RCU read-side critical section.
 */
static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
{
        return !list_empty(&rnp->blkd_tasks);
}

/*
 * Report deferred quiescent states.  The deferral time can
 * be quite short, for example, in the case of the call from
 * rcu_read_unlock_special().
 */
static notrace void
rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
{
        bool empty_exp;
        bool empty_norm;
        bool empty_exp_now;
        struct list_head *np;
        bool drop_boost_mutex = false;
        struct rcu_data *rdp;
        struct rcu_node *rnp;
        union rcu_special special;

        rdp = this_cpu_ptr(&rcu_data);
        if (rdp->defer_qs_pending == DEFER_QS_PENDING)
                rdp->defer_qs_pending = DEFER_QS_IDLE;

        /*
         * If RCU core is waiting for this CPU to exit its critical section,
         * report the fact that it has exited.  Because irqs are disabled,
         * t->rcu_read_unlock_special cannot change.
         */
        special = t->rcu_read_unlock_special;
        if (!special.s && !rdp->cpu_no_qs.b.exp) {
                local_irq_restore(flags);
                return;
        }
        t->rcu_read_unlock_special.s = 0;
        if (special.b.need_qs) {
                if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) {
                        rdp->cpu_no_qs.b.norm = false;
                        rcu_report_qs_rdp(rdp);
                        udelay(rcu_unlock_delay);
                } else {
                        rcu_qs();
                }
        }

        /*
         * Respond to a request by an expedited grace period for a
         * quiescent state from this CPU.  Note that requests from
         * tasks are handled when removing the task from the
         * blocked-tasks list below.
         */
        if (rdp->cpu_no_qs.b.exp)
                rcu_report_exp_rdp(rdp);

        /* Clean up if blocked during RCU read-side critical section. */
        if (special.b.blocked) {

                /*
                 * Remove this task from the list it blocked on.  The task
                 * now remains queued on the rcu_node corresponding to the
                 * CPU it first blocked on, so there is no longer any need
                 * to loop.  Retain a WARN_ON_ONCE() out of sheer paranoia.
                 */
                rnp = t->rcu_blocked_node;
                raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
                WARN_ON_ONCE(rnp != t->rcu_blocked_node);
                WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
                empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);
                WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq &&
                             (!empty_norm || rnp->qsmask));
                empty_exp = sync_rcu_exp_done(rnp);
                np = rcu_next_node_entry(t, rnp);
                list_del_init(&t->rcu_node_entry);
                t->rcu_blocked_node = NULL;
                trace_rcu_unlock_preempted_task(TPS("rcu_preempt"),
                                                rnp->gp_seq, t->pid);
                if (&t->rcu_node_entry == rnp->gp_tasks)
                        WRITE_ONCE(rnp->gp_tasks, np);
                if (&t->rcu_node_entry == rnp->exp_tasks)
                        WRITE_ONCE(rnp->exp_tasks, np);
                if (IS_ENABLED(CONFIG_RCU_BOOST)) {
                        /* Snapshot ->boost_mtx ownership w/rnp->lock held. */
                        drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx.rtmutex) == t;
                        if (&t->rcu_node_entry == rnp->boost_tasks)
                                WRITE_ONCE(rnp->boost_tasks, np);
                }

                /*
                 * If this was the last task on the current list, and if
                 * we aren't waiting on any CPUs, report the quiescent state.
                 * Note that rcu_report_unblock_qs_rnp() releases rnp->lock,
                 * so we must take a snapshot of the expedited state.
                 */
                empty_exp_now = sync_rcu_exp_done(rnp);
                if (!empty_norm && !rcu_preempt_blocked_readers_cgp(rnp)) {
                        trace_rcu_quiescent_state_report(TPS("preempt_rcu"),
                                                         rnp->gp_seq,
                                                         0, rnp->qsmask,
                                                         rnp->level,
                                                         rnp->grplo,
                                                         rnp->grphi,
                                                         !!rnp->gp_tasks);
                        rcu_report_unblock_qs_rnp(rnp, flags);
                } else {
                        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                }

                /*
                 * If this was the last task on the expedited lists,
                 * then we need to report up the rcu_node hierarchy.
                 */
                if (!empty_exp && empty_exp_now)
                        rcu_report_exp_rnp(rnp, true);

                /* Unboost if we were boosted. */
                if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
                        rt_mutex_futex_unlock(&rnp->boost_mtx.rtmutex);
        } else {
                local_irq_restore(flags);
        }
}

/*
 * Is a deferred quiescent-state pending, and are we also not in
 * an RCU read-side critical section?  It is the caller's responsibility
 * to ensure it is otherwise safe to report any deferred quiescent
 * states.  The reason for this is that it is safe to report a
 * quiescent state during context switch even though preemption
 * is disabled.  This function cannot be expected to understand these
 * nuances, so the caller must handle them.
 */
static notrace bool rcu_preempt_need_deferred_qs(struct task_struct *t)
{
        return (__this_cpu_read(rcu_data.cpu_no_qs.b.exp) ||
                READ_ONCE(t->rcu_read_unlock_special.s)) &&
               rcu_preempt_depth() == 0;
}

/*
 * Report a deferred quiescent state if needed and safe to do so.
 * As with rcu_preempt_need_deferred_qs(), "safe" involves only
 * not being in an RCU read-side critical section.  The caller must
 * evaluate safety in terms of interrupt, softirq, and preemption
 * disabling.
 */
notrace void rcu_preempt_deferred_qs(struct task_struct *t)
{
        unsigned long flags;

        if (!rcu_preempt_need_deferred_qs(t))
                return;
        local_irq_save(flags);
        rcu_preempt_deferred_qs_irqrestore(t, flags);
}

/*
 * Minimal handler to give the scheduler a chance to re-evaluate.
 */
static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
{
        struct rcu_data *rdp;

        lockdep_assert_irqs_disabled();
        rdp = container_of(iwp, struct rcu_data, defer_qs_iw);

        /*
         * If the IRQ work handler happens to run in the middle of RCU read-side
         * critical section, it could be ineffective in getting the scheduler's
         * attention to report a deferred quiescent state (the whole point of the
         * IRQ work). For this reason, requeue the IRQ work.
         *
         * Basically, we want to avoid following situation:
         * 1. rcu_read_unlock() queues IRQ work (state -> DEFER_QS_PENDING)
         * 2. CPU enters new rcu_read_lock()
         * 3. IRQ work runs but cannot report QS due to rcu_preempt_depth() > 0
         * 4. rcu_read_unlock() does not re-queue work (state still PENDING)
         * 5. Deferred QS reporting does not happen.
         */
        if (rcu_preempt_depth() > 0)
                WRITE_ONCE(rdp->defer_qs_pending, DEFER_QS_IDLE);
}

/*
 * Check if expedited grace period processing during unlock is needed.
 *
 * This function determines whether expedited handling is required based on:
 * 1. Task blocking an expedited grace period (based on a heuristic, could be
 *    false-positive, see below.)
 * 2. CPU participating in an expedited grace period
 * 3. Strict grace period mode requiring expedited handling
 * 4. RCU priority deboosting needs when interrupts were disabled
 *
 * @t: The task being checked
 * @rdp: The per-CPU RCU data
 * @rnp: The RCU node for this CPU
 * @irqs_were_disabled: Whether interrupts were disabled before rcu_read_unlock()
 *
 * Returns true if expedited processing of the rcu_read_unlock() is needed.
 */
static bool rcu_unlock_needs_exp_handling(struct task_struct *t,
                                      struct rcu_data *rdp,
                                      struct rcu_node *rnp,
                                      bool irqs_were_disabled)
{
        /*
         * Check if this task is blocking an expedited grace period. If the
         * task was preempted within an RCU read-side critical section and is
         * on the expedited grace period blockers list (exp_tasks), we need
         * expedited handling to unblock the expedited GP. This is not an exact
         * check because 't' might not be on the exp_tasks list at all - its
         * just a fast heuristic that can be false-positive sometimes.
         */
        if (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks))
                return true;

        /*
         * Check if this CPU is participating in an expedited grace period.
         * The expmask bitmap tracks which CPUs need to check in for the
         * current expedited GP. If our CPU's bit is set, we need expedited
         * handling to help complete the expedited GP.
         */
        if (rdp->grpmask & READ_ONCE(rnp->expmask))
                return true;

        /*
         * In CONFIG_RCU_STRICT_GRACE_PERIOD=y kernels, all grace periods
         * are treated as short for testing purposes even if that means
         * disturbing the system more. Check if either:
         * - This CPU has not yet reported a quiescent state, or
         * - This task was preempted within an RCU critical section
         * In either case, require expedited handling for strict GP mode.
         */
        if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
            ((rdp->grpmask & READ_ONCE(rnp->qsmask)) || t->rcu_blocked_node))
                return true;

        /*
         * RCU priority boosting case: If a task is subject to RCU priority
         * boosting and exits an RCU read-side critical section with interrupts
         * disabled, we need expedited handling to ensure timely deboosting.
         * Without this, a low-priority task could incorrectly run at high
         * real-time priority for an extended period degrading real-time
         * responsiveness. This applies to all CONFIG_RCU_BOOST=y kernels,
         * not just to PREEMPT_RT.
         */
        if (IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled && t->rcu_blocked_node)
                return true;

        return false;
}

/*
 * Handle special cases during rcu_read_unlock(), such as needing to
 * notify RCU core processing or task having blocked during the RCU
 * read-side critical section.
 */
static void rcu_read_unlock_special(struct task_struct *t)
{
        unsigned long flags;
        bool irqs_were_disabled;
        bool preempt_bh_were_disabled =
                        !!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));

        /* NMI handlers cannot block and cannot safely manipulate state. */
        if (in_nmi())
                return;

        local_irq_save(flags);
        irqs_were_disabled = irqs_disabled_flags(flags);
        if (preempt_bh_were_disabled || irqs_were_disabled) {
                bool needs_exp; // Expedited handling needed.
                struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
                struct rcu_node *rnp = rdp->mynode;

                needs_exp = rcu_unlock_needs_exp_handling(t, rdp, rnp, irqs_were_disabled);

                // Need to defer quiescent state until everything is enabled.
                if (use_softirq && (in_hardirq() || (needs_exp && !irqs_were_disabled))) {
                        // Using softirq, safe to awaken, and either the
                        // wakeup is free or there is either an expedited
                        // GP in flight or a potential need to deboost.
                        if (rdp->defer_qs_pending != DEFER_QS_PENDING) {
                                rdp->defer_qs_pending = DEFER_QS_PENDING;
                                raise_softirq_irqoff(RCU_SOFTIRQ);
                        }
                } else {
                        // Enabling BH or preempt does reschedule, so...
                        // Also if no expediting and no possible deboosting,
                        // slow is OK.  Plus nohz_full CPUs eventually get
                        // tick enabled.
                        set_need_resched_current();
                        if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
                            needs_exp && rdp->defer_qs_pending != DEFER_QS_PENDING &&
                            cpu_online(rdp->cpu)) {
                                // Get scheduler to re-evaluate and call hooks.
                                // If !IRQ_WORK, FQS scan will eventually IPI.
                                rdp->defer_qs_pending = DEFER_QS_PENDING;
                                irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
                        }
                }
                local_irq_restore(flags);
                return;
        }
        rcu_preempt_deferred_qs_irqrestore(t, flags);
}

/*
 * Check that the list of blocked tasks for the newly completed grace
 * period is in fact empty.  It is a serious bug to complete a grace
 * period that still has RCU readers blocked!  This function must be
 * invoked -before- updating this rnp's ->gp_seq.
 *
 * Also, if there are blocked tasks on the list, they automatically
 * block the newly created grace period, so set up ->gp_tasks accordingly.
 */
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
{
        struct task_struct *t;

        RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_check_blocked_tasks() invoked with preemption enabled!!!\n");
        raw_lockdep_assert_held_rcu_node(rnp);
        if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
                dump_blkd_tasks(rnp, 10);
        if (rcu_preempt_has_tasks(rnp) &&
            (rnp->qsmaskinit || rnp->wait_blkd_tasks)) {
                WRITE_ONCE(rnp->gp_tasks, rnp->blkd_tasks.next);
                t = container_of(rnp->gp_tasks, struct task_struct,
                                 rcu_node_entry);
                trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
                                                rnp->gp_seq, t->pid);
        }
        WARN_ON_ONCE(rnp->qsmask);
}

/*
 * Check for a quiescent state from the current CPU, including voluntary
 * context switches for Tasks RCU.  When a task blocks, the task is
 * recorded in the corresponding CPU's rcu_node structure, which is checked
 * elsewhere, hence this function need only check for quiescent states
 * related to the current CPU, not to those related to tasks.
 */
static void rcu_flavor_sched_clock_irq(int user)
{
        struct task_struct *t = current;

        lockdep_assert_irqs_disabled();
        if (rcu_preempt_depth() > 0 ||
            (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {
                /* No QS, force context switch if deferred. */
                if (rcu_preempt_need_deferred_qs(t))
                        set_need_resched_current();
        } else if (rcu_preempt_need_deferred_qs(t)) {
                rcu_preempt_deferred_qs(t); /* Report deferred QS. */
                return;
        } else if (!WARN_ON_ONCE(rcu_preempt_depth())) {
                rcu_qs(); /* Report immediate QS. */
                return;
        }

        /* If GP is oldish, ask for help from rcu_read_unlock_special(). */
        if (rcu_preempt_depth() > 0 &&
            __this_cpu_read(rcu_data.core_needs_qs) &&
            __this_cpu_read(rcu_data.cpu_no_qs.b.norm) &&
            !t->rcu_read_unlock_special.b.need_qs &&
            time_after(jiffies, rcu_state.gp_start + HZ))
                t->rcu_read_unlock_special.b.need_qs = true;
}

/*
 * Check for a task exiting while in a preemptible-RCU read-side
 * critical section, clean up if so.  No need to issue warnings, as
 * debug_check_no_locks_held() already does this if lockdep is enabled.
 * Besides, if this function does anything other than just immediately
 * return, there was a bug of some sort.  Spewing warnings from this
 * function is like as not to simply obscure important prior warnings.
 */
void exit_rcu(void)
{
        struct task_struct *t = current;

        if (unlikely(!list_empty(&current->rcu_node_entry))) {
                rcu_preempt_depth_set(1);
                barrier();
                WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);
        } else if (unlikely(rcu_preempt_depth())) {
                rcu_preempt_depth_set(1);
        } else {
                return;
        }
        __rcu_read_unlock();
        rcu_preempt_deferred_qs(current);
}

/*
 * Dump the blocked-tasks state, but limit the list dump to the
 * specified number of elements.
 */
static void
dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
{
        int cpu;
        int i;
        struct list_head *lhp;
        struct rcu_data *rdp;
        struct rcu_node *rnp1;

        raw_lockdep_assert_held_rcu_node(rnp);
        pr_info("%s: grp: %d-%d level: %d ->gp_seq %ld ->completedqs %ld\n",
                __func__, rnp->grplo, rnp->grphi, rnp->level,
                (long)READ_ONCE(rnp->gp_seq), (long)rnp->completedqs);
        for (rnp1 = rnp; rnp1; rnp1 = rnp1->parent)
                pr_info("%s: %d:%d ->qsmask %#lx ->qsmaskinit %#lx ->qsmaskinitnext %#lx\n",
                        __func__, rnp1->grplo, rnp1->grphi, rnp1->qsmask, rnp1->qsmaskinit, rnp1->qsmaskinitnext);
        pr_info("%s: ->gp_tasks %p ->boost_tasks %p ->exp_tasks %p\n",
                __func__, READ_ONCE(rnp->gp_tasks), data_race(rnp->boost_tasks),
                READ_ONCE(rnp->exp_tasks));
        pr_info("%s: ->blkd_tasks", __func__);
        i = 0;
        list_for_each(lhp, &rnp->blkd_tasks) {
                pr_cont(" %p", lhp);
                if (++i >= ncheck)
                        break;
        }
        pr_cont("\n");
        for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++) {
                rdp = per_cpu_ptr(&rcu_data, cpu);
                pr_info("\t%d: %c online: %ld(%d) offline: %ld(%d)\n",
                        cpu, ".o"[rcu_rdp_cpu_online(rdp)],
                        (long)rdp->rcu_onl_gp_seq, rdp->rcu_onl_gp_state,
                        (long)rdp->rcu_ofl_gp_seq, rdp->rcu_ofl_gp_state);
        }
}

static void rcu_preempt_deferred_qs_init(struct rcu_data *rdp)
{
        rdp->defer_qs_iw = IRQ_WORK_INIT_HARD(rcu_preempt_deferred_qs_handler);
}
#else /* #ifdef CONFIG_PREEMPT_RCU */

/*
 * If strict grace periods are enabled, and if the calling
 * __rcu_read_unlock() marks the beginning of a quiescent state, immediately
 * report that quiescent state and, if requested, spin for a bit.
 */
void rcu_read_unlock_strict(void)
{
        struct rcu_data *rdp;

        if (irqs_disabled() || in_atomic_preempt_off() || !rcu_state.gp_kthread)
                return;

        /*
         * rcu_report_qs_rdp() can only be invoked with a stable rdp and
         * from the local CPU.
         *
         * The in_atomic_preempt_off() check ensures that we come here holding
         * the last preempt_count (which will get dropped once we return to
         * __rcu_read_unlock().
         */
        rdp = this_cpu_ptr(&rcu_data);
        rdp->cpu_no_qs.b.norm = false;
        rcu_report_qs_rdp(rdp);
        udelay(rcu_unlock_delay);
}
EXPORT_SYMBOL_GPL(rcu_read_unlock_strict);

/*
 * Tell them what RCU they are running.
 */
static void __init rcu_bootup_announce(void)
{
        pr_info("Hierarchical RCU implementation.\n");
        rcu_bootup_announce_oddness();
}

/*
 * Note a quiescent state for PREEMPTION=n.  Because we do not need to know
 * how many quiescent states passed, just if there was at least one since
 * the start of the grace period, this just sets a flag.  The caller must
 * have disabled preemption.
 */
static void rcu_qs(void)
{
        RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!");
        if (!__this_cpu_read(rcu_data.cpu_no_qs.s))
                return;
        trace_rcu_grace_period(TPS("rcu_sched"),
                               __this_cpu_read(rcu_data.gp_seq), TPS("cpuqs"));
        __this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
        if (__this_cpu_read(rcu_data.cpu_no_qs.b.exp))
                rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
}

/*
 * Register an urgently needed quiescent state.  If there is an
 * emergency, invoke rcu_momentary_eqs() to do a heavy-weight
 * dyntick-idle quiescent state visible to other CPUs, which will in
 * some cases serve for expedited as well as normal grace periods.
 * Either way, register a lightweight quiescent state.
 */
void rcu_all_qs(void)
{
        unsigned long flags;

        if (!raw_cpu_read(rcu_data.rcu_urgent_qs))
                return;
        preempt_disable();  // For CONFIG_PREEMPT_COUNT=y kernels
        /* Load rcu_urgent_qs before other flags. */
        if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs))) {
                preempt_enable();
                return;
        }
        this_cpu_write(rcu_data.rcu_urgent_qs, false);
        if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs))) {
                local_irq_save(flags);
                rcu_momentary_eqs();
                local_irq_restore(flags);
        }
        rcu_qs();
        preempt_enable();
}
EXPORT_SYMBOL_GPL(rcu_all_qs);

/*
 * Note a PREEMPTION=n context switch. The caller must have disabled interrupts.
 */
void rcu_note_context_switch(bool preempt)
{
        trace_rcu_utilization(TPS("Start context switch"));
        rcu_qs();
        /* Load rcu_urgent_qs before other flags. */
        if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs)))
                goto out;
        this_cpu_write(rcu_data.rcu_urgent_qs, false);
        if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs)))
                rcu_momentary_eqs();
out:
        rcu_tasks_qs(current, preempt);
        trace_rcu_utilization(TPS("End context switch"));
}
EXPORT_SYMBOL_GPL(rcu_note_context_switch);

/*
 * Because preemptible RCU does not exist, there are never any preempted
 * RCU readers.
 */
static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
{
        return 0;
}

/*
 * Because there is no preemptible RCU, there can be no readers blocked.
 */
static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
{
        return false;
}

/*
 * Because there is no preemptible RCU, there can be no deferred quiescent
 * states.
 */
static notrace bool rcu_preempt_need_deferred_qs(struct task_struct *t)
{
        return false;
}

// Except that we do need to respond to a request by an expedited
// grace period for a quiescent state from this CPU.  Note that in
// non-preemptible kernels, there can be no context switches within RCU
// read-side critical sections, which in turn means that the leaf rcu_node
// structure's blocked-tasks list is always empty.  is therefore no need to
// actually check it.  Instead, a quiescent state from this CPU suffices,
// and this function is only called from such a quiescent state.
notrace void rcu_preempt_deferred_qs(struct task_struct *t)
{
        struct rcu_data *rdp = this_cpu_ptr(&rcu_data);

        if (READ_ONCE(rdp->cpu_no_qs.b.exp))
                rcu_report_exp_rdp(rdp);
}

/*
 * Because there is no preemptible RCU, there can be no readers blocked,
 * so there is no need to check for blocked tasks.  So check only for
 * bogus qsmask values.
 */
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
{
        WARN_ON_ONCE(rnp->qsmask);
}

/*
 * Check to see if this CPU is in a non-context-switch quiescent state,
 * namely user mode and idle loop.
 */
static void rcu_flavor_sched_clock_irq(int user)
{
        if (user || rcu_is_cpu_rrupt_from_idle() ||
             (IS_ENABLED(CONFIG_PREEMPT_COUNT) &&
              (preempt_count() == HARDIRQ_OFFSET))) {

                /*
                 * Get here if this CPU took its interrupt from user
                 * mode, from the idle loop without this being a nested
                 * interrupt, or while not holding the task preempt count
                 * (with PREEMPT_COUNT=y). In this case, the CPU is in a
                 * quiescent state, so note it.
                 *
                 * No memory barrier is required here because rcu_qs()
                 * references only CPU-local variables that other CPUs
                 * neither access nor modify, at least not while the
                 * corresponding CPU is online.
                 */
                rcu_qs();
        }
}

/*
 * Because preemptible RCU does not exist, tasks cannot possibly exit
 * while in preemptible RCU read-side critical sections.
 */
void exit_rcu(void)
{
}

/*
 * Dump the guaranteed-empty blocked-tasks state.  Trust but verify.
 */
static void
dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
{
        WARN_ON_ONCE(!list_empty(&rnp->blkd_tasks));
}

static void rcu_preempt_deferred_qs_init(struct rcu_data *rdp) { }

#endif /* #else #ifdef CONFIG_PREEMPT_RCU */

/*
 * If boosting, set rcuc kthreads to realtime priority.
 */
static void rcu_cpu_kthread_setup(unsigned int cpu)
{
        struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
#ifdef CONFIG_RCU_BOOST
        struct sched_param sp;

        sp.sched_priority = kthread_prio;
        sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
#endif /* #ifdef CONFIG_RCU_BOOST */

        WRITE_ONCE(rdp->rcuc_activity, jiffies);
}

static bool rcu_is_callbacks_nocb_kthread(struct rcu_data *rdp)
{
#ifdef CONFIG_RCU_NOCB_CPU
        return rdp->nocb_cb_kthread == current;
#else
        return false;
#endif
}

/*
 * Is the current CPU running the RCU-callbacks kthread?
 * Caller must have preemption disabled.
 */
static bool rcu_is_callbacks_kthread(struct rcu_data *rdp)
{
        return rdp->rcu_cpu_kthread_task == current ||
                        rcu_is_callbacks_nocb_kthread(rdp);
}

#ifdef CONFIG_RCU_BOOST

/*
 * Carry out RCU priority boosting on the task indicated by ->exp_tasks
 * or ->boost_tasks, advancing the pointer to the next task in the
 * ->blkd_tasks list.
 *
 * Note that irqs must be enabled: boosting the task can block.
 * Returns 1 if there are more tasks needing to be boosted.
 */
static int rcu_boost(struct rcu_node *rnp)
{
        unsigned long flags;
        struct task_struct *t;
        struct list_head *tb;

        if (READ_ONCE(rnp->exp_tasks) == NULL &&
            READ_ONCE(rnp->boost_tasks) == NULL)
                return 0;  /* Nothing left to boost. */

        raw_spin_lock_irqsave_rcu_node(rnp, flags);

        /*
         * Recheck under the lock: all tasks in need of boosting
         * might exit their RCU read-side critical sections on their own.
         */
        if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL) {
                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                return 0;
        }

        /*
         * Preferentially boost tasks blocking expedited grace periods.
         * This cannot starve the normal grace periods because a second
         * expedited grace period must boost all blocked tasks, including
         * those blocking the pre-existing normal grace period.
         */
        if (rnp->exp_tasks != NULL)
                tb = rnp->exp_tasks;
        else
                tb = rnp->boost_tasks;

        /*
         * We boost task t by manufacturing an rt_mutex that appears to
         * be held by task t.  We leave a pointer to that rt_mutex where
         * task t can find it, and task t will release the mutex when it
         * exits its outermost RCU read-side critical section.  Then
         * simply acquiring this artificial rt_mutex will boost task
         * t's priority.  (Thanks to tglx for suggesting this approach!)
         *
         * Note that task t must acquire rnp->lock to remove itself from
         * the ->blkd_tasks list, which it will do from exit() if from
         * nowhere else.  We therefore are guaranteed that task t will
         * stay around at least until we drop rnp->lock.  Note that
         * rnp->lock also resolves races between our priority boosting
         * and task t's exiting its outermost RCU read-side critical
         * section.
         */
        t = container_of(tb, struct task_struct, rcu_node_entry);
        rt_mutex_init_proxy_locked(&rnp->boost_mtx.rtmutex, t);
        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
        /* Lock only for side effect: boosts task t's priority. */
        rt_mutex_lock(&rnp->boost_mtx);
        rt_mutex_unlock(&rnp->boost_mtx);  /* Then keep lockdep happy. */
        rnp->n_boosts++;

        return READ_ONCE(rnp->exp_tasks) != NULL ||
               READ_ONCE(rnp->boost_tasks) != NULL;
}

/*
 * Priority-boosting kthread, one per leaf rcu_node.
 */
static int rcu_boost_kthread(void *arg)
{
        struct rcu_node *rnp = (struct rcu_node *)arg;
        int spincnt = 0;
        int more2boost;

        trace_rcu_utilization(TPS("Start boost kthread@init"));
        for (;;) {
                WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_WAITING);
                trace_rcu_utilization(TPS("End boost kthread@rcu_wait"));
                rcu_wait(READ_ONCE(rnp->boost_tasks) ||
                         READ_ONCE(rnp->exp_tasks));
                trace_rcu_utilization(TPS("Start boost kthread@rcu_wait"));
                WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_RUNNING);
                more2boost = rcu_boost(rnp);
                if (more2boost)
                        spincnt++;
                else
                        spincnt = 0;
                if (spincnt > 10) {
                        WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_YIELDING);
                        trace_rcu_utilization(TPS("End boost kthread@rcu_yield"));
                        schedule_timeout_idle(2);
                        trace_rcu_utilization(TPS("Start boost kthread@rcu_yield"));
                        spincnt = 0;
                }
        }
        /* NOTREACHED */
        trace_rcu_utilization(TPS("End boost kthread@notreached"));
        return 0;
}

/*
 * Check to see if it is time to start boosting RCU readers that are
 * blocking the current grace period, and, if so, tell the per-rcu_node
 * kthread to start boosting them.  If there is an expedited grace
 * period in progress, it is always time to boost.
 *
 * The caller must hold rnp->lock, which this function releases.
 * The ->boost_kthread_task is immortal, so we don't need to worry
 * about it going away.
 */
static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
        __releases(rnp->lock)
{
        raw_lockdep_assert_held_rcu_node(rnp);
        if (!rnp->boost_kthread_task ||
            (!rcu_preempt_blocked_readers_cgp(rnp) && !rnp->exp_tasks)) {
                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                return;
        }
        if (rnp->exp_tasks != NULL ||
            (rnp->gp_tasks != NULL &&
             rnp->boost_tasks == NULL &&
             rnp->qsmask == 0 &&
             (!time_after(rnp->boost_time, jiffies) || rcu_state.cbovld ||
              IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)))) {
                if (rnp->exp_tasks == NULL)
                        WRITE_ONCE(rnp->boost_tasks, rnp->gp_tasks);
                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                rcu_wake_cond(rnp->boost_kthread_task,
                              READ_ONCE(rnp->boost_kthread_status));
        } else {
                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
        }
}

#define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)

/*
 * Do priority-boost accounting for the start of a new grace period.
 */
static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
{
        rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
}

/*
 * Create an RCU-boost kthread for the specified node if one does not
 * already exist.  We only create this kthread for preemptible RCU.
 */
static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
{
        unsigned long flags;
        int rnp_index = rnp - rcu_get_root();
        struct sched_param sp;
        struct task_struct *t;

        if (rnp->boost_kthread_task)
                return;

        t = kthread_create(rcu_boost_kthread, (void *)rnp,
                           "rcub/%d", rnp_index);
        if (WARN_ON_ONCE(IS_ERR(t)))
                return;

        raw_spin_lock_irqsave_rcu_node(rnp, flags);
        rnp->boost_kthread_task = t;
        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

        sp.sched_priority = kthread_prio;
        sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
        rcu_thread_affine_rnp(t, rnp);
        wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
}

#else /* #ifdef CONFIG_RCU_BOOST */

static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
        __releases(rnp->lock)
{
        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
}

static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
{
}

static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
{
}

#endif /* #else #ifdef CONFIG_RCU_BOOST */

/*
 * Is this CPU a NO_HZ_FULL CPU that should ignore RCU so that the
 * grace-period kthread will do force_quiescent_state() processing?
 * The idea is to avoid waking up RCU core processing on such a
 * CPU unless the grace period has extended for too long.
 *
 * This code relies on the fact that all NO_HZ_FULL CPUs are also
 * RCU_NOCB_CPU CPUs.
 */
static bool rcu_nohz_full_cpu(void)
{
#ifdef CONFIG_NO_HZ_FULL
        if (tick_nohz_full_cpu(smp_processor_id()) &&
            (!rcu_gp_in_progress() ||
             time_before(jiffies, READ_ONCE(rcu_state.gp_start) + HZ)))
                return true;
#endif /* #ifdef CONFIG_NO_HZ_FULL */
        return false;
}

/*
 * Bind the RCU grace-period kthreads to the housekeeping CPU.
 */
static void rcu_bind_gp_kthread(void)
{
        if (!tick_nohz_full_enabled())
                return;
        housekeeping_affine(current, HK_TYPE_RCU);
}