rcu: Improve boost selection when moving tasks to root rcu_node

[firefly-linux-kernel-4.4.55.git] / kernel / rcutree_plugin.h

/*
 * Read-Copy Update mechanism for mutual exclusion (tree-based version)
 * Internal non-public definitions that provide either classic
 * or preemptible semantics.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 *
 * Copyright Red Hat, 2009
 * Copyright IBM Corporation, 2009
 *
 * Author: Ingo Molnar <mingo@elte.hu>
 *         Paul E. McKenney <paulmck@linux.vnet.ibm.com>
 */

#include <linux/delay.h>

#define RCU_KTHREAD_PRIO 1

#ifdef CONFIG_RCU_BOOST
#define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
#else
#define RCU_BOOST_PRIO RCU_KTHREAD_PRIO
#endif

/*
 * Check the RCU kernel configuration parameters and print informative
 * messages about anything out of the ordinary.  If you like #ifdef, you
 * will love this function.
 */
static void __init rcu_bootup_announce_oddness(void)
{
#ifdef CONFIG_RCU_TRACE
        printk(KERN_INFO "\tRCU debugfs-based tracing is enabled.\n");
#endif
#if (defined(CONFIG_64BIT) && CONFIG_RCU_FANOUT != 64) || (!defined(CONFIG_64BIT) && CONFIG_RCU_FANOUT != 32)
        printk(KERN_INFO "\tCONFIG_RCU_FANOUT set to non-default value of %d\n",
               CONFIG_RCU_FANOUT);
#endif
#ifdef CONFIG_RCU_FANOUT_EXACT
        printk(KERN_INFO "\tHierarchical RCU autobalancing is disabled.\n");
#endif
#ifdef CONFIG_RCU_FAST_NO_HZ
        printk(KERN_INFO
               "\tRCU dyntick-idle grace-period acceleration is enabled.\n");
#endif
#ifdef CONFIG_PROVE_RCU
        printk(KERN_INFO "\tRCU lockdep checking is enabled.\n");
#endif
#ifdef CONFIG_RCU_TORTURE_TEST_RUNNABLE
        printk(KERN_INFO "\tRCU torture testing starts during boot.\n");
#endif
#if defined(CONFIG_TREE_PREEMPT_RCU) && !defined(CONFIG_RCU_CPU_STALL_VERBOSE)
        printk(KERN_INFO "\tDump stacks of tasks blocking RCU-preempt GP.\n");
#endif
#if defined(CONFIG_RCU_CPU_STALL_INFO)
        printk(KERN_INFO "\tAdditional per-CPU info printed with stalls.\n");
#endif
#if NUM_RCU_LVL_4 != 0
        printk(KERN_INFO "\tFour-level hierarchy is enabled.\n");
#endif
        if (rcu_fanout_leaf != CONFIG_RCU_FANOUT_LEAF)
                printk(KERN_INFO "\tExperimental boot-time adjustment of leaf fanout to %d.\n", rcu_fanout_leaf);
        if (nr_cpu_ids != NR_CPUS)
                printk(KERN_INFO "\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%d.\n", NR_CPUS, nr_cpu_ids);
}

#ifdef CONFIG_TREE_PREEMPT_RCU

struct rcu_state rcu_preempt_state =
        RCU_STATE_INITIALIZER(rcu_preempt, call_rcu);
DEFINE_PER_CPU(struct rcu_data, rcu_preempt_data);
static struct rcu_state *rcu_state = &rcu_preempt_state;

static int rcu_preempted_readers_exp(struct rcu_node *rnp);

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

/*
 * Return the number of RCU-preempt batches processed thus far
 * for debug and statistics.
 */
long rcu_batches_completed_preempt(void)
{
        return rcu_preempt_state.completed;
}
EXPORT_SYMBOL_GPL(rcu_batches_completed_preempt);

/*
 * Return the number of RCU batches processed thus far for debug & stats.
 */
long rcu_batches_completed(void)
{
        return rcu_batches_completed_preempt();
}
EXPORT_SYMBOL_GPL(rcu_batches_completed);

/*
 * Force a quiescent state for preemptible RCU.
 */
void rcu_force_quiescent_state(void)
{
        force_quiescent_state(&rcu_preempt_state, 0);
}
EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);

/*
 * Record a preemptible-RCU quiescent state for the specified CPU.  Note
 * that this just means that the task currently running on the CPU is
 * not in a quiescent state.  There might be any number of tasks blocked
 * while in an RCU read-side critical section.
 *
 * Unlike the other rcu_*_qs() functions, callers to this function
 * must disable irqs in order to protect the assignment to
 * ->rcu_read_unlock_special.
 */
static void rcu_preempt_qs(int cpu)
{
        struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);

        rdp->passed_quiesce_gpnum = rdp->gpnum;
        barrier();
        if (rdp->passed_quiesce == 0)
                trace_rcu_grace_period("rcu_preempt", rdp->gpnum, "cpuqs");
        rdp->passed_quiesce = 1;
        current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
}

/*
 * 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 preemption.
 */
static void rcu_preempt_note_context_switch(int cpu)
{
        struct task_struct *t = current;
        unsigned long flags;
        struct rcu_data *rdp;
        struct rcu_node *rnp;

        if (t->rcu_read_lock_nesting > 0 &&
            (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {

                /* Possibly blocking in an RCU read-side critical section. */
                rdp = per_cpu_ptr(rcu_preempt_state.rda, cpu);
                rnp = rdp->mynode;
                raw_spin_lock_irqsave(&rnp->lock, flags);
                t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
                t->rcu_blocked_node = rnp;

                /*
                 * If this CPU has already checked in, then this task
                 * will hold up the next grace period rather than the
                 * current grace period.  Queue the task accordingly.
                 * If the task is queued for the current grace period
                 * (i.e., this CPU has not yet passed through a quiescent
                 * state for the current grace period), then as long
                 * as that task remains queued, the current grace period
                 * cannot end.  Note that there is some uncertainty as
                 * to exactly when the current grace period started.
                 * We take a conservative approach, which can result
                 * in unnecessarily waiting on tasks that started very
                 * slightly after the current grace period began.  C'est
                 * la vie!!!
                 *
                 * But first, note that the current CPU must still be
                 * on line!
                 */
                WARN_ON_ONCE((rdp->grpmask & rnp->qsmaskinit) == 0);
                WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
                if ((rnp->qsmask & rdp->grpmask) && rnp->gp_tasks != NULL) {
                        list_add(&t->rcu_node_entry, rnp->gp_tasks->prev);
                        rnp->gp_tasks = &t->rcu_node_entry;
#ifdef CONFIG_RCU_BOOST
                        if (rnp->boost_tasks != NULL)
                                rnp->boost_tasks = rnp->gp_tasks;
#endif /* #ifdef CONFIG_RCU_BOOST */
                } else {
                        list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
                        if (rnp->qsmask & rdp->grpmask)
                                rnp->gp_tasks = &t->rcu_node_entry;
                }
                trace_rcu_preempt_task(rdp->rsp->name,
                                       t->pid,
                                       (rnp->qsmask & rdp->grpmask)
                                       ? rnp->gpnum
                                       : rnp->gpnum + 1);
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
        } else if (t->rcu_read_lock_nesting < 0 &&
                   t->rcu_read_unlock_special) {

                /*
                 * Complete exit from RCU read-side critical section on
                 * behalf of preempted instance of __rcu_read_unlock().
                 */
                rcu_read_unlock_special(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.
         */
        local_irq_save(flags);
        rcu_preempt_qs(cpu);
        local_irq_restore(flags);
}

/*
 * 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 rnp->gp_tasks != NULL;
}

/*
 * Record a quiescent state for all tasks that were previously queued
 * on the specified rcu_node structure and that were blocking the current
 * RCU grace period.  The caller must hold the specified rnp->lock with
 * irqs disabled, and this lock is released upon return, but irqs remain
 * disabled.
 */
static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp, unsigned long flags)
        __releases(rnp->lock)
{
        unsigned long mask;
        struct rcu_node *rnp_p;

        if (rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
                return;  /* Still need more quiescent states! */
        }

        rnp_p = rnp->parent;
        if (rnp_p == NULL) {
                /*
                 * Either there is only one rcu_node in the tree,
                 * or tasks were kicked up to root rcu_node due to
                 * CPUs going offline.
                 */
                rcu_report_qs_rsp(&rcu_preempt_state, flags);
                return;
        }

        /* Report up the rest of the hierarchy. */
        mask = rnp->grpmask;
        raw_spin_unlock(&rnp->lock);    /* irqs remain disabled. */
        raw_spin_lock(&rnp_p->lock);    /* irqs already disabled. */
        rcu_report_qs_rnp(mask, &rcu_preempt_state, rnp_p, flags);
}

/*
 * 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;
}

/*
 * 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.
 */
void rcu_read_unlock_special(struct task_struct *t)
{
        int empty;
        int empty_exp;
        int empty_exp_now;
        unsigned long flags;
        struct list_head *np;
#ifdef CONFIG_RCU_BOOST
        struct rt_mutex *rbmp = NULL;
#endif /* #ifdef CONFIG_RCU_BOOST */
        struct rcu_node *rnp;
        int special;

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

        local_irq_save(flags);

        /*
         * If RCU core is waiting for this CPU to exit critical section,
         * let it know that we have done so.
         */
        special = t->rcu_read_unlock_special;
        if (special & RCU_READ_UNLOCK_NEED_QS) {
                rcu_preempt_qs(smp_processor_id());
        }

        /* Hardware IRQ handlers cannot block. */
        if (in_irq() || in_serving_softirq()) {
                local_irq_restore(flags);
                return;
        }

        /* Clean up if blocked during RCU read-side critical section. */
        if (special & RCU_READ_UNLOCK_BLOCKED) {
                t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;

                /*
                 * Remove this task from the list it blocked on.  The
                 * task can migrate while we acquire the lock, but at
                 * most one time.  So at most two passes through loop.
                 */
                for (;;) {
                        rnp = t->rcu_blocked_node;
                        raw_spin_lock(&rnp->lock);  /* irqs already disabled. */
                        if (rnp == t->rcu_blocked_node)
                                break;
                        raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
                }
                empty = !rcu_preempt_blocked_readers_cgp(rnp);
                empty_exp = !rcu_preempted_readers_exp(rnp);
                smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
                np = rcu_next_node_entry(t, rnp);
                list_del_init(&t->rcu_node_entry);
                t->rcu_blocked_node = NULL;
                trace_rcu_unlock_preempted_task("rcu_preempt",
                                                rnp->gpnum, t->pid);
                if (&t->rcu_node_entry == rnp->gp_tasks)
                        rnp->gp_tasks = np;
                if (&t->rcu_node_entry == rnp->exp_tasks)
                        rnp->exp_tasks = np;
#ifdef CONFIG_RCU_BOOST
                if (&t->rcu_node_entry == rnp->boost_tasks)
                        rnp->boost_tasks = np;
                /* Snapshot/clear ->rcu_boost_mutex with rcu_node lock held. */
                if (t->rcu_boost_mutex) {
                        rbmp = t->rcu_boost_mutex;
                        t->rcu_boost_mutex = NULL;
                }
#endif /* #ifdef CONFIG_RCU_BOOST */

                /*
                 * 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 = !rcu_preempted_readers_exp(rnp);
                if (!empty && !rcu_preempt_blocked_readers_cgp(rnp)) {
                        trace_rcu_quiescent_state_report("preempt_rcu",
                                                         rnp->gpnum,
                                                         0, rnp->qsmask,
                                                         rnp->level,
                                                         rnp->grplo,
                                                         rnp->grphi,
                                                         !!rnp->gp_tasks);
                        rcu_report_unblock_qs_rnp(rnp, flags);
                } else {
                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
                }

#ifdef CONFIG_RCU_BOOST
                /* Unboost if we were boosted. */
                if (rbmp)
                        rt_mutex_unlock(rbmp);
#endif /* #ifdef CONFIG_RCU_BOOST */

                /*
                 * 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(&rcu_preempt_state, rnp, true);
        } else {
                local_irq_restore(flags);
        }
}

#ifdef CONFIG_RCU_CPU_STALL_VERBOSE

/*
 * Dump detailed information for all tasks blocking the current RCU
 * grace period on the specified rcu_node structure.
 */
static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
{
        unsigned long flags;
        struct task_struct *t;

        if (!rcu_preempt_blocked_readers_cgp(rnp))
                return;
        raw_spin_lock_irqsave(&rnp->lock, flags);
        t = list_entry(rnp->gp_tasks,
                       struct task_struct, rcu_node_entry);
        list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
                sched_show_task(t);
        raw_spin_unlock_irqrestore(&rnp->lock, flags);
}

/*
 * Dump detailed information for all tasks blocking the current RCU
 * grace period.
 */
static void rcu_print_detail_task_stall(struct rcu_state *rsp)
{
        struct rcu_node *rnp = rcu_get_root(rsp);

        rcu_print_detail_task_stall_rnp(rnp);
        rcu_for_each_leaf_node(rsp, rnp)
                rcu_print_detail_task_stall_rnp(rnp);
}

#else /* #ifdef CONFIG_RCU_CPU_STALL_VERBOSE */

static void rcu_print_detail_task_stall(struct rcu_state *rsp)
{
}

#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_VERBOSE */

#ifdef CONFIG_RCU_CPU_STALL_INFO

static void rcu_print_task_stall_begin(struct rcu_node *rnp)
{
        printk(KERN_ERR "\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
               rnp->level, rnp->grplo, rnp->grphi);
}

static void rcu_print_task_stall_end(void)
{
        printk(KERN_CONT "\n");
}

#else /* #ifdef CONFIG_RCU_CPU_STALL_INFO */

static void rcu_print_task_stall_begin(struct rcu_node *rnp)
{
}

static void rcu_print_task_stall_end(void)
{
}

#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_INFO */

/*
 * Scan the current list of tasks blocked within RCU read-side critical
 * sections, printing out the tid of each.
 */
static int rcu_print_task_stall(struct rcu_node *rnp)
{
        struct task_struct *t;
        int ndetected = 0;

        if (!rcu_preempt_blocked_readers_cgp(rnp))
                return 0;
        rcu_print_task_stall_begin(rnp);
        t = list_entry(rnp->gp_tasks,
                       struct task_struct, rcu_node_entry);
        list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
                printk(KERN_CONT " P%d", t->pid);
                ndetected++;
        }
        rcu_print_task_stall_end();
        return ndetected;
}

/*
 * 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 ->gpnum, and the rnp's ->lock
 * must be held by the caller.
 *
 * 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)
{
        WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp));
        if (!list_empty(&rnp->blkd_tasks))
                rnp->gp_tasks = rnp->blkd_tasks.next;
        WARN_ON_ONCE(rnp->qsmask);
}

#ifdef CONFIG_HOTPLUG_CPU

/*
 * Handle tasklist migration for case in which all CPUs covered by the
 * specified rcu_node have gone offline.  Move them up to the root
 * rcu_node.  The reason for not just moving them to the immediate
 * parent is to remove the need for rcu_read_unlock_special() to
 * make more than two attempts to acquire the target rcu_node's lock.
 * Returns true if there were tasks blocking the current RCU grace
 * period.
 *
 * Returns 1 if there was previously a task blocking the current grace
 * period on the specified rcu_node structure.
 *
 * The caller must hold rnp->lock with irqs disabled.
 */
static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
                                     struct rcu_node *rnp,
                                     struct rcu_data *rdp)
{
        struct list_head *lp;
        struct list_head *lp_root;
        int retval = 0;
        struct rcu_node *rnp_root = rcu_get_root(rsp);
        struct task_struct *t;

        if (rnp == rnp_root) {
                WARN_ONCE(1, "Last CPU thought to be offlined?");
                return 0;  /* Shouldn't happen: at least one CPU online. */
        }

        /* If we are on an internal node, complain bitterly. */
        WARN_ON_ONCE(rnp != rdp->mynode);

        /*
         * Move tasks up to root rcu_node.  Don't try to get fancy for
         * this corner-case operation -- just put this node's tasks
         * at the head of the root node's list, and update the root node's
         * ->gp_tasks and ->exp_tasks pointers to those of this node's,
         * if non-NULL.  This might result in waiting for more tasks than
         * absolutely necessary, but this is a good performance/complexity
         * tradeoff.
         */
        if (rcu_preempt_blocked_readers_cgp(rnp) && rnp->qsmask == 0)
                retval |= RCU_OFL_TASKS_NORM_GP;
        if (rcu_preempted_readers_exp(rnp))
                retval |= RCU_OFL_TASKS_EXP_GP;
        lp = &rnp->blkd_tasks;
        lp_root = &rnp_root->blkd_tasks;
        while (!list_empty(lp)) {
                t = list_entry(lp->next, typeof(*t), rcu_node_entry);
                raw_spin_lock(&rnp_root->lock); /* irqs already disabled */
                list_del(&t->rcu_node_entry);
                t->rcu_blocked_node = rnp_root;
                list_add(&t->rcu_node_entry, lp_root);
                if (&t->rcu_node_entry == rnp->gp_tasks)
                        rnp_root->gp_tasks = rnp->gp_tasks;
                if (&t->rcu_node_entry == rnp->exp_tasks)
                        rnp_root->exp_tasks = rnp->exp_tasks;
#ifdef CONFIG_RCU_BOOST
                if (&t->rcu_node_entry == rnp->boost_tasks)
                        rnp_root->boost_tasks = rnp->boost_tasks;
#endif /* #ifdef CONFIG_RCU_BOOST */
                raw_spin_unlock(&rnp_root->lock); /* irqs still disabled */
        }

        rnp->gp_tasks = NULL;
        rnp->exp_tasks = NULL;
#ifdef CONFIG_RCU_BOOST
        rnp->boost_tasks = NULL;
        /*
         * In case root is being boosted and leaf was not.  Make sure
         * that we boost the tasks blocking the current grace period
         * in this case.
         */
        raw_spin_lock(&rnp_root->lock); /* irqs already disabled */
        if (rnp_root->boost_tasks != NULL &&
            rnp_root->boost_tasks != rnp_root->gp_tasks &&
            rnp_root->boost_tasks != rnp_root->exp_tasks)
                rnp_root->boost_tasks = rnp_root->gp_tasks;
        raw_spin_unlock(&rnp_root->lock); /* irqs still disabled */
#endif /* #ifdef CONFIG_RCU_BOOST */

        return retval;
}

#endif /* #ifdef CONFIG_HOTPLUG_CPU */

/*
 * Check for a quiescent state from the current CPU.  When a task blocks,
 * the task is recorded in the corresponding CPU's rcu_node structure,
 * which is checked elsewhere.
 *
 * Caller must disable hard irqs.
 */
static void rcu_preempt_check_callbacks(int cpu)
{
        struct task_struct *t = current;

        if (t->rcu_read_lock_nesting == 0) {
                rcu_preempt_qs(cpu);
                return;
        }
        if (t->rcu_read_lock_nesting > 0 &&
            per_cpu(rcu_preempt_data, cpu).qs_pending)
                t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
}

#ifdef CONFIG_RCU_BOOST

static void rcu_preempt_do_callbacks(void)
{
        rcu_do_batch(&rcu_preempt_state, &__get_cpu_var(rcu_preempt_data));
}

#endif /* #ifdef CONFIG_RCU_BOOST */

/*
 * Queue a preemptible-RCU callback for invocation after a grace period.
 */
void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
{
        __call_rcu(head, func, &rcu_preempt_state, 0);
}
EXPORT_SYMBOL_GPL(call_rcu);

/*
 * Queue an RCU callback for lazy invocation after a grace period.
 * This will likely be later named something like "call_rcu_lazy()",
 * but this change will require some way of tagging the lazy RCU
 * callbacks in the list of pending callbacks.  Until then, this
 * function may only be called from __kfree_rcu().
 */
void kfree_call_rcu(struct rcu_head *head,
                    void (*func)(struct rcu_head *rcu))
{
        __call_rcu(head, func, &rcu_preempt_state, 1);
}
EXPORT_SYMBOL_GPL(kfree_call_rcu);

/**
 * synchronize_rcu - wait until a grace period has elapsed.
 *
 * Control will return to the caller some time after a full grace
 * period has elapsed, in other words after all currently executing RCU
 * read-side critical sections have completed.  Note, however, that
 * upon return from synchronize_rcu(), the caller might well be executing
 * concurrently with new RCU read-side critical sections that began while
 * synchronize_rcu() was waiting.  RCU read-side critical sections are
 * delimited by rcu_read_lock() and rcu_read_unlock(), and may be nested.
 */
void synchronize_rcu(void)
{
        rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map) &&
                           !lock_is_held(&rcu_lock_map) &&
                           !lock_is_held(&rcu_sched_lock_map),
                           "Illegal synchronize_rcu() in RCU read-side critical section");
        if (!rcu_scheduler_active)
                return;
        wait_rcu_gp(call_rcu);
}
EXPORT_SYMBOL_GPL(synchronize_rcu);

static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
static long sync_rcu_preempt_exp_count;
static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);

/*
 * Return non-zero if there are any tasks in RCU read-side critical
 * sections blocking the current preemptible-RCU expedited grace period.
 * If there is no preemptible-RCU expedited grace period currently in
 * progress, returns zero unconditionally.
 */
static int rcu_preempted_readers_exp(struct rcu_node *rnp)
{
        return rnp->exp_tasks != NULL;
}

/*
 * return non-zero if there is no RCU expedited grace period in progress
 * for the specified rcu_node structure, in other words, if all CPUs and
 * tasks covered by the specified rcu_node structure have done their bit
 * for the current expedited grace period.  Works only for preemptible
 * RCU -- other RCU implementation use other means.
 *
 * Caller must hold sync_rcu_preempt_exp_mutex.
 */
static int sync_rcu_preempt_exp_done(struct rcu_node *rnp)
{
        return !rcu_preempted_readers_exp(rnp) &&
               ACCESS_ONCE(rnp->expmask) == 0;
}

/*
 * Report the exit from RCU read-side critical section for the last task
 * that queued itself during or before the current expedited preemptible-RCU
 * grace period.  This event is reported either to the rcu_node structure on
 * which the task was queued or to one of that rcu_node structure's ancestors,
 * recursively up the tree.  (Calm down, calm down, we do the recursion
 * iteratively!)
 *
 * Most callers will set the "wake" flag, but the task initiating the
 * expedited grace period need not wake itself.
 *
 * Caller must hold sync_rcu_preempt_exp_mutex.
 */
static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
                               bool wake)
{
        unsigned long flags;
        unsigned long mask;

        raw_spin_lock_irqsave(&rnp->lock, flags);
        for (;;) {
                if (!sync_rcu_preempt_exp_done(rnp)) {
                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
                        break;
                }
                if (rnp->parent == NULL) {
                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
                        if (wake)
                                wake_up(&sync_rcu_preempt_exp_wq);
                        break;
                }
                mask = rnp->grpmask;
                raw_spin_unlock(&rnp->lock); /* irqs remain disabled */
                rnp = rnp->parent;
                raw_spin_lock(&rnp->lock); /* irqs already disabled */
                rnp->expmask &= ~mask;
        }
}

/*
 * Snapshot the tasks blocking the newly started preemptible-RCU expedited
 * grace period for the specified rcu_node structure.  If there are no such
 * tasks, report it up the rcu_node hierarchy.
 *
 * Caller must hold sync_rcu_preempt_exp_mutex and rsp->onofflock.
 */
static void
sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
{
        unsigned long flags;
        int must_wait = 0;

        raw_spin_lock_irqsave(&rnp->lock, flags);
        if (list_empty(&rnp->blkd_tasks)) {
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
        } else {
                rnp->exp_tasks = rnp->blkd_tasks.next;
                rcu_initiate_boost(rnp, flags);  /* releases rnp->lock */
                must_wait = 1;
        }
        if (!must_wait)
                rcu_report_exp_rnp(rsp, rnp, false); /* Don't wake self. */
}

/**
 * synchronize_rcu_expedited - Brute-force RCU grace period
 *
 * Wait for an RCU-preempt grace period, but expedite it.  The basic
 * idea is to invoke synchronize_sched_expedited() to push all the tasks to
 * the ->blkd_tasks lists and wait for this list to drain.  This consumes
 * significant time on all CPUs and is unfriendly to real-time workloads,
 * so is thus not recommended for any sort of common-case code.
 * In fact, if you are using synchronize_rcu_expedited() in a loop,
 * please restructure your code to batch your updates, and then Use a
 * single synchronize_rcu() instead.
 *
 * Note that it is illegal to call this function while holding any lock
 * that is acquired by a CPU-hotplug notifier.  And yes, it is also illegal
 * to call this function from a CPU-hotplug notifier.  Failing to observe
 * these restriction will result in deadlock.
 */
void synchronize_rcu_expedited(void)
{
        unsigned long flags;
        struct rcu_node *rnp;
        struct rcu_state *rsp = &rcu_preempt_state;
        long snap;
        int trycount = 0;

        smp_mb(); /* Caller's modifications seen first by other CPUs. */
        snap = ACCESS_ONCE(sync_rcu_preempt_exp_count) + 1;
        smp_mb(); /* Above access cannot bleed into critical section. */

        /*
         * Acquire lock, falling back to synchronize_rcu() if too many
         * lock-acquisition failures.  Of course, if someone does the
         * expedited grace period for us, just leave.
         */
        while (!mutex_trylock(&sync_rcu_preempt_exp_mutex)) {
                if (trycount++ < 10) {
                        udelay(trycount * num_online_cpus());
                } else {
                        synchronize_rcu();
                        return;
                }
                if ((ACCESS_ONCE(sync_rcu_preempt_exp_count) - snap) > 0)
                        goto mb_ret; /* Others did our work for us. */
        }
        if ((ACCESS_ONCE(sync_rcu_preempt_exp_count) - snap) > 0)
                goto unlock_mb_ret; /* Others did our work for us. */

        /* force all RCU readers onto ->blkd_tasks lists. */
        synchronize_sched_expedited();

        raw_spin_lock_irqsave(&rsp->onofflock, flags);

        /* Initialize ->expmask for all non-leaf rcu_node structures. */
        rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) {
                raw_spin_lock(&rnp->lock); /* irqs already disabled. */
                rnp->expmask = rnp->qsmaskinit;
                raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
        }

        /* Snapshot current state of ->blkd_tasks lists. */
        rcu_for_each_leaf_node(rsp, rnp)
                sync_rcu_preempt_exp_init(rsp, rnp);
        if (NUM_RCU_NODES > 1)
                sync_rcu_preempt_exp_init(rsp, rcu_get_root(rsp));

        raw_spin_unlock_irqrestore(&rsp->onofflock, flags);

        /* Wait for snapshotted ->blkd_tasks lists to drain. */
        rnp = rcu_get_root(rsp);
        wait_event(sync_rcu_preempt_exp_wq,
                   sync_rcu_preempt_exp_done(rnp));

        /* Clean up and exit. */
        smp_mb(); /* ensure expedited GP seen before counter increment. */
        ACCESS_ONCE(sync_rcu_preempt_exp_count)++;
unlock_mb_ret:
        mutex_unlock(&sync_rcu_preempt_exp_mutex);
mb_ret:
        smp_mb(); /* ensure subsequent action seen after grace period. */
}
EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);

/**
 * rcu_barrier - Wait until all in-flight call_rcu() callbacks complete.
 */
void rcu_barrier(void)
{
        _rcu_barrier(&rcu_preempt_state);
}
EXPORT_SYMBOL_GPL(rcu_barrier);

/*
 * Initialize preemptible RCU's state structures.
 */
static void __init __rcu_init_preempt(void)
{
        rcu_init_one(&rcu_preempt_state, &rcu_preempt_data);
}

#else /* #ifdef CONFIG_TREE_PREEMPT_RCU */

static struct rcu_state *rcu_state = &rcu_sched_state;

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

/*
 * Return the number of RCU batches processed thus far for debug & stats.
 */
long rcu_batches_completed(void)
{
        return rcu_batches_completed_sched();
}
EXPORT_SYMBOL_GPL(rcu_batches_completed);

/*
 * Force a quiescent state for RCU, which, because there is no preemptible
 * RCU, becomes the same as rcu-sched.
 */
void rcu_force_quiescent_state(void)
{
        rcu_sched_force_quiescent_state();
}
EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);

/*
 * Because preemptible RCU does not exist, we never have to check for
 * CPUs being in quiescent states.
 */
static void rcu_preempt_note_context_switch(int cpu)
{
}

/*
 * 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;
}

#ifdef CONFIG_HOTPLUG_CPU

/* Because preemptible RCU does not exist, no quieting of tasks. */
static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp, unsigned long flags)
{
        raw_spin_unlock_irqrestore(&rnp->lock, flags);
}

#endif /* #ifdef CONFIG_HOTPLUG_CPU */

/*
 * Because preemptible RCU does not exist, we never have to check for
 * tasks blocked within RCU read-side critical sections.
 */
static void rcu_print_detail_task_stall(struct rcu_state *rsp)
{
}

/*
 * Because preemptible RCU does not exist, we never have to check for
 * tasks blocked within RCU read-side critical sections.
 */
static int rcu_print_task_stall(struct rcu_node *rnp)
{
        return 0;
}

/*
 * 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);
}

#ifdef CONFIG_HOTPLUG_CPU

/*
 * Because preemptible RCU does not exist, it never needs to migrate
 * tasks that were blocked within RCU read-side critical sections, and
 * such non-existent tasks cannot possibly have been blocking the current
 * grace period.
 */
static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
                                     struct rcu_node *rnp,
                                     struct rcu_data *rdp)
{
        return 0;
}

#endif /* #ifdef CONFIG_HOTPLUG_CPU */

/*
 * Because preemptible RCU does not exist, it never has any callbacks
 * to check.
 */
static void rcu_preempt_check_callbacks(int cpu)
{
}

/*
 * Queue an RCU callback for lazy invocation after a grace period.
 * This will likely be later named something like "call_rcu_lazy()",
 * but this change will require some way of tagging the lazy RCU
 * callbacks in the list of pending callbacks.  Until then, this
 * function may only be called from __kfree_rcu().
 *
 * Because there is no preemptible RCU, we use RCU-sched instead.
 */
void kfree_call_rcu(struct rcu_head *head,
                    void (*func)(struct rcu_head *rcu))
{
        __call_rcu(head, func, &rcu_sched_state, 1);
}
EXPORT_SYMBOL_GPL(kfree_call_rcu);

/*
 * Wait for an rcu-preempt grace period, but make it happen quickly.
 * But because preemptible RCU does not exist, map to rcu-sched.
 */
void synchronize_rcu_expedited(void)
{
        synchronize_sched_expedited();
}
EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);

#ifdef CONFIG_HOTPLUG_CPU

/*
 * Because preemptible RCU does not exist, there is never any need to
 * report on tasks preempted in RCU read-side critical sections during
 * expedited RCU grace periods.
 */
static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
                               bool wake)
{
}

#endif /* #ifdef CONFIG_HOTPLUG_CPU */

/*
 * Because preemptible RCU does not exist, rcu_barrier() is just
 * another name for rcu_barrier_sched().
 */
void rcu_barrier(void)
{
        rcu_barrier_sched();
}
EXPORT_SYMBOL_GPL(rcu_barrier);

/*
 * Because preemptible RCU does not exist, it need not be initialized.
 */
static void __init __rcu_init_preempt(void)
{
}

#endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */

#ifdef CONFIG_RCU_BOOST

#include "rtmutex_common.h"

#ifdef CONFIG_RCU_TRACE

static void rcu_initiate_boost_trace(struct rcu_node *rnp)
{
        if (list_empty(&rnp->blkd_tasks))
                rnp->n_balk_blkd_tasks++;
        else if (rnp->exp_tasks == NULL && rnp->gp_tasks == NULL)
                rnp->n_balk_exp_gp_tasks++;
        else if (rnp->gp_tasks != NULL && rnp->boost_tasks != NULL)
                rnp->n_balk_boost_tasks++;
        else if (rnp->gp_tasks != NULL && rnp->qsmask != 0)
                rnp->n_balk_notblocked++;
        else if (rnp->gp_tasks != NULL &&
                 ULONG_CMP_LT(jiffies, rnp->boost_time))
                rnp->n_balk_notyet++;
        else
                rnp->n_balk_nos++;
}

#else /* #ifdef CONFIG_RCU_TRACE */

static void rcu_initiate_boost_trace(struct rcu_node *rnp)
{
}

#endif /* #else #ifdef CONFIG_RCU_TRACE */

/*
 * 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 rt_mutex mtx;
        struct task_struct *t;
        struct list_head *tb;

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

        raw_spin_lock_irqsave(&rnp->lock, 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(&rnp->lock, 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;
                rnp->n_exp_boosts++;
        } else {
                tb = rnp->boost_tasks;
                rnp->n_normal_boosts++;
        }
        rnp->n_tasks_boosted++;

        /*
         * 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(&mtx, t);
        t->rcu_boost_mutex = &mtx;
        raw_spin_unlock_irqrestore(&rnp->lock, flags);
        rt_mutex_lock(&mtx);  /* Side effect: boosts task t's priority. */
        rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */

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

/*
 * Timer handler to initiate waking up of boost kthreads that
 * have yielded the CPU due to excessive numbers of tasks to
 * boost.  We wake up the per-rcu_node kthread, which in turn
 * will wake up the booster kthread.
 */
static void rcu_boost_kthread_timer(unsigned long arg)
{
        invoke_rcu_node_kthread((struct rcu_node *)arg);
}

/*
 * Priority-boosting kthread.  One per leaf rcu_node and one for the
 * root 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("Start boost kthread@init");
        for (;;) {
                rnp->boost_kthread_status = RCU_KTHREAD_WAITING;
                trace_rcu_utilization("End boost kthread@rcu_wait");
                rcu_wait(rnp->boost_tasks || rnp->exp_tasks);
                trace_rcu_utilization("Start boost kthread@rcu_wait");
                rnp->boost_kthread_status = RCU_KTHREAD_RUNNING;
                more2boost = rcu_boost(rnp);
                if (more2boost)
                        spincnt++;
                else
                        spincnt = 0;
                if (spincnt > 10) {
                        trace_rcu_utilization("End boost kthread@rcu_yield");
                        rcu_yield(rcu_boost_kthread_timer, (unsigned long)rnp);
                        trace_rcu_utilization("Start boost kthread@rcu_yield");
                        spincnt = 0;
                }
        }
        /* NOTREACHED */
        trace_rcu_utilization("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,
 * but irqs remain disabled.  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)
{
        struct task_struct *t;

        if (!rcu_preempt_blocked_readers_cgp(rnp) && rnp->exp_tasks == NULL) {
                rnp->n_balk_exp_gp_tasks++;
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
                return;
        }
        if (rnp->exp_tasks != NULL ||
            (rnp->gp_tasks != NULL &&
             rnp->boost_tasks == NULL &&
             rnp->qsmask == 0 &&
             ULONG_CMP_GE(jiffies, rnp->boost_time))) {
                if (rnp->exp_tasks == NULL)
                        rnp->boost_tasks = rnp->gp_tasks;
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
                t = rnp->boost_kthread_task;
                if (t != NULL)
                        wake_up_process(t);
        } else {
                rcu_initiate_boost_trace(rnp);
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
        }
}

/*
 * Wake up the per-CPU kthread to invoke RCU callbacks.
 */
static void invoke_rcu_callbacks_kthread(void)
{
        unsigned long flags;

        local_irq_save(flags);
        __this_cpu_write(rcu_cpu_has_work, 1);
        if (__this_cpu_read(rcu_cpu_kthread_task) != NULL &&
            current != __this_cpu_read(rcu_cpu_kthread_task))
                wake_up_process(__this_cpu_read(rcu_cpu_kthread_task));
        local_irq_restore(flags);
}

/*
 * Is the current CPU running the RCU-callbacks kthread?
 * Caller must have preemption disabled.
 */
static bool rcu_is_callbacks_kthread(void)
{
        return __get_cpu_var(rcu_cpu_kthread_task) == current;
}

/*
 * Set the affinity of the boost kthread.  The CPU-hotplug locks are
 * held, so no one should be messing with the existence of the boost
 * kthread.
 */
static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp,
                                          cpumask_var_t cm)
{
        struct task_struct *t;

        t = rnp->boost_kthread_task;
        if (t != NULL)
                set_cpus_allowed_ptr(rnp->boost_kthread_task, cm);
}

#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.
 * Returns zero if all is well, a negated errno otherwise.
 */
static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
                                                 struct rcu_node *rnp,
                                                 int rnp_index)
{
        unsigned long flags;
        struct sched_param sp;
        struct task_struct *t;

        if (&rcu_preempt_state != rsp)
                return 0;
        rsp->boost = 1;
        if (rnp->boost_kthread_task != NULL)
                return 0;
        t = kthread_create(rcu_boost_kthread, (void *)rnp,
                           "rcub/%d", rnp_index);
        if (IS_ERR(t))
                return PTR_ERR(t);
        raw_spin_lock_irqsave(&rnp->lock, flags);
        rnp->boost_kthread_task = t;
        raw_spin_unlock_irqrestore(&rnp->lock, flags);
        sp.sched_priority = RCU_BOOST_PRIO;
        sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
        wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
        return 0;
}

#ifdef CONFIG_HOTPLUG_CPU

/*
 * Stop the RCU's per-CPU kthread when its CPU goes offline,.
 */
static void rcu_stop_cpu_kthread(int cpu)
{
        struct task_struct *t;

        /* Stop the CPU's kthread. */
        t = per_cpu(rcu_cpu_kthread_task, cpu);
        if (t != NULL) {
                per_cpu(rcu_cpu_kthread_task, cpu) = NULL;
                kthread_stop(t);
        }
}

#endif /* #ifdef CONFIG_HOTPLUG_CPU */

static void rcu_kthread_do_work(void)
{
        rcu_do_batch(&rcu_sched_state, &__get_cpu_var(rcu_sched_data));
        rcu_do_batch(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
        rcu_preempt_do_callbacks();
}

/*
 * Wake up the specified per-rcu_node-structure kthread.
 * Because the per-rcu_node kthreads are immortal, we don't need
 * to do anything to keep them alive.
 */
static void invoke_rcu_node_kthread(struct rcu_node *rnp)
{
        struct task_struct *t;

        t = rnp->node_kthread_task;
        if (t != NULL)
                wake_up_process(t);
}

/*
 * Set the specified CPU's kthread to run RT or not, as specified by
 * the to_rt argument.  The CPU-hotplug locks are held, so the task
 * is not going away.
 */
static void rcu_cpu_kthread_setrt(int cpu, int to_rt)
{
        int policy;
        struct sched_param sp;
        struct task_struct *t;

        t = per_cpu(rcu_cpu_kthread_task, cpu);
        if (t == NULL)
                return;
        if (to_rt) {
                policy = SCHED_FIFO;
                sp.sched_priority = RCU_KTHREAD_PRIO;
        } else {
                policy = SCHED_NORMAL;
                sp.sched_priority = 0;
        }
        sched_setscheduler_nocheck(t, policy, &sp);
}

/*
 * Timer handler to initiate the waking up of per-CPU kthreads that
 * have yielded the CPU due to excess numbers of RCU callbacks.
 * We wake up the per-rcu_node kthread, which in turn will wake up
 * the booster kthread.
 */
static void rcu_cpu_kthread_timer(unsigned long arg)
{
        struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, arg);
        struct rcu_node *rnp = rdp->mynode;

        atomic_or(rdp->grpmask, &rnp->wakemask);
        invoke_rcu_node_kthread(rnp);
}

/*
 * Drop to non-real-time priority and yield, but only after posting a
 * timer that will cause us to regain our real-time priority if we
 * remain preempted.  Either way, we restore our real-time priority
 * before returning.
 */
static void rcu_yield(void (*f)(unsigned long), unsigned long arg)
{
        struct sched_param sp;
        struct timer_list yield_timer;
        int prio = current->rt_priority;

        setup_timer_on_stack(&yield_timer, f, arg);
        mod_timer(&yield_timer, jiffies + 2);
        sp.sched_priority = 0;
        sched_setscheduler_nocheck(current, SCHED_NORMAL, &sp);
        set_user_nice(current, 19);
        schedule();
        set_user_nice(current, 0);
        sp.sched_priority = prio;
        sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
        del_timer(&yield_timer);
}

/*
 * Handle cases where the rcu_cpu_kthread() ends up on the wrong CPU.
 * This can happen while the corresponding CPU is either coming online
 * or going offline.  We cannot wait until the CPU is fully online
 * before starting the kthread, because the various notifier functions
 * can wait for RCU grace periods.  So we park rcu_cpu_kthread() until
 * the corresponding CPU is online.
 *
 * Return 1 if the kthread needs to stop, 0 otherwise.
 *
 * Caller must disable bh.  This function can momentarily enable it.
 */
static int rcu_cpu_kthread_should_stop(int cpu)
{
        while (cpu_is_offline(cpu) ||
               !cpumask_equal(&current->cpus_allowed, cpumask_of(cpu)) ||
               smp_processor_id() != cpu) {
                if (kthread_should_stop())
                        return 1;
                per_cpu(rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
                per_cpu(rcu_cpu_kthread_cpu, cpu) = raw_smp_processor_id();
                local_bh_enable();
                schedule_timeout_uninterruptible(1);
                if (!cpumask_equal(&current->cpus_allowed, cpumask_of(cpu)))
                        set_cpus_allowed_ptr(current, cpumask_of(cpu));
                local_bh_disable();
        }
        per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
        return 0;
}

/*
 * Per-CPU kernel thread that invokes RCU callbacks.  This replaces the
 * RCU softirq used in flavors and configurations of RCU that do not
 * support RCU priority boosting.
 */
static int rcu_cpu_kthread(void *arg)
{
        int cpu = (int)(long)arg;
        unsigned long flags;
        int spincnt = 0;
        unsigned int *statusp = &per_cpu(rcu_cpu_kthread_status, cpu);
        char work;
        char *workp = &per_cpu(rcu_cpu_has_work, cpu);

        trace_rcu_utilization("Start CPU kthread@init");
        for (;;) {
                *statusp = RCU_KTHREAD_WAITING;
                trace_rcu_utilization("End CPU kthread@rcu_wait");
                rcu_wait(*workp != 0 || kthread_should_stop());
                trace_rcu_utilization("Start CPU kthread@rcu_wait");
                local_bh_disable();
                if (rcu_cpu_kthread_should_stop(cpu)) {
                        local_bh_enable();
                        break;
                }
                *statusp = RCU_KTHREAD_RUNNING;
                per_cpu(rcu_cpu_kthread_loops, cpu)++;
                local_irq_save(flags);
                work = *workp;
                *workp = 0;
                local_irq_restore(flags);
                if (work)
                        rcu_kthread_do_work();
                local_bh_enable();
                if (*workp != 0)
                        spincnt++;
                else
                        spincnt = 0;
                if (spincnt > 10) {
                        *statusp = RCU_KTHREAD_YIELDING;
                        trace_rcu_utilization("End CPU kthread@rcu_yield");
                        rcu_yield(rcu_cpu_kthread_timer, (unsigned long)cpu);
                        trace_rcu_utilization("Start CPU kthread@rcu_yield");
                        spincnt = 0;
                }
        }
        *statusp = RCU_KTHREAD_STOPPED;
        trace_rcu_utilization("End CPU kthread@term");
        return 0;
}

/*
 * Spawn a per-CPU kthread, setting up affinity and priority.
 * Because the CPU hotplug lock is held, no other CPU will be attempting
 * to manipulate rcu_cpu_kthread_task.  There might be another CPU
 * attempting to access it during boot, but the locking in kthread_bind()
 * will enforce sufficient ordering.
 *
 * Please note that we cannot simply refuse to wake up the per-CPU
 * kthread because kthreads are created in TASK_UNINTERRUPTIBLE state,
 * which can result in softlockup complaints if the task ends up being
 * idle for more than a couple of minutes.
 *
 * However, please note also that we cannot bind the per-CPU kthread to its
 * CPU until that CPU is fully online.  We also cannot wait until the
 * CPU is fully online before we create its per-CPU kthread, as this would
 * deadlock the system when CPU notifiers tried waiting for grace
 * periods.  So we bind the per-CPU kthread to its CPU only if the CPU
 * is online.  If its CPU is not yet fully online, then the code in
 * rcu_cpu_kthread() will wait until it is fully online, and then do
 * the binding.
 */
static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu)
{
        struct sched_param sp;
        struct task_struct *t;

        if (!rcu_scheduler_fully_active ||
            per_cpu(rcu_cpu_kthread_task, cpu) != NULL)
                return 0;
        t = kthread_create_on_node(rcu_cpu_kthread,
                                   (void *)(long)cpu,
                                   cpu_to_node(cpu),
                                   "rcuc/%d", cpu);
        if (IS_ERR(t))
                return PTR_ERR(t);
        if (cpu_online(cpu))
                kthread_bind(t, cpu);
        per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
        WARN_ON_ONCE(per_cpu(rcu_cpu_kthread_task, cpu) != NULL);
        sp.sched_priority = RCU_KTHREAD_PRIO;
        sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
        per_cpu(rcu_cpu_kthread_task, cpu) = t;
        wake_up_process(t); /* Get to TASK_INTERRUPTIBLE quickly. */
        return 0;
}

/*
 * Per-rcu_node kthread, which is in charge of waking up the per-CPU
 * kthreads when needed.  We ignore requests to wake up kthreads
 * for offline CPUs, which is OK because force_quiescent_state()
 * takes care of this case.
 */
static int rcu_node_kthread(void *arg)
{
        int cpu;
        unsigned long flags;
        unsigned long mask;
        struct rcu_node *rnp = (struct rcu_node *)arg;
        struct sched_param sp;
        struct task_struct *t;

        for (;;) {
                rnp->node_kthread_status = RCU_KTHREAD_WAITING;
                rcu_wait(atomic_read(&rnp->wakemask) != 0);
                rnp->node_kthread_status = RCU_KTHREAD_RUNNING;
                raw_spin_lock_irqsave(&rnp->lock, flags);
                mask = atomic_xchg(&rnp->wakemask, 0);
                rcu_initiate_boost(rnp, flags); /* releases rnp->lock. */
                for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask >>= 1) {
                        if ((mask & 0x1) == 0)
                                continue;
                        preempt_disable();
                        t = per_cpu(rcu_cpu_kthread_task, cpu);
                        if (!cpu_online(cpu) || t == NULL) {
                                preempt_enable();
                                continue;
                        }
                        per_cpu(rcu_cpu_has_work, cpu) = 1;
                        sp.sched_priority = RCU_KTHREAD_PRIO;
                        sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
                        preempt_enable();
                }
        }
        /* NOTREACHED */
        rnp->node_kthread_status = RCU_KTHREAD_STOPPED;
        return 0;
}

/*
 * Set the per-rcu_node kthread's affinity to cover all CPUs that are
 * served by the rcu_node in question.  The CPU hotplug lock is still
 * held, so the value of rnp->qsmaskinit will be stable.
 *
 * We don't include outgoingcpu in the affinity set, use -1 if there is
 * no outgoing CPU.  If there are no CPUs left in the affinity set,
 * this function allows the kthread to execute on any CPU.
 */
static void rcu_node_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
{
        cpumask_var_t cm;
        int cpu;
        unsigned long mask = rnp->qsmaskinit;

        if (rnp->node_kthread_task == NULL)
                return;
        if (!alloc_cpumask_var(&cm, GFP_KERNEL))
                return;
        cpumask_clear(cm);
        for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask >>= 1)
                if ((mask & 0x1) && cpu != outgoingcpu)
                        cpumask_set_cpu(cpu, cm);
        if (cpumask_weight(cm) == 0) {
                cpumask_setall(cm);
                for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++)
                        cpumask_clear_cpu(cpu, cm);
                WARN_ON_ONCE(cpumask_weight(cm) == 0);
        }
        set_cpus_allowed_ptr(rnp->node_kthread_task, cm);
        rcu_boost_kthread_setaffinity(rnp, cm);
        free_cpumask_var(cm);
}

/*
 * Spawn a per-rcu_node kthread, setting priority and affinity.
 * Called during boot before online/offline can happen, or, if
 * during runtime, with the main CPU-hotplug locks held.  So only
 * one of these can be executing at a time.
 */
static int __cpuinit rcu_spawn_one_node_kthread(struct rcu_state *rsp,
                                                struct rcu_node *rnp)
{
        unsigned long flags;
        int rnp_index = rnp - &rsp->node[0];
        struct sched_param sp;
        struct task_struct *t;

        if (!rcu_scheduler_fully_active ||
            rnp->qsmaskinit == 0)
                return 0;
        if (rnp->node_kthread_task == NULL) {
                t = kthread_create(rcu_node_kthread, (void *)rnp,
                                   "rcun/%d", rnp_index);
                if (IS_ERR(t))
                        return PTR_ERR(t);
                raw_spin_lock_irqsave(&rnp->lock, flags);
                rnp->node_kthread_task = t;
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
                sp.sched_priority = 99;
                sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
                wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
        }
        return rcu_spawn_one_boost_kthread(rsp, rnp, rnp_index);
}

/*
 * Spawn all kthreads -- called as soon as the scheduler is running.
 */
static int __init rcu_spawn_kthreads(void)
{
        int cpu;
        struct rcu_node *rnp;

        rcu_scheduler_fully_active = 1;
        for_each_possible_cpu(cpu) {
                per_cpu(rcu_cpu_has_work, cpu) = 0;
                if (cpu_online(cpu))
                        (void)rcu_spawn_one_cpu_kthread(cpu);
        }
        rnp = rcu_get_root(rcu_state);
        (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
        if (NUM_RCU_NODES > 1) {
                rcu_for_each_leaf_node(rcu_state, rnp)
                        (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
        }
        return 0;
}
early_initcall(rcu_spawn_kthreads);

static void __cpuinit rcu_prepare_kthreads(int cpu)
{
        struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
        struct rcu_node *rnp = rdp->mynode;

        /* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
        if (rcu_scheduler_fully_active) {
                (void)rcu_spawn_one_cpu_kthread(cpu);
                if (rnp->node_kthread_task == NULL)
                        (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
        }
}

#else /* #ifdef CONFIG_RCU_BOOST */

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

static void invoke_rcu_callbacks_kthread(void)
{
        WARN_ON_ONCE(1);
}

static bool rcu_is_callbacks_kthread(void)
{
        return false;
}

static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
{
}

#ifdef CONFIG_HOTPLUG_CPU

static void rcu_stop_cpu_kthread(int cpu)
{
}

#endif /* #ifdef CONFIG_HOTPLUG_CPU */

static void rcu_node_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
{
}

static void rcu_cpu_kthread_setrt(int cpu, int to_rt)
{
}

static int __init rcu_scheduler_really_started(void)
{
        rcu_scheduler_fully_active = 1;
        return 0;
}
early_initcall(rcu_scheduler_really_started);

static void __cpuinit rcu_prepare_kthreads(int cpu)
{
}

#endif /* #else #ifdef CONFIG_RCU_BOOST */

#if !defined(CONFIG_RCU_FAST_NO_HZ)

/*
 * Check to see if any future RCU-related work will need to be done
 * by the current CPU, even if none need be done immediately, returning
 * 1 if so.  This function is part of the RCU implementation; it is -not-
 * an exported member of the RCU API.
 *
 * Because we not have RCU_FAST_NO_HZ, just check whether this CPU needs
 * any flavor of RCU.
 */
int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)
{
        *delta_jiffies = ULONG_MAX;
        return rcu_cpu_has_callbacks(cpu);
}

/*
 * Because we do not have RCU_FAST_NO_HZ, don't bother initializing for it.
 */
static void rcu_prepare_for_idle_init(int cpu)
{
}

/*
 * Because we do not have RCU_FAST_NO_HZ, don't bother cleaning up
 * after it.
 */
static void rcu_cleanup_after_idle(int cpu)
{
}

/*
 * Do the idle-entry grace-period work, which, because CONFIG_RCU_FAST_NO_HZ=n,
 * is nothing.
 */
static void rcu_prepare_for_idle(int cpu)
{
}

/*
 * Don't bother keeping a running count of the number of RCU callbacks
 * posted because CONFIG_RCU_FAST_NO_HZ=n.
 */
static void rcu_idle_count_callbacks_posted(void)
{
}

#else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */

/*
 * This code is invoked when a CPU goes idle, at which point we want
 * to have the CPU do everything required for RCU so that it can enter
 * the energy-efficient dyntick-idle mode.  This is handled by a
 * state machine implemented by rcu_prepare_for_idle() below.
 *
 * The following three proprocessor symbols control this state machine:
 *
 * RCU_IDLE_FLUSHES gives the maximum number of times that we will attempt
 *      to satisfy RCU.  Beyond this point, it is better to incur a periodic
 *      scheduling-clock interrupt than to loop through the state machine
 *      at full power.
 * RCU_IDLE_OPT_FLUSHES gives the number of RCU_IDLE_FLUSHES that are
 *      optional if RCU does not need anything immediately from this
 *      CPU, even if this CPU still has RCU callbacks queued.  The first
 *      times through the state machine are mandatory: we need to give
 *      the state machine a chance to communicate a quiescent state
 *      to the RCU core.
 * RCU_IDLE_GP_DELAY gives the number of jiffies that a CPU is permitted
 *      to sleep in dyntick-idle mode with RCU callbacks pending.  This
 *      is sized to be roughly one RCU grace period.  Those energy-efficiency
 *      benchmarkers who might otherwise be tempted to set this to a large
 *      number, be warned: Setting RCU_IDLE_GP_DELAY too high can hang your
 *      system.  And if you are -that- concerned about energy efficiency,
 *      just power the system down and be done with it!
 * RCU_IDLE_LAZY_GP_DELAY gives the number of jiffies that a CPU is
 *      permitted to sleep in dyntick-idle mode with only lazy RCU
 *      callbacks pending.  Setting this too high can OOM your system.
 *
 * The values below work well in practice.  If future workloads require
 * adjustment, they can be converted into kernel config parameters, though
 * making the state machine smarter might be a better option.
 */
#define RCU_IDLE_FLUSHES 5              /* Number of dyntick-idle tries. */
#define RCU_IDLE_OPT_FLUSHES 3          /* Optional dyntick-idle tries. */
#define RCU_IDLE_GP_DELAY 4             /* Roughly one grace period. */
#define RCU_IDLE_LAZY_GP_DELAY (6 * HZ) /* Roughly six seconds. */

extern int tick_nohz_enabled;

/*
 * Does the specified flavor of RCU have non-lazy callbacks pending on
 * the specified CPU?  Both RCU flavor and CPU are specified by the
 * rcu_data structure.
 */
static bool __rcu_cpu_has_nonlazy_callbacks(struct rcu_data *rdp)
{
        return rdp->qlen != rdp->qlen_lazy;
}

#ifdef CONFIG_TREE_PREEMPT_RCU

/*
 * Are there non-lazy RCU-preempt callbacks?  (There cannot be if there
 * is no RCU-preempt in the kernel.)
 */
static bool rcu_preempt_cpu_has_nonlazy_callbacks(int cpu)
{
        struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);

        return __rcu_cpu_has_nonlazy_callbacks(rdp);
}

#else /* #ifdef CONFIG_TREE_PREEMPT_RCU */

static bool rcu_preempt_cpu_has_nonlazy_callbacks(int cpu)
{
        return 0;
}

#endif /* else #ifdef CONFIG_TREE_PREEMPT_RCU */

/*
 * Does any flavor of RCU have non-lazy callbacks on the specified CPU?
 */
static bool rcu_cpu_has_nonlazy_callbacks(int cpu)
{
        return __rcu_cpu_has_nonlazy_callbacks(&per_cpu(rcu_sched_data, cpu)) ||
               __rcu_cpu_has_nonlazy_callbacks(&per_cpu(rcu_bh_data, cpu)) ||
               rcu_preempt_cpu_has_nonlazy_callbacks(cpu);
}

/*
 * Allow the CPU to enter dyntick-idle mode if either: (1) There are no
 * callbacks on this CPU, (2) this CPU has not yet attempted to enter
 * dyntick-idle mode, or (3) this CPU is in the process of attempting to
 * enter dyntick-idle mode.  Otherwise, if we have recently tried and failed
 * to enter dyntick-idle mode, we refuse to try to enter it.  After all,
 * it is better to incur scheduling-clock interrupts than to spin
 * continuously for the same time duration!
 *
 * The delta_jiffies argument is used to store the time when RCU is
 * going to need the CPU again if it still has callbacks.  The reason
 * for this is that rcu_prepare_for_idle() might need to post a timer,
 * but if so, it will do so after tick_nohz_stop_sched_tick() has set
 * the wakeup time for this CPU.  This means that RCU's timer can be
 * delayed until the wakeup time, which defeats the purpose of posting
 * a timer.
 */
int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)
{
        struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);

        /* Flag a new idle sojourn to the idle-entry state machine. */
        rdtp->idle_first_pass = 1;
        /* If no callbacks, RCU doesn't need the CPU. */
        if (!rcu_cpu_has_callbacks(cpu)) {
                *delta_jiffies = ULONG_MAX;
                return 0;
        }
        if (rdtp->dyntick_holdoff == jiffies) {
                /* RCU recently tried and failed, so don't try again. */
                *delta_jiffies = 1;
                return 1;
        }
        /* Set up for the possibility that RCU will post a timer. */
        if (rcu_cpu_has_nonlazy_callbacks(cpu)) {
                *delta_jiffies = round_up(RCU_IDLE_GP_DELAY + jiffies,
                                          RCU_IDLE_GP_DELAY) - jiffies;
        } else {
                *delta_jiffies = jiffies + RCU_IDLE_LAZY_GP_DELAY;
                *delta_jiffies = round_jiffies(*delta_jiffies) - jiffies;
        }
        return 0;
}

/*
 * Handler for smp_call_function_single().  The only point of this
 * handler is to wake the CPU up, so the handler does only tracing.
 */
void rcu_idle_demigrate(void *unused)
{
        trace_rcu_prep_idle("Demigrate");
}

/*
 * Timer handler used to force CPU to start pushing its remaining RCU
 * callbacks in the case where it entered dyntick-idle mode with callbacks
 * pending.  The hander doesn't really need to do anything because the
 * real work is done upon re-entry to idle, or by the next scheduling-clock
 * interrupt should idle not be re-entered.
 *
 * One special case: the timer gets migrated without awakening the CPU
 * on which the timer was scheduled on.  In this case, we must wake up
 * that CPU.  We do so with smp_call_function_single().
 */
static void rcu_idle_gp_timer_func(unsigned long cpu_in)
{
        int cpu = (int)cpu_in;

        trace_rcu_prep_idle("Timer");
        if (cpu != smp_processor_id())
                smp_call_function_single(cpu, rcu_idle_demigrate, NULL, 0);
        else
                WARN_ON_ONCE(1); /* Getting here can hang the system... */
}

/*
 * Initialize the timer used to pull CPUs out of dyntick-idle mode.
 */
static void rcu_prepare_for_idle_init(int cpu)
{
        struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);

        rdtp->dyntick_holdoff = jiffies - 1;
        setup_timer(&rdtp->idle_gp_timer, rcu_idle_gp_timer_func, cpu);
        rdtp->idle_gp_timer_expires = jiffies - 1;
        rdtp->idle_first_pass = 1;
}

/*
 * Clean up for exit from idle.  Because we are exiting from idle, there
 * is no longer any point to ->idle_gp_timer, so cancel it.  This will
 * do nothing if this timer is not active, so just cancel it unconditionally.
 */
static void rcu_cleanup_after_idle(int cpu)
{
        struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);

        del_timer(&rdtp->idle_gp_timer);
        trace_rcu_prep_idle("Cleanup after idle");
        rdtp->tick_nohz_enabled_snap = ACCESS_ONCE(tick_nohz_enabled);
}

/*
 * Check to see if any RCU-related work can be done by the current CPU,
 * and if so, schedule a softirq to get it done.  This function is part
 * of the RCU implementation; it is -not- an exported member of the RCU API.
 *
 * The idea is for the current CPU to clear out all work required by the
 * RCU core for the current grace period, so that this CPU can be permitted
 * to enter dyntick-idle mode.  In some cases, it will need to be awakened
 * at the end of the grace period by whatever CPU ends the grace period.
 * This allows CPUs to go dyntick-idle more quickly, and to reduce the
 * number of wakeups by a modest integer factor.
 *
 * Because it is not legal to invoke rcu_process_callbacks() with irqs
 * disabled, we do one pass of force_quiescent_state(), then do a
 * invoke_rcu_core() to cause rcu_process_callbacks() to be invoked
 * later.  The ->dyntick_drain field controls the sequencing.
 *
 * The caller must have disabled interrupts.
 */
static void rcu_prepare_for_idle(int cpu)
{
        struct timer_list *tp;
        struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
        int tne;

        /* Handle nohz enablement switches conservatively. */
        tne = ACCESS_ONCE(tick_nohz_enabled);
        if (tne != rdtp->tick_nohz_enabled_snap) {
                if (rcu_cpu_has_callbacks(cpu))
                        invoke_rcu_core(); /* force nohz to see update. */
                rdtp->tick_nohz_enabled_snap = tne;
                return;
        }
        if (!tne)
                return;

        /*
         * If this is an idle re-entry, for example, due to use of
         * RCU_NONIDLE() or the new idle-loop tracing API within the idle
         * loop, then don't take any state-machine actions, unless the
         * momentary exit from idle queued additional non-lazy callbacks.
         * Instead, repost the ->idle_gp_timer if this CPU has callbacks
         * pending.
         */
        if (!rdtp->idle_first_pass &&
            (rdtp->nonlazy_posted == rdtp->nonlazy_posted_snap)) {
                if (rcu_cpu_has_callbacks(cpu)) {
                        tp = &rdtp->idle_gp_timer;
                        mod_timer_pinned(tp, rdtp->idle_gp_timer_expires);
                }
                return;
        }
        rdtp->idle_first_pass = 0;
        rdtp->nonlazy_posted_snap = rdtp->nonlazy_posted - 1;

        /*
         * If there are no callbacks on this CPU, enter dyntick-idle mode.
         * Also reset state to avoid prejudicing later attempts.
         */
        if (!rcu_cpu_has_callbacks(cpu)) {
                rdtp->dyntick_holdoff = jiffies - 1;
                rdtp->dyntick_drain = 0;
                trace_rcu_prep_idle("No callbacks");
                return;
        }

        /*
         * If in holdoff mode, just return.  We will presumably have
         * refrained from disabling the scheduling-clock tick.
         */
        if (rdtp->dyntick_holdoff == jiffies) {
                trace_rcu_prep_idle("In holdoff");
                return;
        }

        /* Check and update the ->dyntick_drain sequencing. */
        if (rdtp->dyntick_drain <= 0) {
                /* First time through, initialize the counter. */
                rdtp->dyntick_drain = RCU_IDLE_FLUSHES;
        } else if (rdtp->dyntick_drain <= RCU_IDLE_OPT_FLUSHES &&
                   !rcu_pending(cpu) &&
                   !local_softirq_pending()) {
                /* Can we go dyntick-idle despite still having callbacks? */
                rdtp->dyntick_drain = 0;
                rdtp->dyntick_holdoff = jiffies;
                if (rcu_cpu_has_nonlazy_callbacks(cpu)) {
                        trace_rcu_prep_idle("Dyntick with callbacks");
                        rdtp->idle_gp_timer_expires =
                                round_up(jiffies + RCU_IDLE_GP_DELAY,
                                         RCU_IDLE_GP_DELAY);
                } else {
                        rdtp->idle_gp_timer_expires =
                                round_jiffies(jiffies + RCU_IDLE_LAZY_GP_DELAY);
                        trace_rcu_prep_idle("Dyntick with lazy callbacks");
                }
                tp = &rdtp->idle_gp_timer;
                mod_timer_pinned(tp, rdtp->idle_gp_timer_expires);
                rdtp->nonlazy_posted_snap = rdtp->nonlazy_posted;
                return; /* Nothing more to do immediately. */
        } else if (--(rdtp->dyntick_drain) <= 0) {
                /* We have hit the limit, so time to give up. */
                rdtp->dyntick_holdoff = jiffies;
                trace_rcu_prep_idle("Begin holdoff");
                invoke_rcu_core();  /* Force the CPU out of dyntick-idle. */
                return;
        }

        /*
         * Do one step of pushing the remaining RCU callbacks through
         * the RCU core state machine.
         */
#ifdef CONFIG_TREE_PREEMPT_RCU
        if (per_cpu(rcu_preempt_data, cpu).nxtlist) {
                rcu_preempt_qs(cpu);
                force_quiescent_state(&rcu_preempt_state, 0);
        }
#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
        if (per_cpu(rcu_sched_data, cpu).nxtlist) {
                rcu_sched_qs(cpu);
                force_quiescent_state(&rcu_sched_state, 0);
        }
        if (per_cpu(rcu_bh_data, cpu).nxtlist) {
                rcu_bh_qs(cpu);
                force_quiescent_state(&rcu_bh_state, 0);
        }

        /*
         * If RCU callbacks are still pending, RCU still needs this CPU.
         * So try forcing the callbacks through the grace period.
         */
        if (rcu_cpu_has_callbacks(cpu)) {
                trace_rcu_prep_idle("More callbacks");
                invoke_rcu_core();
        } else {
                trace_rcu_prep_idle("Callbacks drained");
        }
}

/*
 * Keep a running count of the number of non-lazy callbacks posted
 * on this CPU.  This running counter (which is never decremented) allows
 * rcu_prepare_for_idle() to detect when something out of the idle loop
 * posts a callback, even if an equal number of callbacks are invoked.
 * Of course, callbacks should only be posted from within a trace event
 * designed to be called from idle or from within RCU_NONIDLE().
 */
static void rcu_idle_count_callbacks_posted(void)
{
        __this_cpu_add(rcu_dynticks.nonlazy_posted, 1);
}

#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */

#ifdef CONFIG_RCU_CPU_STALL_INFO

#ifdef CONFIG_RCU_FAST_NO_HZ

static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
{
        struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
        struct timer_list *tltp = &rdtp->idle_gp_timer;

        sprintf(cp, "drain=%d %c timer=%lu",
                rdtp->dyntick_drain,
                rdtp->dyntick_holdoff == jiffies ? 'H' : '.',
                timer_pending(tltp) ? tltp->expires - jiffies : -1);
}

#else /* #ifdef CONFIG_RCU_FAST_NO_HZ */

static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
{
        *cp = '\0';
}

#endif /* #else #ifdef CONFIG_RCU_FAST_NO_HZ */

/* Initiate the stall-info list. */
static void print_cpu_stall_info_begin(void)
{
        printk(KERN_CONT "\n");
}

/*
 * Print out diagnostic information for the specified stalled CPU.
 *
 * If the specified CPU is aware of the current RCU grace period
 * (flavor specified by rsp), then print the number of scheduling
 * clock interrupts the CPU has taken during the time that it has
 * been aware.  Otherwise, print the number of RCU grace periods
 * that this CPU is ignorant of, for example, "1" if the CPU was
 * aware of the previous grace period.
 *
 * Also print out idle and (if CONFIG_RCU_FAST_NO_HZ) idle-entry info.
 */
static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
{
        char fast_no_hz[72];
        struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
        struct rcu_dynticks *rdtp = rdp->dynticks;
        char *ticks_title;
        unsigned long ticks_value;

        if (rsp->gpnum == rdp->gpnum) {
                ticks_title = "ticks this GP";
                ticks_value = rdp->ticks_this_gp;
        } else {
                ticks_title = "GPs behind";
                ticks_value = rsp->gpnum - rdp->gpnum;
        }
        print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
        printk(KERN_ERR "\t%d: (%lu %s) idle=%03x/%llx/%d %s\n",
               cpu, ticks_value, ticks_title,
               atomic_read(&rdtp->dynticks) & 0xfff,
               rdtp->dynticks_nesting, rdtp->dynticks_nmi_nesting,
               fast_no_hz);
}

/* Terminate the stall-info list. */
static void print_cpu_stall_info_end(void)
{
        printk(KERN_ERR "\t");
}

/* Zero ->ticks_this_gp for all flavors of RCU. */
static void zero_cpu_stall_ticks(struct rcu_data *rdp)
{
        rdp->ticks_this_gp = 0;
}

/* Increment ->ticks_this_gp for all flavors of RCU. */
static void increment_cpu_stall_ticks(void)
{
        __get_cpu_var(rcu_sched_data).ticks_this_gp++;
        __get_cpu_var(rcu_bh_data).ticks_this_gp++;
#ifdef CONFIG_TREE_PREEMPT_RCU
        __get_cpu_var(rcu_preempt_data).ticks_this_gp++;
#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
}

#else /* #ifdef CONFIG_RCU_CPU_STALL_INFO */

static void print_cpu_stall_info_begin(void)
{
        printk(KERN_CONT " {");
}

static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
{
        printk(KERN_CONT " %d", cpu);
}

static void print_cpu_stall_info_end(void)
{
        printk(KERN_CONT "} ");
}

static void zero_cpu_stall_ticks(struct rcu_data *rdp)
{
}

static void increment_cpu_stall_ticks(void)
{
}

#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_INFO */