rcu: Make force_quiescent_state() start grace period if needed

[safe/jmp/linux-2.6] / kernel / rcutree.c

/*
 * Read-Copy Update mechanism for mutual exclusion
 *
 * 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 IBM Corporation, 2008
 *
 * Authors: Dipankar Sarma <dipankar@in.ibm.com>
 *          Manfred Spraul <manfred@colorfullife.com>
 *          Paul E. McKenney <paulmck@linux.vnet.ibm.com> Hierarchical version
 *
 * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
 *
 * For detailed explanation of Read-Copy Update mechanism see -
 *      Documentation/RCU
 */
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/spinlock.h>
#include <linux/smp.h>
#include <linux/rcupdate.h>
#include <linux/interrupt.h>
#include <linux/sched.h>
#include <linux/nmi.h>
#include <asm/atomic.h>
#include <linux/bitops.h>
#include <linux/module.h>
#include <linux/completion.h>
#include <linux/moduleparam.h>
#include <linux/percpu.h>
#include <linux/notifier.h>
#include <linux/cpu.h>
#include <linux/mutex.h>
#include <linux/time.h>
#include <linux/kernel_stat.h>

#include "rcutree.h"

/* Data structures. */

static struct lock_class_key rcu_node_class[NUM_RCU_LVLS];

#define RCU_STATE_INITIALIZER(name) { \
        .level = { &name.node[0] }, \
        .levelcnt = { \
                NUM_RCU_LVL_0,  /* root of hierarchy. */ \
                NUM_RCU_LVL_1, \
                NUM_RCU_LVL_2, \
                NUM_RCU_LVL_3, \
                NUM_RCU_LVL_4, /* == MAX_RCU_LVLS */ \
        }, \
        .signaled = RCU_GP_IDLE, \
        .gpnum = -300, \
        .completed = -300, \
        .onofflock = __SPIN_LOCK_UNLOCKED(&name.onofflock), \
        .orphan_cbs_list = NULL, \
        .orphan_cbs_tail = &name.orphan_cbs_list, \
        .orphan_qlen = 0, \
        .fqslock = __SPIN_LOCK_UNLOCKED(&name.fqslock), \
        .n_force_qs = 0, \
        .n_force_qs_ngp = 0, \
}

struct rcu_state rcu_sched_state = RCU_STATE_INITIALIZER(rcu_sched_state);
DEFINE_PER_CPU(struct rcu_data, rcu_sched_data);

struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh_state);
DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);

static int rcu_scheduler_active __read_mostly;


/*
 * Return true if an RCU grace period is in progress.  The ACCESS_ONCE()s
 * permit this function to be invoked without holding the root rcu_node
 * structure's ->lock, but of course results can be subject to change.
 */
static int rcu_gp_in_progress(struct rcu_state *rsp)
{
        return ACCESS_ONCE(rsp->completed) != ACCESS_ONCE(rsp->gpnum);
}

/*
 * Note a quiescent state.  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.
 */
void rcu_sched_qs(int cpu)
{
        struct rcu_data *rdp;

        rdp = &per_cpu(rcu_sched_data, cpu);
        rdp->passed_quiesc_completed = rdp->gpnum - 1;
        barrier();
        rdp->passed_quiesc = 1;
        rcu_preempt_note_context_switch(cpu);
}

void rcu_bh_qs(int cpu)
{
        struct rcu_data *rdp;

        rdp = &per_cpu(rcu_bh_data, cpu);
        rdp->passed_quiesc_completed = rdp->gpnum - 1;
        barrier();
        rdp->passed_quiesc = 1;
}

#ifdef CONFIG_NO_HZ
DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
        .dynticks_nesting = 1,
        .dynticks = 1,
};
#endif /* #ifdef CONFIG_NO_HZ */

static int blimit = 10;         /* Maximum callbacks per softirq. */
static int qhimark = 10000;     /* If this many pending, ignore blimit. */
static int qlowmark = 100;      /* Once only this many pending, use blimit. */

module_param(blimit, int, 0);
module_param(qhimark, int, 0);
module_param(qlowmark, int, 0);

static void force_quiescent_state(struct rcu_state *rsp, int relaxed);
static int rcu_pending(int cpu);

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

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

/*
 * Does the CPU have callbacks ready to be invoked?
 */
static int
cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp)
{
        return &rdp->nxtlist != rdp->nxttail[RCU_DONE_TAIL];
}

/*
 * Does the current CPU require a yet-as-unscheduled grace period?
 */
static int
cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
{
        return *rdp->nxttail[RCU_DONE_TAIL] && !rcu_gp_in_progress(rsp);
}

/*
 * Return the root node of the specified rcu_state structure.
 */
static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
{
        return &rsp->node[0];
}

#ifdef CONFIG_SMP

/*
 * If the specified CPU is offline, tell the caller that it is in
 * a quiescent state.  Otherwise, whack it with a reschedule IPI.
 * Grace periods can end up waiting on an offline CPU when that
 * CPU is in the process of coming online -- it will be added to the
 * rcu_node bitmasks before it actually makes it online.  The same thing
 * can happen while a CPU is in the process of coming online.  Because this
 * race is quite rare, we check for it after detecting that the grace
 * period has been delayed rather than checking each and every CPU
 * each and every time we start a new grace period.
 */
static int rcu_implicit_offline_qs(struct rcu_data *rdp)
{
        /*
         * If the CPU is offline, it is in a quiescent state.  We can
         * trust its state not to change because interrupts are disabled.
         */
        if (cpu_is_offline(rdp->cpu)) {
                rdp->offline_fqs++;
                return 1;
        }

        /* If preemptable RCU, no point in sending reschedule IPI. */
        if (rdp->preemptable)
                return 0;

        /* The CPU is online, so send it a reschedule IPI. */
        if (rdp->cpu != smp_processor_id())
                smp_send_reschedule(rdp->cpu);
        else
                set_need_resched();
        rdp->resched_ipi++;
        return 0;
}

#endif /* #ifdef CONFIG_SMP */

#ifdef CONFIG_NO_HZ

/**
 * rcu_enter_nohz - inform RCU that current CPU is entering nohz
 *
 * Enter nohz mode, in other words, -leave- the mode in which RCU
 * read-side critical sections can occur.  (Though RCU read-side
 * critical sections can occur in irq handlers in nohz mode, a possibility
 * handled by rcu_irq_enter() and rcu_irq_exit()).
 */
void rcu_enter_nohz(void)
{
        unsigned long flags;
        struct rcu_dynticks *rdtp;

        smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
        local_irq_save(flags);
        rdtp = &__get_cpu_var(rcu_dynticks);
        rdtp->dynticks++;
        rdtp->dynticks_nesting--;
        WARN_ON_ONCE(rdtp->dynticks & 0x1);
        local_irq_restore(flags);
}

/*
 * rcu_exit_nohz - inform RCU that current CPU is leaving nohz
 *
 * Exit nohz mode, in other words, -enter- the mode in which RCU
 * read-side critical sections normally occur.
 */
void rcu_exit_nohz(void)
{
        unsigned long flags;
        struct rcu_dynticks *rdtp;

        local_irq_save(flags);
        rdtp = &__get_cpu_var(rcu_dynticks);
        rdtp->dynticks++;
        rdtp->dynticks_nesting++;
        WARN_ON_ONCE(!(rdtp->dynticks & 0x1));
        local_irq_restore(flags);
        smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
}

/**
 * rcu_nmi_enter - inform RCU of entry to NMI context
 *
 * If the CPU was idle with dynamic ticks active, and there is no
 * irq handler running, this updates rdtp->dynticks_nmi to let the
 * RCU grace-period handling know that the CPU is active.
 */
void rcu_nmi_enter(void)
{
        struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);

        if (rdtp->dynticks & 0x1)
                return;
        rdtp->dynticks_nmi++;
        WARN_ON_ONCE(!(rdtp->dynticks_nmi & 0x1));
        smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
}

/**
 * rcu_nmi_exit - inform RCU of exit from NMI context
 *
 * If the CPU was idle with dynamic ticks active, and there is no
 * irq handler running, this updates rdtp->dynticks_nmi to let the
 * RCU grace-period handling know that the CPU is no longer active.
 */
void rcu_nmi_exit(void)
{
        struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);

        if (rdtp->dynticks & 0x1)
                return;
        smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
        rdtp->dynticks_nmi++;
        WARN_ON_ONCE(rdtp->dynticks_nmi & 0x1);
}

/**
 * rcu_irq_enter - inform RCU of entry to hard irq context
 *
 * If the CPU was idle with dynamic ticks active, this updates the
 * rdtp->dynticks to let the RCU handling know that the CPU is active.
 */
void rcu_irq_enter(void)
{
        struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);

        if (rdtp->dynticks_nesting++)
                return;
        rdtp->dynticks++;
        WARN_ON_ONCE(!(rdtp->dynticks & 0x1));
        smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
}

/**
 * rcu_irq_exit - inform RCU of exit from hard irq context
 *
 * If the CPU was idle with dynamic ticks active, update the rdp->dynticks
 * to put let the RCU handling be aware that the CPU is going back to idle
 * with no ticks.
 */
void rcu_irq_exit(void)
{
        struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);

        if (--rdtp->dynticks_nesting)
                return;
        smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
        rdtp->dynticks++;
        WARN_ON_ONCE(rdtp->dynticks & 0x1);

        /* If the interrupt queued a callback, get out of dyntick mode. */
        if (__get_cpu_var(rcu_sched_data).nxtlist ||
            __get_cpu_var(rcu_bh_data).nxtlist)
                set_need_resched();
}

#ifdef CONFIG_SMP

/*
 * Snapshot the specified CPU's dynticks counter so that we can later
 * credit them with an implicit quiescent state.  Return 1 if this CPU
 * is in dynticks idle mode, which is an extended quiescent state.
 */
static int dyntick_save_progress_counter(struct rcu_data *rdp)
{
        int ret;
        int snap;
        int snap_nmi;

        snap = rdp->dynticks->dynticks;
        snap_nmi = rdp->dynticks->dynticks_nmi;
        smp_mb();       /* Order sampling of snap with end of grace period. */
        rdp->dynticks_snap = snap;
        rdp->dynticks_nmi_snap = snap_nmi;
        ret = ((snap & 0x1) == 0) && ((snap_nmi & 0x1) == 0);
        if (ret)
                rdp->dynticks_fqs++;
        return ret;
}

/*
 * Return true if the specified CPU has passed through a quiescent
 * state by virtue of being in or having passed through an dynticks
 * idle state since the last call to dyntick_save_progress_counter()
 * for this same CPU.
 */
static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
{
        long curr;
        long curr_nmi;
        long snap;
        long snap_nmi;

        curr = rdp->dynticks->dynticks;
        snap = rdp->dynticks_snap;
        curr_nmi = rdp->dynticks->dynticks_nmi;
        snap_nmi = rdp->dynticks_nmi_snap;
        smp_mb(); /* force ordering with cpu entering/leaving dynticks. */

        /*
         * If the CPU passed through or entered a dynticks idle phase with
         * no active irq/NMI handlers, then we can safely pretend that the CPU
         * already acknowledged the request to pass through a quiescent
         * state.  Either way, that CPU cannot possibly be in an RCU
         * read-side critical section that started before the beginning
         * of the current RCU grace period.
         */
        if ((curr != snap || (curr & 0x1) == 0) &&
            (curr_nmi != snap_nmi || (curr_nmi & 0x1) == 0)) {
                rdp->dynticks_fqs++;
                return 1;
        }

        /* Go check for the CPU being offline. */
        return rcu_implicit_offline_qs(rdp);
}

#endif /* #ifdef CONFIG_SMP */

#else /* #ifdef CONFIG_NO_HZ */

#ifdef CONFIG_SMP

static int dyntick_save_progress_counter(struct rcu_data *rdp)
{
        return 0;
}

static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
{
        return rcu_implicit_offline_qs(rdp);
}

#endif /* #ifdef CONFIG_SMP */

#endif /* #else #ifdef CONFIG_NO_HZ */

#ifdef CONFIG_RCU_CPU_STALL_DETECTOR

static void record_gp_stall_check_time(struct rcu_state *rsp)
{
        rsp->gp_start = jiffies;
        rsp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_CHECK;
}

static void print_other_cpu_stall(struct rcu_state *rsp)
{
        int cpu;
        long delta;
        unsigned long flags;
        struct rcu_node *rnp = rcu_get_root(rsp);

        /* Only let one CPU complain about others per time interval. */

        spin_lock_irqsave(&rnp->lock, flags);
        delta = jiffies - rsp->jiffies_stall;
        if (delta < RCU_STALL_RAT_DELAY || !rcu_gp_in_progress(rsp)) {
                spin_unlock_irqrestore(&rnp->lock, flags);
                return;
        }
        rsp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_RECHECK;

        /*
         * Now rat on any tasks that got kicked up to the root rcu_node
         * due to CPU offlining.
         */
        rcu_print_task_stall(rnp);
        spin_unlock_irqrestore(&rnp->lock, flags);

        /* OK, time to rat on our buddy... */

        printk(KERN_ERR "INFO: RCU detected CPU stalls:");
        rcu_for_each_leaf_node(rsp, rnp) {
                rcu_print_task_stall(rnp);
                if (rnp->qsmask == 0)
                        continue;
                for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
                        if (rnp->qsmask & (1UL << cpu))
                                printk(" %d", rnp->grplo + cpu);
        }
        printk(" (detected by %d, t=%ld jiffies)\n",
               smp_processor_id(), (long)(jiffies - rsp->gp_start));
        trigger_all_cpu_backtrace();

        force_quiescent_state(rsp, 0);  /* Kick them all. */
}

static void print_cpu_stall(struct rcu_state *rsp)
{
        unsigned long flags;
        struct rcu_node *rnp = rcu_get_root(rsp);

        printk(KERN_ERR "INFO: RCU detected CPU %d stall (t=%lu jiffies)\n",
                        smp_processor_id(), jiffies - rsp->gp_start);
        trigger_all_cpu_backtrace();

        spin_lock_irqsave(&rnp->lock, flags);
        if ((long)(jiffies - rsp->jiffies_stall) >= 0)
                rsp->jiffies_stall =
                        jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
        spin_unlock_irqrestore(&rnp->lock, flags);

        set_need_resched();  /* kick ourselves to get things going. */
}

static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
{
        long delta;
        struct rcu_node *rnp;

        delta = jiffies - rsp->jiffies_stall;
        rnp = rdp->mynode;
        if ((rnp->qsmask & rdp->grpmask) && delta >= 0) {

                /* We haven't checked in, so go dump stack. */
                print_cpu_stall(rsp);

        } else if (rcu_gp_in_progress(rsp) && delta >= RCU_STALL_RAT_DELAY) {

                /* They had two time units to dump stack, so complain. */
                print_other_cpu_stall(rsp);
        }
}

#else /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */

static void record_gp_stall_check_time(struct rcu_state *rsp)
{
}

static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
{
}

#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */

/*
 * Update CPU-local rcu_data state to record the newly noticed grace period.
 * This is used both when we started the grace period and when we notice
 * that someone else started the grace period.  The caller must hold the
 * ->lock of the leaf rcu_node structure corresponding to the current CPU,
 *  and must have irqs disabled.
 */
static void __note_new_gpnum(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
{
        if (rdp->gpnum != rnp->gpnum) {
                rdp->qs_pending = 1;
                rdp->passed_quiesc = 0;
                rdp->gpnum = rnp->gpnum;
        }
}

static void note_new_gpnum(struct rcu_state *rsp, struct rcu_data *rdp)
{
        unsigned long flags;
        struct rcu_node *rnp;

        local_irq_save(flags);
        rnp = rdp->mynode;
        if (rdp->gpnum == ACCESS_ONCE(rnp->gpnum) || /* outside lock. */
            !spin_trylock(&rnp->lock)) { /* irqs already off, retry later. */
                local_irq_restore(flags);
                return;
        }
        __note_new_gpnum(rsp, rnp, rdp);
        spin_unlock_irqrestore(&rnp->lock, flags);
}

/*
 * Did someone else start a new RCU grace period start since we last
 * checked?  Update local state appropriately if so.  Must be called
 * on the CPU corresponding to rdp.
 */
static int
check_for_new_grace_period(struct rcu_state *rsp, struct rcu_data *rdp)
{
        unsigned long flags;
        int ret = 0;

        local_irq_save(flags);
        if (rdp->gpnum != rsp->gpnum) {
                note_new_gpnum(rsp, rdp);
                ret = 1;
        }
        local_irq_restore(flags);
        return ret;
}

/*
 * Advance this CPU's callbacks, but only if the current grace period
 * has ended.  This may be called only from the CPU to whom the rdp
 * belongs.  In addition, the corresponding leaf rcu_node structure's
 * ->lock must be held by the caller, with irqs disabled.
 */
static void
__rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
{
        /* Did another grace period end? */
        if (rdp->completed != rnp->completed) {

                /* Advance callbacks.  No harm if list empty. */
                rdp->nxttail[RCU_DONE_TAIL] = rdp->nxttail[RCU_WAIT_TAIL];
                rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_READY_TAIL];
                rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];

                /* Remember that we saw this grace-period completion. */
                rdp->completed = rnp->completed;
        }
}

/*
 * Advance this CPU's callbacks, but only if the current grace period
 * has ended.  This may be called only from the CPU to whom the rdp
 * belongs.
 */
static void
rcu_process_gp_end(struct rcu_state *rsp, struct rcu_data *rdp)
{
        unsigned long flags;
        struct rcu_node *rnp;

        local_irq_save(flags);
        rnp = rdp->mynode;
        if (rdp->completed == ACCESS_ONCE(rnp->completed) || /* outside lock. */
            !spin_trylock(&rnp->lock)) { /* irqs already off, retry later. */
                local_irq_restore(flags);
                return;
        }
        __rcu_process_gp_end(rsp, rnp, rdp);
        spin_unlock_irqrestore(&rnp->lock, flags);
}

/*
 * Do per-CPU grace-period initialization for running CPU.  The caller
 * must hold the lock of the leaf rcu_node structure corresponding to
 * this CPU.
 */
static void
rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
{
        /* Prior grace period ended, so advance callbacks for current CPU. */
        __rcu_process_gp_end(rsp, rnp, rdp);

        /*
         * Because this CPU just now started the new grace period, we know
         * that all of its callbacks will be covered by this upcoming grace
         * period, even the ones that were registered arbitrarily recently.
         * Therefore, advance all outstanding callbacks to RCU_WAIT_TAIL.
         *
         * Other CPUs cannot be sure exactly when the grace period started.
         * Therefore, their recently registered callbacks must pass through
         * an additional RCU_NEXT_READY stage, so that they will be handled
         * by the next RCU grace period.
         */
        rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
        rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];

        /* Set state so that this CPU will detect the next quiescent state. */
        __note_new_gpnum(rsp, rnp, rdp);
}

/*
 * Start a new RCU grace period if warranted, re-initializing the hierarchy
 * in preparation for detecting the next grace period.  The caller must hold
 * the root node's ->lock, which is released before return.  Hard irqs must
 * be disabled.
 */
static void
rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
        __releases(rcu_get_root(rsp)->lock)
{
        struct rcu_data *rdp = rsp->rda[smp_processor_id()];
        struct rcu_node *rnp = rcu_get_root(rsp);

        if (!cpu_needs_another_gp(rsp, rdp) || rsp->fqs_active) {
                if (cpu_needs_another_gp(rsp, rdp))
                        rsp->fqs_need_gp = 1;
                if (rnp->completed == rsp->completed) {
                        spin_unlock_irqrestore(&rnp->lock, flags);
                        return;
                }
                spin_unlock(&rnp->lock);         /* irqs remain disabled. */

                /*
                 * Propagate new ->completed value to rcu_node structures
                 * so that other CPUs don't have to wait until the start
                 * of the next grace period to process their callbacks.
                 */
                rcu_for_each_node_breadth_first(rsp, rnp) {
                        spin_lock(&rnp->lock);   /* irqs already disabled. */
                        rnp->completed = rsp->completed;
                        spin_unlock(&rnp->lock); /* irqs remain disabled. */
                }
                local_irq_restore(flags);
                return;
        }

        /* Advance to a new grace period and initialize state. */
        rsp->gpnum++;
        WARN_ON_ONCE(rsp->signaled == RCU_GP_INIT);
        rsp->signaled = RCU_GP_INIT; /* Hold off force_quiescent_state. */
        rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
        record_gp_stall_check_time(rsp);

        /* Special-case the common single-level case. */
        if (NUM_RCU_NODES == 1) {
                rcu_preempt_check_blocked_tasks(rnp);
                rnp->qsmask = rnp->qsmaskinit;
                rnp->gpnum = rsp->gpnum;
                rnp->completed = rsp->completed;
                rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state OK. */
                rcu_start_gp_per_cpu(rsp, rnp, rdp);
                spin_unlock_irqrestore(&rnp->lock, flags);
                return;
        }

        spin_unlock(&rnp->lock);  /* leave irqs disabled. */


        /* Exclude any concurrent CPU-hotplug operations. */
        spin_lock(&rsp->onofflock);  /* irqs already disabled. */

        /*
         * Set the quiescent-state-needed bits in all the rcu_node
         * structures for all currently online CPUs in breadth-first
         * order, starting from the root rcu_node structure.  This
         * operation relies on the layout of the hierarchy within the
         * rsp->node[] array.  Note that other CPUs will access only
         * the leaves of the hierarchy, which still indicate that no
         * grace period is in progress, at least until the corresponding
         * leaf node has been initialized.  In addition, we have excluded
         * CPU-hotplug operations.
         *
         * Note that the grace period cannot complete until we finish
         * the initialization process, as there will be at least one
         * qsmask bit set in the root node until that time, namely the
         * one corresponding to this CPU, due to the fact that we have
         * irqs disabled.
         */
        rcu_for_each_node_breadth_first(rsp, rnp) {
                spin_lock(&rnp->lock);          /* irqs already disabled. */
                rcu_preempt_check_blocked_tasks(rnp);
                rnp->qsmask = rnp->qsmaskinit;
                rnp->gpnum = rsp->gpnum;
                rnp->completed = rsp->completed;
                if (rnp == rdp->mynode)
                        rcu_start_gp_per_cpu(rsp, rnp, rdp);
                spin_unlock(&rnp->lock);        /* irqs remain disabled. */
        }

        rnp = rcu_get_root(rsp);
        spin_lock(&rnp->lock);                  /* irqs already disabled. */
        rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state now OK. */
        spin_unlock(&rnp->lock);                /* irqs remain disabled. */
        spin_unlock_irqrestore(&rsp->onofflock, flags);
}

/*
 * Report a full set of quiescent states to the specified rcu_state
 * data structure.  This involves cleaning up after the prior grace
 * period and letting rcu_start_gp() start up the next grace period
 * if one is needed.  Note that the caller must hold rnp->lock, as
 * required by rcu_start_gp(), which will release it.
 */
static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
        __releases(rcu_get_root(rsp)->lock)
{
        WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
        rsp->completed = rsp->gpnum;
        rsp->signaled = RCU_GP_IDLE;
        rcu_start_gp(rsp, flags);  /* releases root node's rnp->lock. */
}

/*
 * Similar to rcu_report_qs_rdp(), for which it is a helper function.
 * Allows quiescent states for a group of CPUs to be reported at one go
 * to the specified rcu_node structure, though all the CPUs in the group
 * must be represented by the same rcu_node structure (which need not be
 * a leaf rcu_node structure, though it often will be).  That structure's
 * lock must be held upon entry, and it is released before return.
 */
static void
rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
                  struct rcu_node *rnp, unsigned long flags)
        __releases(rnp->lock)
{
        struct rcu_node *rnp_c;

        /* Walk up the rcu_node hierarchy. */
        for (;;) {
                if (!(rnp->qsmask & mask)) {

                        /* Our bit has already been cleared, so done. */
                        spin_unlock_irqrestore(&rnp->lock, flags);
                        return;
                }
                rnp->qsmask &= ~mask;
                if (rnp->qsmask != 0 || rcu_preempted_readers(rnp)) {

                        /* Other bits still set at this level, so done. */
                        spin_unlock_irqrestore(&rnp->lock, flags);
                        return;
                }
                mask = rnp->grpmask;
                if (rnp->parent == NULL) {

                        /* No more levels.  Exit loop holding root lock. */

                        break;
                }
                spin_unlock_irqrestore(&rnp->lock, flags);
                rnp_c = rnp;
                rnp = rnp->parent;
                spin_lock_irqsave(&rnp->lock, flags);
                WARN_ON_ONCE(rnp_c->qsmask);
        }

        /*
         * Get here if we are the last CPU to pass through a quiescent
         * state for this grace period.  Invoke rcu_report_qs_rsp()
         * to clean up and start the next grace period if one is needed.
         */
        rcu_report_qs_rsp(rsp, flags); /* releases rnp->lock. */
}

/*
 * Record a quiescent state for the specified CPU to that CPU's rcu_data
 * structure.  This must be either called from the specified CPU, or
 * called when the specified CPU is known to be offline (and when it is
 * also known that no other CPU is concurrently trying to help the offline
 * CPU).  The lastcomp argument is used to make sure we are still in the
 * grace period of interest.  We don't want to end the current grace period
 * based on quiescent states detected in an earlier grace period!
 */
static void
rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp, long lastcomp)
{
        unsigned long flags;
        unsigned long mask;
        struct rcu_node *rnp;

        rnp = rdp->mynode;
        spin_lock_irqsave(&rnp->lock, flags);
        if (lastcomp != rnp->completed) {

                /*
                 * Someone beat us to it for this grace period, so leave.
                 * The race with GP start is resolved by the fact that we
                 * hold the leaf rcu_node lock, so that the per-CPU bits
                 * cannot yet be initialized -- so we would simply find our
                 * CPU's bit already cleared in rcu_report_qs_rnp() if this
                 * race occurred.
                 */
                rdp->passed_quiesc = 0; /* try again later! */
                spin_unlock_irqrestore(&rnp->lock, flags);
                return;
        }
        mask = rdp->grpmask;
        if ((rnp->qsmask & mask) == 0) {
                spin_unlock_irqrestore(&rnp->lock, flags);
        } else {
                rdp->qs_pending = 0;

                /*
                 * This GP can't end until cpu checks in, so all of our
                 * callbacks can be processed during the next GP.
                 */
                rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];

                rcu_report_qs_rnp(mask, rsp, rnp, flags); /* rlses rnp->lock */
        }
}

/*
 * Check to see if there is a new grace period of which this CPU
 * is not yet aware, and if so, set up local rcu_data state for it.
 * Otherwise, see if this CPU has just passed through its first
 * quiescent state for this grace period, and record that fact if so.
 */
static void
rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
{
        /* If there is now a new grace period, record and return. */
        if (check_for_new_grace_period(rsp, rdp))
                return;

        /*
         * Does this CPU still need to do its part for current grace period?
         * If no, return and let the other CPUs do their part as well.
         */
        if (!rdp->qs_pending)
                return;

        /*
         * Was there a quiescent state since the beginning of the grace
         * period? If no, then exit and wait for the next call.
         */
        if (!rdp->passed_quiesc)
                return;

        /*
         * Tell RCU we are done (but rcu_report_qs_rdp() will be the
         * judge of that).
         */
        rcu_report_qs_rdp(rdp->cpu, rsp, rdp, rdp->passed_quiesc_completed);
}

#ifdef CONFIG_HOTPLUG_CPU

/*
 * Move a dying CPU's RCU callbacks to the ->orphan_cbs_list for the
 * specified flavor of RCU.  The callbacks will be adopted by the next
 * _rcu_barrier() invocation or by the CPU_DEAD notifier, whichever
 * comes first.  Because this is invoked from the CPU_DYING notifier,
 * irqs are already disabled.
 */
static void rcu_send_cbs_to_orphanage(struct rcu_state *rsp)
{
        int i;
        struct rcu_data *rdp = rsp->rda[smp_processor_id()];

        if (rdp->nxtlist == NULL)
                return;  /* irqs disabled, so comparison is stable. */
        spin_lock(&rsp->onofflock);  /* irqs already disabled. */
        *rsp->orphan_cbs_tail = rdp->nxtlist;
        rsp->orphan_cbs_tail = rdp->nxttail[RCU_NEXT_TAIL];
        rdp->nxtlist = NULL;
        for (i = 0; i < RCU_NEXT_SIZE; i++)
                rdp->nxttail[i] = &rdp->nxtlist;
        rsp->orphan_qlen += rdp->qlen;
        rdp->qlen = 0;
        spin_unlock(&rsp->onofflock);  /* irqs remain disabled. */
}

/*
 * Adopt previously orphaned RCU callbacks.
 */
static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
{
        unsigned long flags;
        struct rcu_data *rdp;

        spin_lock_irqsave(&rsp->onofflock, flags);
        rdp = rsp->rda[smp_processor_id()];
        if (rsp->orphan_cbs_list == NULL) {
                spin_unlock_irqrestore(&rsp->onofflock, flags);
                return;
        }
        *rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_cbs_list;
        rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_cbs_tail;
        rdp->qlen += rsp->orphan_qlen;
        rsp->orphan_cbs_list = NULL;
        rsp->orphan_cbs_tail = &rsp->orphan_cbs_list;
        rsp->orphan_qlen = 0;
        spin_unlock_irqrestore(&rsp->onofflock, flags);
}

/*
 * Remove the outgoing CPU from the bitmasks in the rcu_node hierarchy
 * and move all callbacks from the outgoing CPU to the current one.
 */
static void __rcu_offline_cpu(int cpu, struct rcu_state *rsp)
{
        unsigned long flags;
        unsigned long mask;
        int need_report = 0;
        struct rcu_data *rdp = rsp->rda[cpu];
        struct rcu_node *rnp;

        /* Exclude any attempts to start a new grace period. */
        spin_lock_irqsave(&rsp->onofflock, flags);

        /* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
        rnp = rdp->mynode;      /* this is the outgoing CPU's rnp. */
        mask = rdp->grpmask;    /* rnp->grplo is constant. */
        do {
                spin_lock(&rnp->lock);          /* irqs already disabled. */
                rnp->qsmaskinit &= ~mask;
                if (rnp->qsmaskinit != 0) {
                        if (rnp != rdp->mynode)
                                spin_unlock(&rnp->lock); /* irqs remain disabled. */
                        break;
                }
                if (rnp == rdp->mynode)
                        need_report = rcu_preempt_offline_tasks(rsp, rnp, rdp);
                else
                        spin_unlock(&rnp->lock); /* irqs remain disabled. */
                mask = rnp->grpmask;
                rnp = rnp->parent;
        } while (rnp != NULL);

        /*
         * We still hold the leaf rcu_node structure lock here, and
         * irqs are still disabled.  The reason for this subterfuge is
         * because invoking rcu_report_unblock_qs_rnp() with ->onofflock
         * held leads to deadlock.
         */
        spin_unlock(&rsp->onofflock); /* irqs remain disabled. */
        rnp = rdp->mynode;
        if (need_report & RCU_OFL_TASKS_NORM_GP)
                rcu_report_unblock_qs_rnp(rnp, flags);
        else
                spin_unlock_irqrestore(&rnp->lock, flags);
        if (need_report & RCU_OFL_TASKS_EXP_GP)
                rcu_report_exp_rnp(rsp, rnp);

        rcu_adopt_orphan_cbs(rsp);
}

/*
 * Remove the specified CPU from the RCU hierarchy and move any pending
 * callbacks that it might have to the current CPU.  This code assumes
 * that at least one CPU in the system will remain running at all times.
 * Any attempt to offline -all- CPUs is likely to strand RCU callbacks.
 */
static void rcu_offline_cpu(int cpu)
{
        __rcu_offline_cpu(cpu, &rcu_sched_state);
        __rcu_offline_cpu(cpu, &rcu_bh_state);
        rcu_preempt_offline_cpu(cpu);
}

#else /* #ifdef CONFIG_HOTPLUG_CPU */

static void rcu_send_cbs_to_orphanage(struct rcu_state *rsp)
{
}

static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
{
}

static void rcu_offline_cpu(int cpu)
{
}

#endif /* #else #ifdef CONFIG_HOTPLUG_CPU */

/*
 * Invoke any RCU callbacks that have made it to the end of their grace
 * period.  Thottle as specified by rdp->blimit.
 */
static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
{
        unsigned long flags;
        struct rcu_head *next, *list, **tail;
        int count;

        /* If no callbacks are ready, just return.*/
        if (!cpu_has_callbacks_ready_to_invoke(rdp))
                return;

        /*
         * Extract the list of ready callbacks, disabling to prevent
         * races with call_rcu() from interrupt handlers.
         */
        local_irq_save(flags);
        list = rdp->nxtlist;
        rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
        *rdp->nxttail[RCU_DONE_TAIL] = NULL;
        tail = rdp->nxttail[RCU_DONE_TAIL];
        for (count = RCU_NEXT_SIZE - 1; count >= 0; count--)
                if (rdp->nxttail[count] == rdp->nxttail[RCU_DONE_TAIL])
                        rdp->nxttail[count] = &rdp->nxtlist;
        local_irq_restore(flags);

        /* Invoke callbacks. */
        count = 0;
        while (list) {
                next = list->next;
                prefetch(next);
                list->func(list);
                list = next;
                if (++count >= rdp->blimit)
                        break;
        }

        local_irq_save(flags);

        /* Update count, and requeue any remaining callbacks. */
        rdp->qlen -= count;
        if (list != NULL) {
                *tail = rdp->nxtlist;
                rdp->nxtlist = list;
                for (count = 0; count < RCU_NEXT_SIZE; count++)
                        if (&rdp->nxtlist == rdp->nxttail[count])
                                rdp->nxttail[count] = tail;
                        else
                                break;
        }

        /* Reinstate batch limit if we have worked down the excess. */
        if (rdp->blimit == LONG_MAX && rdp->qlen <= qlowmark)
                rdp->blimit = blimit;

        /* Reset ->qlen_last_fqs_check trigger if enough CBs have drained. */
        if (rdp->qlen == 0 && rdp->qlen_last_fqs_check != 0) {
                rdp->qlen_last_fqs_check = 0;
                rdp->n_force_qs_snap = rsp->n_force_qs;
        } else if (rdp->qlen < rdp->qlen_last_fqs_check - qhimark)
                rdp->qlen_last_fqs_check = rdp->qlen;

        local_irq_restore(flags);

        /* Re-raise the RCU softirq if there are callbacks remaining. */
        if (cpu_has_callbacks_ready_to_invoke(rdp))
                raise_softirq(RCU_SOFTIRQ);
}

/*
 * Check to see if this CPU is in a non-context-switch quiescent state
 * (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
 * Also schedule the RCU softirq handler.
 *
 * This function must be called with hardirqs disabled.  It is normally
 * invoked from the scheduling-clock interrupt.  If rcu_pending returns
 * false, there is no point in invoking rcu_check_callbacks().
 */
void rcu_check_callbacks(int cpu, int user)
{
        if (!rcu_pending(cpu))
                return; /* if nothing for RCU to do. */
        if (user ||
            (idle_cpu(cpu) && rcu_scheduler_active &&
             !in_softirq() && hardirq_count() <= (1 << HARDIRQ_SHIFT))) {

                /*
                 * Get here if this CPU took its interrupt from user
                 * mode or from the idle loop, and if this is not a
                 * nested interrupt.  In this case, the CPU is in
                 * a quiescent state, so note it.
                 *
                 * No memory barrier is required here because both
                 * rcu_sched_qs() and rcu_bh_qs() reference only CPU-local
                 * variables that other CPUs neither access nor modify,
                 * at least not while the corresponding CPU is online.
                 */

                rcu_sched_qs(cpu);
                rcu_bh_qs(cpu);

        } else if (!in_softirq()) {

                /*
                 * Get here if this CPU did not take its interrupt from
                 * softirq, in other words, if it is not interrupting
                 * a rcu_bh read-side critical section.  This is an _bh
                 * critical section, so note it.
                 */

                rcu_bh_qs(cpu);
        }
        rcu_preempt_check_callbacks(cpu);
        raise_softirq(RCU_SOFTIRQ);
}

#ifdef CONFIG_SMP

/*
 * Scan the leaf rcu_node structures, processing dyntick state for any that
 * have not yet encountered a quiescent state, using the function specified.
 * The caller must have suppressed start of new grace periods.
 */
static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
{
        unsigned long bit;
        int cpu;
        unsigned long flags;
        unsigned long mask;
        struct rcu_node *rnp;

        rcu_for_each_leaf_node(rsp, rnp) {
                mask = 0;
                spin_lock_irqsave(&rnp->lock, flags);
                if (!rcu_gp_in_progress(rsp)) {
                        spin_unlock_irqrestore(&rnp->lock, flags);
                        return;
                }
                if (rnp->qsmask == 0) {
                        spin_unlock_irqrestore(&rnp->lock, flags);
                        continue;
                }
                cpu = rnp->grplo;
                bit = 1;
                for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {
                        if ((rnp->qsmask & bit) != 0 && f(rsp->rda[cpu]))
                                mask |= bit;
                }
                if (mask != 0) {

                        /* rcu_report_qs_rnp() releases rnp->lock. */
                        rcu_report_qs_rnp(mask, rsp, rnp, flags);
                        continue;
                }
                spin_unlock_irqrestore(&rnp->lock, flags);
        }
}

/*
 * Force quiescent states on reluctant CPUs, and also detect which
 * CPUs are in dyntick-idle mode.
 */
static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
{
        unsigned long flags;
        struct rcu_node *rnp = rcu_get_root(rsp);

        if (!rcu_gp_in_progress(rsp))
                return;  /* No grace period in progress, nothing to force. */
        if (!spin_trylock_irqsave(&rsp->fqslock, flags)) {
                rsp->n_force_qs_lh++; /* Inexact, can lose counts.  Tough! */
                return; /* Someone else is already on the job. */
        }
        if (relaxed &&
            (long)(rsp->jiffies_force_qs - jiffies) >= 0)
                goto unlock_fqs_ret; /* no emergency and done recently. */
        rsp->n_force_qs++;
        spin_lock(&rnp->lock);  /* irqs already disabled */
        rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
        if(!rcu_gp_in_progress(rsp)) {
                rsp->n_force_qs_ngp++;
                spin_unlock(&rnp->lock);  /* irqs remain disabled */
                goto unlock_fqs_ret;  /* no GP in progress, time updated. */
        }
        rsp->fqs_active = 1;
        switch (rsp->signaled) {
        case RCU_GP_IDLE:
        case RCU_GP_INIT:

                break; /* grace period idle or initializing, ignore. */

        case RCU_SAVE_DYNTICK:

                spin_unlock(&rnp->lock);  /* irqs remain disabled */
                if (RCU_SIGNAL_INIT != RCU_SAVE_DYNTICK)
                        break; /* So gcc recognizes the dead code. */

                /* Record dyntick-idle state. */
                force_qs_rnp(rsp, dyntick_save_progress_counter);
                spin_lock(&rnp->lock);  /* irqs already disabled */
                if (rcu_gp_in_progress(rsp))
                        rsp->signaled = RCU_FORCE_QS;
                break;

        case RCU_FORCE_QS:

                /* Check dyntick-idle state, send IPI to laggarts. */
                spin_unlock(&rnp->lock);  /* irqs remain disabled */
                force_qs_rnp(rsp, rcu_implicit_dynticks_qs);

                /* Leave state in case more forcing is required. */

                spin_lock(&rnp->lock);  /* irqs already disabled */
                break;
        }
        rsp->fqs_active = 0;
        if (rsp->fqs_need_gp) {
                spin_unlock(&rsp->fqslock); /* irqs remain disabled */
                rsp->fqs_need_gp = 0;
                rcu_start_gp(rsp, flags); /* releases rnp->lock */
                return;
        }
        spin_unlock(&rnp->lock);  /* irqs remain disabled */
unlock_fqs_ret:
        spin_unlock_irqrestore(&rsp->fqslock, flags);
}

#else /* #ifdef CONFIG_SMP */

static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
{
        set_need_resched();
}

#endif /* #else #ifdef CONFIG_SMP */

/*
 * This does the RCU processing work from softirq context for the
 * specified rcu_state and rcu_data structures.  This may be called
 * only from the CPU to whom the rdp belongs.
 */
static void
__rcu_process_callbacks(struct rcu_state *rsp, struct rcu_data *rdp)
{
        unsigned long flags;

        WARN_ON_ONCE(rdp->beenonline == 0);

        /*
         * If an RCU GP has gone long enough, go check for dyntick
         * idle CPUs and, if needed, send resched IPIs.
         */
        if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)
                force_quiescent_state(rsp, 1);

        /*
         * Advance callbacks in response to end of earlier grace
         * period that some other CPU ended.
         */
        rcu_process_gp_end(rsp, rdp);

        /* Update RCU state based on any recent quiescent states. */
        rcu_check_quiescent_state(rsp, rdp);

        /* Does this CPU require a not-yet-started grace period? */
        if (cpu_needs_another_gp(rsp, rdp)) {
                spin_lock_irqsave(&rcu_get_root(rsp)->lock, flags);
                rcu_start_gp(rsp, flags);  /* releases above lock */
        }

        /* If there are callbacks ready, invoke them. */
        rcu_do_batch(rsp, rdp);
}

/*
 * Do softirq processing for the current CPU.
 */
static void rcu_process_callbacks(struct softirq_action *unused)
{
        /*
         * Memory references from any prior RCU read-side critical sections
         * executed by the interrupted code must be seen before any RCU
         * grace-period manipulations below.
         */
        smp_mb(); /* See above block comment. */

        __rcu_process_callbacks(&rcu_sched_state,
                                &__get_cpu_var(rcu_sched_data));
        __rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
        rcu_preempt_process_callbacks();

        /*
         * Memory references from any later RCU read-side critical sections
         * executed by the interrupted code must be seen after any RCU
         * grace-period manipulations above.
         */
        smp_mb(); /* See above block comment. */
}

static void
__call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
           struct rcu_state *rsp)
{
        unsigned long flags;
        struct rcu_data *rdp;

        head->func = func;
        head->next = NULL;

        smp_mb(); /* Ensure RCU update seen before callback registry. */

        /*
         * Opportunistically note grace-period endings and beginnings.
         * Note that we might see a beginning right after we see an
         * end, but never vice versa, since this CPU has to pass through
         * a quiescent state betweentimes.
         */
        local_irq_save(flags);
        rdp = rsp->rda[smp_processor_id()];
        rcu_process_gp_end(rsp, rdp);
        check_for_new_grace_period(rsp, rdp);

        /* Add the callback to our list. */
        *rdp->nxttail[RCU_NEXT_TAIL] = head;
        rdp->nxttail[RCU_NEXT_TAIL] = &head->next;

        /* Start a new grace period if one not already started. */
        if (!rcu_gp_in_progress(rsp)) {
                unsigned long nestflag;
                struct rcu_node *rnp_root = rcu_get_root(rsp);

                spin_lock_irqsave(&rnp_root->lock, nestflag);
                rcu_start_gp(rsp, nestflag);  /* releases rnp_root->lock. */
        }

        /*
         * Force the grace period if too many callbacks or too long waiting.
         * Enforce hysteresis, and don't invoke force_quiescent_state()
         * if some other CPU has recently done so.  Also, don't bother
         * invoking force_quiescent_state() if the newly enqueued callback
         * is the only one waiting for a grace period to complete.
         */
        if (unlikely(++rdp->qlen > rdp->qlen_last_fqs_check + qhimark)) {
                rdp->blimit = LONG_MAX;
                if (rsp->n_force_qs == rdp->n_force_qs_snap &&
                    *rdp->nxttail[RCU_DONE_TAIL] != head)
                        force_quiescent_state(rsp, 0);
                rdp->n_force_qs_snap = rsp->n_force_qs;
                rdp->qlen_last_fqs_check = rdp->qlen;
        } else if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)
                force_quiescent_state(rsp, 1);
        local_irq_restore(flags);
}

/*
 * Queue an RCU-sched callback for invocation after a grace period.
 */
void call_rcu_sched(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
{
        __call_rcu(head, func, &rcu_sched_state);
}
EXPORT_SYMBOL_GPL(call_rcu_sched);

/*
 * Queue an RCU for invocation after a quicker grace period.
 */
void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
{
        __call_rcu(head, func, &rcu_bh_state);
}
EXPORT_SYMBOL_GPL(call_rcu_bh);

/**
 * synchronize_sched - wait until an rcu-sched grace period has elapsed.
 *
 * Control will return to the caller some time after a full rcu-sched
 * grace period has elapsed, in other words after all currently executing
 * rcu-sched read-side critical sections have completed.   These read-side
 * critical sections are delimited by rcu_read_lock_sched() and
 * rcu_read_unlock_sched(), and may be nested.  Note that preempt_disable(),
 * local_irq_disable(), and so on may be used in place of
 * rcu_read_lock_sched().
 *
 * This means that all preempt_disable code sequences, including NMI and
 * hardware-interrupt handlers, in progress on entry will have completed
 * before this primitive returns.  However, this does not guarantee that
 * softirq handlers will have completed, since in some kernels, these
 * handlers can run in process context, and can block.
 *
 * This primitive provides the guarantees made by the (now removed)
 * synchronize_kernel() API.  In contrast, synchronize_rcu() only
 * guarantees that rcu_read_lock() sections will have completed.
 * In "classic RCU", these two guarantees happen to be one and
 * the same, but can differ in realtime RCU implementations.
 */
void synchronize_sched(void)
{
        struct rcu_synchronize rcu;

        if (rcu_blocking_is_gp())
                return;

        init_completion(&rcu.completion);
        /* Will wake me after RCU finished. */
        call_rcu_sched(&rcu.head, wakeme_after_rcu);
        /* Wait for it. */
        wait_for_completion(&rcu.completion);
}
EXPORT_SYMBOL_GPL(synchronize_sched);

/**
 * synchronize_rcu_bh - wait until an rcu_bh grace period has elapsed.
 *
 * Control will return to the caller some time after a full rcu_bh grace
 * period has elapsed, in other words after all currently executing rcu_bh
 * read-side critical sections have completed.  RCU read-side critical
 * sections are delimited by rcu_read_lock_bh() and rcu_read_unlock_bh(),
 * and may be nested.
 */
void synchronize_rcu_bh(void)
{
        struct rcu_synchronize rcu;

        if (rcu_blocking_is_gp())
                return;

        init_completion(&rcu.completion);
        /* Will wake me after RCU finished. */
        call_rcu_bh(&rcu.head, wakeme_after_rcu);
        /* Wait for it. */
        wait_for_completion(&rcu.completion);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_bh);

/*
 * Check to see if there is any immediate RCU-related work to be done
 * by the current CPU, for the specified type of RCU, returning 1 if so.
 * The checks are in order of increasing expense: checks that can be
 * carried out against CPU-local state are performed first.  However,
 * we must check for CPU stalls first, else we might not get a chance.
 */
static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
{
        struct rcu_node *rnp = rdp->mynode;

        rdp->n_rcu_pending++;

        /* Check for CPU stalls, if enabled. */
        check_cpu_stall(rsp, rdp);

        /* Is the RCU core waiting for a quiescent state from this CPU? */
        if (rdp->qs_pending) {
                rdp->n_rp_qs_pending++;
                return 1;
        }

        /* Does this CPU have callbacks ready to invoke? */
        if (cpu_has_callbacks_ready_to_invoke(rdp)) {
                rdp->n_rp_cb_ready++;
                return 1;
        }

        /* Has RCU gone idle with this CPU needing another grace period? */
        if (cpu_needs_another_gp(rsp, rdp)) {
                rdp->n_rp_cpu_needs_gp++;
                return 1;
        }

        /* Has another RCU grace period completed?  */
        if (ACCESS_ONCE(rnp->completed) != rdp->completed) { /* outside lock */
                rdp->n_rp_gp_completed++;
                return 1;
        }

        /* Has a new RCU grace period started? */
        if (ACCESS_ONCE(rnp->gpnum) != rdp->gpnum) { /* outside lock */
                rdp->n_rp_gp_started++;
                return 1;
        }

        /* Has an RCU GP gone long enough to send resched IPIs &c? */
        if (rcu_gp_in_progress(rsp) &&
            ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)) {
                rdp->n_rp_need_fqs++;
                return 1;
        }

        /* nothing to do */
        rdp->n_rp_need_nothing++;
        return 0;
}

/*
 * Check to see if there is any immediate RCU-related work to be done
 * by the current CPU, returning 1 if so.  This function is part of the
 * RCU implementation; it is -not- an exported member of the RCU API.
 */
static int rcu_pending(int cpu)
{
        return __rcu_pending(&rcu_sched_state, &per_cpu(rcu_sched_data, cpu)) ||
               __rcu_pending(&rcu_bh_state, &per_cpu(rcu_bh_data, cpu)) ||
               rcu_preempt_pending(cpu);
}

/*
 * 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.
 */
int rcu_needs_cpu(int cpu)
{
        /* RCU callbacks either ready or pending? */
        return per_cpu(rcu_sched_data, cpu).nxtlist ||
               per_cpu(rcu_bh_data, cpu).nxtlist ||
               rcu_preempt_needs_cpu(cpu);
}

/*
 * This function is invoked towards the end of the scheduler's initialization
 * process.  Before this is called, the idle task might contain
 * RCU read-side critical sections (during which time, this idle
 * task is booting the system).  After this function is called, the
 * idle tasks are prohibited from containing RCU read-side critical
 * sections.
 */
void rcu_scheduler_starting(void)
{
        WARN_ON(num_online_cpus() != 1);
        WARN_ON(nr_context_switches() > 0);
        rcu_scheduler_active = 1;
}

static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
static atomic_t rcu_barrier_cpu_count;
static DEFINE_MUTEX(rcu_barrier_mutex);
static struct completion rcu_barrier_completion;

static void rcu_barrier_callback(struct rcu_head *notused)
{
        if (atomic_dec_and_test(&rcu_barrier_cpu_count))
                complete(&rcu_barrier_completion);
}

/*
 * Called with preemption disabled, and from cross-cpu IRQ context.
 */
static void rcu_barrier_func(void *type)
{
        int cpu = smp_processor_id();
        struct rcu_head *head = &per_cpu(rcu_barrier_head, cpu);
        void (*call_rcu_func)(struct rcu_head *head,
                              void (*func)(struct rcu_head *head));

        atomic_inc(&rcu_barrier_cpu_count);
        call_rcu_func = type;
        call_rcu_func(head, rcu_barrier_callback);
}

/*
 * Orchestrate the specified type of RCU barrier, waiting for all
 * RCU callbacks of the specified type to complete.
 */
static void _rcu_barrier(struct rcu_state *rsp,
                         void (*call_rcu_func)(struct rcu_head *head,
                                               void (*func)(struct rcu_head *head)))
{
        BUG_ON(in_interrupt());
        /* Take mutex to serialize concurrent rcu_barrier() requests. */
        mutex_lock(&rcu_barrier_mutex);
        init_completion(&rcu_barrier_completion);
        /*
         * Initialize rcu_barrier_cpu_count to 1, then invoke
         * rcu_barrier_func() on each CPU, so that each CPU also has
         * incremented rcu_barrier_cpu_count.  Only then is it safe to
         * decrement rcu_barrier_cpu_count -- otherwise the first CPU
         * might complete its grace period before all of the other CPUs
         * did their increment, causing this function to return too
         * early.
         */
        atomic_set(&rcu_barrier_cpu_count, 1);
        preempt_disable(); /* stop CPU_DYING from filling orphan_cbs_list */
        rcu_adopt_orphan_cbs(rsp);
        on_each_cpu(rcu_barrier_func, (void *)call_rcu_func, 1);
        preempt_enable(); /* CPU_DYING can again fill orphan_cbs_list */
        if (atomic_dec_and_test(&rcu_barrier_cpu_count))
                complete(&rcu_barrier_completion);
        wait_for_completion(&rcu_barrier_completion);
        mutex_unlock(&rcu_barrier_mutex);
}

/**
 * rcu_barrier_bh - Wait until all in-flight call_rcu_bh() callbacks complete.
 */
void rcu_barrier_bh(void)
{
        _rcu_barrier(&rcu_bh_state, call_rcu_bh);
}
EXPORT_SYMBOL_GPL(rcu_barrier_bh);

/**
 * rcu_barrier_sched - Wait for in-flight call_rcu_sched() callbacks.
 */
void rcu_barrier_sched(void)
{
        _rcu_barrier(&rcu_sched_state, call_rcu_sched);
}
EXPORT_SYMBOL_GPL(rcu_barrier_sched);

/*
 * Do boot-time initialization of a CPU's per-CPU RCU data.
 */
static void __init
rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp)
{
        unsigned long flags;
        int i;
        struct rcu_data *rdp = rsp->rda[cpu];
        struct rcu_node *rnp = rcu_get_root(rsp);

        /* Set up local state, ensuring consistent view of global state. */
        spin_lock_irqsave(&rnp->lock, flags);
        rdp->grpmask = 1UL << (cpu - rdp->mynode->grplo);
        rdp->nxtlist = NULL;
        for (i = 0; i < RCU_NEXT_SIZE; i++)
                rdp->nxttail[i] = &rdp->nxtlist;
        rdp->qlen = 0;
#ifdef CONFIG_NO_HZ
        rdp->dynticks = &per_cpu(rcu_dynticks, cpu);
#endif /* #ifdef CONFIG_NO_HZ */
        rdp->cpu = cpu;
        spin_unlock_irqrestore(&rnp->lock, flags);
}

/*
 * Initialize a CPU's per-CPU RCU data.  Note that only one online or
 * offline event can be happening at a given time.  Note also that we
 * can accept some slop in the rsp->completed access due to the fact
 * that this CPU cannot possibly have any RCU callbacks in flight yet.
 */
static void __cpuinit
rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptable)
{
        unsigned long flags;
        unsigned long mask;
        struct rcu_data *rdp = rsp->rda[cpu];
        struct rcu_node *rnp = rcu_get_root(rsp);

        /* Set up local state, ensuring consistent view of global state. */
        spin_lock_irqsave(&rnp->lock, flags);
        rdp->passed_quiesc = 0;  /* We could be racing with new GP, */
        rdp->qs_pending = 1;     /*  so set up to respond to current GP. */
        rdp->beenonline = 1;     /* We have now been online. */
        rdp->preemptable = preemptable;
        rdp->qlen_last_fqs_check = 0;
        rdp->n_force_qs_snap = rsp->n_force_qs;
        rdp->blimit = blimit;
        spin_unlock(&rnp->lock);                /* irqs remain disabled. */

        /*
         * A new grace period might start here.  If so, we won't be part
         * of it, but that is OK, as we are currently in a quiescent state.
         */

        /* Exclude any attempts to start a new GP on large systems. */
        spin_lock(&rsp->onofflock);             /* irqs already disabled. */

        /* Add CPU to rcu_node bitmasks. */
        rnp = rdp->mynode;
        mask = rdp->grpmask;
        do {
                /* Exclude any attempts to start a new GP on small systems. */
                spin_lock(&rnp->lock);  /* irqs already disabled. */
                rnp->qsmaskinit |= mask;
                mask = rnp->grpmask;
                if (rnp == rdp->mynode) {
                        rdp->gpnum = rnp->completed; /* if GP in progress... */
                        rdp->completed = rnp->completed;
                        rdp->passed_quiesc_completed = rnp->completed - 1;
                }
                spin_unlock(&rnp->lock); /* irqs already disabled. */
                rnp = rnp->parent;
        } while (rnp != NULL && !(rnp->qsmaskinit & mask));

        spin_unlock_irqrestore(&rsp->onofflock, flags);
}

static void __cpuinit rcu_online_cpu(int cpu)
{
        rcu_init_percpu_data(cpu, &rcu_sched_state, 0);
        rcu_init_percpu_data(cpu, &rcu_bh_state, 0);
        rcu_preempt_init_percpu_data(cpu);
}

/*
 * Handle CPU online/offline notification events.
 */
static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
                                    unsigned long action, void *hcpu)
{
        long cpu = (long)hcpu;

        switch (action) {
        case CPU_UP_PREPARE:
        case CPU_UP_PREPARE_FROZEN:
                rcu_online_cpu(cpu);
                break;
        case CPU_DYING:
        case CPU_DYING_FROZEN:
                /*
                 * preempt_disable() in _rcu_barrier() prevents stop_machine(),
                 * so when "on_each_cpu(rcu_barrier_func, (void *)type, 1);"
                 * returns, all online cpus have queued rcu_barrier_func().
                 * The dying CPU clears its cpu_online_mask bit and
                 * moves all of its RCU callbacks to ->orphan_cbs_list
                 * in the context of stop_machine(), so subsequent calls
                 * to _rcu_barrier() will adopt these callbacks and only
                 * then queue rcu_barrier_func() on all remaining CPUs.
                 */
                rcu_send_cbs_to_orphanage(&rcu_bh_state);
                rcu_send_cbs_to_orphanage(&rcu_sched_state);
                rcu_preempt_send_cbs_to_orphanage();
                break;
        case CPU_DEAD:
        case CPU_DEAD_FROZEN:
        case CPU_UP_CANCELED:
        case CPU_UP_CANCELED_FROZEN:
                rcu_offline_cpu(cpu);
                break;
        default:
                break;
        }
        return NOTIFY_OK;
}

/*
 * Compute the per-level fanout, either using the exact fanout specified
 * or balancing the tree, depending on CONFIG_RCU_FANOUT_EXACT.
 */
#ifdef CONFIG_RCU_FANOUT_EXACT
static void __init rcu_init_levelspread(struct rcu_state *rsp)
{
        int i;

        for (i = NUM_RCU_LVLS - 1; i >= 0; i--)
                rsp->levelspread[i] = CONFIG_RCU_FANOUT;
}
#else /* #ifdef CONFIG_RCU_FANOUT_EXACT */
static void __init rcu_init_levelspread(struct rcu_state *rsp)
{
        int ccur;
        int cprv;
        int i;

        cprv = NR_CPUS;
        for (i = NUM_RCU_LVLS - 1; i >= 0; i--) {
                ccur = rsp->levelcnt[i];
                rsp->levelspread[i] = (cprv + ccur - 1) / ccur;
                cprv = ccur;
        }
}
#endif /* #else #ifdef CONFIG_RCU_FANOUT_EXACT */

/*
 * Helper function for rcu_init() that initializes one rcu_state structure.
 */
static void __init rcu_init_one(struct rcu_state *rsp)
{
        int cpustride = 1;
        int i;
        int j;
        struct rcu_node *rnp;

        /* Initialize the level-tracking arrays. */

        for (i = 1; i < NUM_RCU_LVLS; i++)
                rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
        rcu_init_levelspread(rsp);

        /* Initialize the elements themselves, starting from the leaves. */

        for (i = NUM_RCU_LVLS - 1; i >= 0; i--) {
                cpustride *= rsp->levelspread[i];
                rnp = rsp->level[i];
                for (j = 0; j < rsp->levelcnt[i]; j++, rnp++) {
                        spin_lock_init(&rnp->lock);
                        lockdep_set_class(&rnp->lock, &rcu_node_class[i]);
                        rnp->gpnum = 0;
                        rnp->qsmask = 0;
                        rnp->qsmaskinit = 0;
                        rnp->grplo = j * cpustride;
                        rnp->grphi = (j + 1) * cpustride - 1;
                        if (rnp->grphi >= NR_CPUS)
                                rnp->grphi = NR_CPUS - 1;
                        if (i == 0) {
                                rnp->grpnum = 0;
                                rnp->grpmask = 0;
                                rnp->parent = NULL;
                        } else {
                                rnp->grpnum = j % rsp->levelspread[i - 1];
                                rnp->grpmask = 1UL << rnp->grpnum;
                                rnp->parent = rsp->level[i - 1] +
                                              j / rsp->levelspread[i - 1];
                        }
                        rnp->level = i;
                        INIT_LIST_HEAD(&rnp->blocked_tasks[0]);
                        INIT_LIST_HEAD(&rnp->blocked_tasks[1]);
                        INIT_LIST_HEAD(&rnp->blocked_tasks[2]);
                        INIT_LIST_HEAD(&rnp->blocked_tasks[3]);
                }
        }
}

/*
 * Helper macro for __rcu_init() and __rcu_init_preempt().  To be used
 * nowhere else!  Assigns leaf node pointers into each CPU's rcu_data
 * structure.
 */
#define RCU_INIT_FLAVOR(rsp, rcu_data) \
do { \
        int i; \
        int j; \
        struct rcu_node *rnp; \
        \
        rcu_init_one(rsp); \
        rnp = (rsp)->level[NUM_RCU_LVLS - 1]; \
        j = 0; \
        for_each_possible_cpu(i) { \
                if (i > rnp[j].grphi) \
                        j++; \
                per_cpu(rcu_data, i).mynode = &rnp[j]; \
                (rsp)->rda[i] = &per_cpu(rcu_data, i); \
                rcu_boot_init_percpu_data(i, rsp); \
        } \
} while (0)

void __init rcu_init(void)
{
        int i;

        rcu_bootup_announce();
#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
        printk(KERN_INFO "RCU-based detection of stalled CPUs is enabled.\n");
#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
#if NUM_RCU_LVL_4 != 0
        printk(KERN_INFO "Experimental four-level hierarchy is enabled.\n");
#endif /* #if NUM_RCU_LVL_4 != 0 */
        RCU_INIT_FLAVOR(&rcu_sched_state, rcu_sched_data);
        RCU_INIT_FLAVOR(&rcu_bh_state, rcu_bh_data);
        __rcu_init_preempt();
        open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);

        /*
         * We don't need protection against CPU-hotplug here because
         * this is called early in boot, before either interrupts
         * or the scheduler are operational.
         */
        cpu_notifier(rcu_cpu_notify, 0);
        for_each_online_cpu(i)
                rcu_cpu_notify(NULL, CPU_UP_PREPARE, (void *)(long)i);
}

#include "rcutree_plugin.h"