Committer: Michael Beasley <mike@snafu.setup>

[mikesnafu-overlay.git] / kernel / rcupreempt.c

/*
 * Read-Copy Update mechanism for mutual exclusion, realtime implementation
 *
 * 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, 2006
 *
 * Authors: Paul E. McKenney <paulmck@us.ibm.com>
 *              With thanks to Esben Nielsen, Bill Huey, and Ingo Molnar
 *              for pushing me away from locks and towards counters, and
 *              to Suparna Bhattacharya for pushing me completely away
 *              from atomic instructions on the read side.
 *
 *  - Added handling of Dynamic Ticks
 *      Copyright 2007 - Paul E. Mckenney <paulmck@us.ibm.com>
 *                     - Steven Rostedt <srostedt@redhat.com>
 *
 * Papers:  http://www.rdrop.com/users/paulmck/RCU
 *
 * Design Document: http://lwn.net/Articles/253651/
 *
 * For detailed explanation of Read-Copy Update mechanism see -
 *              Documentation/RCU/ *.txt
 *
 */
#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 <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/rcupdate.h>
#include <linux/cpu.h>
#include <linux/random.h>
#include <linux/delay.h>
#include <linux/byteorder/swabb.h>
#include <linux/cpumask.h>
#include <linux/rcupreempt_trace.h>

/*
 * Macro that prevents the compiler from reordering accesses, but does
 * absolutely -nothing- to prevent CPUs from reordering.  This is used
 * only to mediate communication between mainline code and hardware
 * interrupt and NMI handlers.
 */
#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))

/*
 * PREEMPT_RCU data structures.
 */

/*
 * GP_STAGES specifies the number of times the state machine has
 * to go through the all the rcu_try_flip_states (see below)
 * in a single Grace Period.
 *
 * GP in GP_STAGES stands for Grace Period ;)
 */
#define GP_STAGES    2
struct rcu_data {
        spinlock_t      lock;           /* Protect rcu_data fields. */
        long            completed;      /* Number of last completed batch. */
        int             waitlistcount;
        struct tasklet_struct rcu_tasklet;
        struct rcu_head *nextlist;
        struct rcu_head **nexttail;
        struct rcu_head *waitlist[GP_STAGES];
        struct rcu_head **waittail[GP_STAGES];
        struct rcu_head *donelist;
        struct rcu_head **donetail;
        long rcu_flipctr[2];
#ifdef CONFIG_RCU_TRACE
        struct rcupreempt_trace trace;
#endif /* #ifdef CONFIG_RCU_TRACE */
};

/*
 * States for rcu_try_flip() and friends.
 */

enum rcu_try_flip_states {

        /*
         * Stay here if nothing is happening. Flip the counter if somthing
         * starts happening. Denoted by "I"
         */
        rcu_try_flip_idle_state,

        /*
         * Wait here for all CPUs to notice that the counter has flipped. This
         * prevents the old set of counters from ever being incremented once
         * we leave this state, which in turn is necessary because we cannot
         * test any individual counter for zero -- we can only check the sum.
         * Denoted by "A".
         */
        rcu_try_flip_waitack_state,

        /*
         * Wait here for the sum of the old per-CPU counters to reach zero.
         * Denoted by "Z".
         */
        rcu_try_flip_waitzero_state,

        /*
         * Wait here for each of the other CPUs to execute a memory barrier.
         * This is necessary to ensure that these other CPUs really have
         * completed executing their RCU read-side critical sections, despite
         * their CPUs wildly reordering memory. Denoted by "M".
         */
        rcu_try_flip_waitmb_state,
};

struct rcu_ctrlblk {
        spinlock_t      fliplock;       /* Protect state-machine transitions. */
        long            completed;      /* Number of last completed batch. */
        enum rcu_try_flip_states rcu_try_flip_state; /* The current state of
                                                        the rcu state machine */
};

static DEFINE_PER_CPU(struct rcu_data, rcu_data);
static struct rcu_ctrlblk rcu_ctrlblk = {
        .fliplock = __SPIN_LOCK_UNLOCKED(rcu_ctrlblk.fliplock),
        .completed = 0,
        .rcu_try_flip_state = rcu_try_flip_idle_state,
};


#ifdef CONFIG_RCU_TRACE
static char *rcu_try_flip_state_names[] =
        { "idle", "waitack", "waitzero", "waitmb" };
#endif /* #ifdef CONFIG_RCU_TRACE */

static cpumask_t rcu_cpu_online_map __read_mostly = CPU_MASK_NONE;

/*
 * Enum and per-CPU flag to determine when each CPU has seen
 * the most recent counter flip.
 */

enum rcu_flip_flag_values {
        rcu_flip_seen,          /* Steady/initial state, last flip seen. */
                                /* Only GP detector can update. */
        rcu_flipped             /* Flip just completed, need confirmation. */
                                /* Only corresponding CPU can update. */
};
static DEFINE_PER_CPU_SHARED_ALIGNED(enum rcu_flip_flag_values, rcu_flip_flag)
                                                                = rcu_flip_seen;

/*
 * Enum and per-CPU flag to determine when each CPU has executed the
 * needed memory barrier to fence in memory references from its last RCU
 * read-side critical section in the just-completed grace period.
 */

enum rcu_mb_flag_values {
        rcu_mb_done,            /* Steady/initial state, no mb()s required. */
                                /* Only GP detector can update. */
        rcu_mb_needed           /* Flip just completed, need an mb(). */
                                /* Only corresponding CPU can update. */
};
static DEFINE_PER_CPU_SHARED_ALIGNED(enum rcu_mb_flag_values, rcu_mb_flag)
                                                                = rcu_mb_done;

/*
 * RCU_DATA_ME: find the current CPU's rcu_data structure.
 * RCU_DATA_CPU: find the specified CPU's rcu_data structure.
 */
#define RCU_DATA_ME()           (&__get_cpu_var(rcu_data))
#define RCU_DATA_CPU(cpu)       (&per_cpu(rcu_data, cpu))

/*
 * Helper macro for tracing when the appropriate rcu_data is not
 * cached in a local variable, but where the CPU number is so cached.
 */
#define RCU_TRACE_CPU(f, cpu) RCU_TRACE(f, &(RCU_DATA_CPU(cpu)->trace));

/*
 * Helper macro for tracing when the appropriate rcu_data is not
 * cached in a local variable.
 */
#define RCU_TRACE_ME(f) RCU_TRACE(f, &(RCU_DATA_ME()->trace));

/*
 * Helper macro for tracing when the appropriate rcu_data is pointed
 * to by a local variable.
 */
#define RCU_TRACE_RDP(f, rdp) RCU_TRACE(f, &((rdp)->trace));

/*
 * Return the number of RCU batches processed thus far.  Useful
 * for debug and statistics.
 */
long rcu_batches_completed(void)
{
        return rcu_ctrlblk.completed;
}
EXPORT_SYMBOL_GPL(rcu_batches_completed);

EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);

void __rcu_read_lock(void)
{
        int idx;
        struct task_struct *t = current;
        int nesting;

        nesting = ACCESS_ONCE(t->rcu_read_lock_nesting);
        if (nesting != 0) {

                /* An earlier rcu_read_lock() covers us, just count it. */

                t->rcu_read_lock_nesting = nesting + 1;

        } else {
                unsigned long flags;

                /*
                 * We disable interrupts for the following reasons:
                 * - If we get scheduling clock interrupt here, and we
                 *   end up acking the counter flip, it's like a promise
                 *   that we will never increment the old counter again.
                 *   Thus we will break that promise if that
                 *   scheduling clock interrupt happens between the time
                 *   we pick the .completed field and the time that we
                 *   increment our counter.
                 *
                 * - We don't want to be preempted out here.
                 *
                 * NMIs can still occur, of course, and might themselves
                 * contain rcu_read_lock().
                 */

                local_irq_save(flags);

                /*
                 * Outermost nesting of rcu_read_lock(), so increment
                 * the current counter for the current CPU.  Use volatile
                 * casts to prevent the compiler from reordering.
                 */

                idx = ACCESS_ONCE(rcu_ctrlblk.completed) & 0x1;
                ACCESS_ONCE(RCU_DATA_ME()->rcu_flipctr[idx])++;

                /*
                 * Now that the per-CPU counter has been incremented, we
                 * are protected from races with rcu_read_lock() invoked
                 * from NMI handlers on this CPU.  We can therefore safely
                 * increment the nesting counter, relieving further NMIs
                 * of the need to increment the per-CPU counter.
                 */

                ACCESS_ONCE(t->rcu_read_lock_nesting) = nesting + 1;

                /*
                 * Now that we have preventing any NMIs from storing
                 * to the ->rcu_flipctr_idx, we can safely use it to
                 * remember which counter to decrement in the matching
                 * rcu_read_unlock().
                 */

                ACCESS_ONCE(t->rcu_flipctr_idx) = idx;
                local_irq_restore(flags);
        }
}
EXPORT_SYMBOL_GPL(__rcu_read_lock);

void __rcu_read_unlock(void)
{
        int idx;
        struct task_struct *t = current;
        int nesting;

        nesting = ACCESS_ONCE(t->rcu_read_lock_nesting);
        if (nesting > 1) {

                /*
                 * We are still protected by the enclosing rcu_read_lock(),
                 * so simply decrement the counter.
                 */

                t->rcu_read_lock_nesting = nesting - 1;

        } else {
                unsigned long flags;

                /*
                 * Disable local interrupts to prevent the grace-period
                 * detection state machine from seeing us half-done.
                 * NMIs can still occur, of course, and might themselves
                 * contain rcu_read_lock() and rcu_read_unlock().
                 */

                local_irq_save(flags);

                /*
                 * Outermost nesting of rcu_read_unlock(), so we must
                 * decrement the current counter for the current CPU.
                 * This must be done carefully, because NMIs can
                 * occur at any point in this code, and any rcu_read_lock()
                 * and rcu_read_unlock() pairs in the NMI handlers
                 * must interact non-destructively with this code.
                 * Lots of volatile casts, and -very- careful ordering.
                 *
                 * Changes to this code, including this one, must be
                 * inspected, validated, and tested extremely carefully!!!
                 */

                /*
                 * First, pick up the index.
                 */

                idx = ACCESS_ONCE(t->rcu_flipctr_idx);

                /*
                 * Now that we have fetched the counter index, it is
                 * safe to decrement the per-task RCU nesting counter.
                 * After this, any interrupts or NMIs will increment and
                 * decrement the per-CPU counters.
                 */
                ACCESS_ONCE(t->rcu_read_lock_nesting) = nesting - 1;

                /*
                 * It is now safe to decrement this task's nesting count.
                 * NMIs that occur after this statement will route their
                 * rcu_read_lock() calls through this "else" clause, and
                 * will thus start incrementing the per-CPU counter on
                 * their own.  They will also clobber ->rcu_flipctr_idx,
                 * but that is OK, since we have already fetched it.
                 */

                ACCESS_ONCE(RCU_DATA_ME()->rcu_flipctr[idx])--;
                local_irq_restore(flags);
        }
}
EXPORT_SYMBOL_GPL(__rcu_read_unlock);

/*
 * If a global counter flip has occurred since the last time that we
 * advanced callbacks, advance them.  Hardware interrupts must be
 * disabled when calling this function.
 */
static void __rcu_advance_callbacks(struct rcu_data *rdp)
{
        int cpu;
        int i;
        int wlc = 0;

        if (rdp->completed != rcu_ctrlblk.completed) {
                if (rdp->waitlist[GP_STAGES - 1] != NULL) {
                        *rdp->donetail = rdp->waitlist[GP_STAGES - 1];
                        rdp->donetail = rdp->waittail[GP_STAGES - 1];
                        RCU_TRACE_RDP(rcupreempt_trace_move2done, rdp);
                }
                for (i = GP_STAGES - 2; i >= 0; i--) {
                        if (rdp->waitlist[i] != NULL) {
                                rdp->waitlist[i + 1] = rdp->waitlist[i];
                                rdp->waittail[i + 1] = rdp->waittail[i];
                                wlc++;
                        } else {
                                rdp->waitlist[i + 1] = NULL;
                                rdp->waittail[i + 1] =
                                        &rdp->waitlist[i + 1];
                        }
                }
                if (rdp->nextlist != NULL) {
                        rdp->waitlist[0] = rdp->nextlist;
                        rdp->waittail[0] = rdp->nexttail;
                        wlc++;
                        rdp->nextlist = NULL;
                        rdp->nexttail = &rdp->nextlist;
                        RCU_TRACE_RDP(rcupreempt_trace_move2wait, rdp);
                } else {
                        rdp->waitlist[0] = NULL;
                        rdp->waittail[0] = &rdp->waitlist[0];
                }
                rdp->waitlistcount = wlc;
                rdp->completed = rcu_ctrlblk.completed;
        }

        /*
         * Check to see if this CPU needs to report that it has seen
         * the most recent counter flip, thereby declaring that all
         * subsequent rcu_read_lock() invocations will respect this flip.
         */

        cpu = raw_smp_processor_id();
        if (per_cpu(rcu_flip_flag, cpu) == rcu_flipped) {
                smp_mb();  /* Subsequent counter accesses must see new value */
                per_cpu(rcu_flip_flag, cpu) = rcu_flip_seen;
                smp_mb();  /* Subsequent RCU read-side critical sections */
                           /*  seen -after- acknowledgement. */
        }
}

#ifdef CONFIG_NO_HZ

DEFINE_PER_CPU(long, dynticks_progress_counter) = 1;
static DEFINE_PER_CPU(long, rcu_dyntick_snapshot);
static DEFINE_PER_CPU(int, rcu_update_flag);

/**
 * rcu_irq_enter - Called from Hard irq handlers and NMI/SMI.
 *
 * If the CPU was idle with dynamic ticks active, this updates the
 * dynticks_progress_counter to let the RCU handling know that the
 * CPU is active.
 */
void rcu_irq_enter(void)
{
        int cpu = smp_processor_id();

        if (per_cpu(rcu_update_flag, cpu))
                per_cpu(rcu_update_flag, cpu)++;

        /*
         * Only update if we are coming from a stopped ticks mode
         * (dynticks_progress_counter is even).
         */
        if (!in_interrupt() &&
            (per_cpu(dynticks_progress_counter, cpu) & 0x1) == 0) {
                /*
                 * The following might seem like we could have a race
                 * with NMI/SMIs. But this really isn't a problem.
                 * Here we do a read/modify/write, and the race happens
                 * when an NMI/SMI comes in after the read and before
                 * the write. But NMI/SMIs will increment this counter
                 * twice before returning, so the zero bit will not
                 * be corrupted by the NMI/SMI which is the most important
                 * part.
                 *
                 * The only thing is that we would bring back the counter
                 * to a postion that it was in during the NMI/SMI.
                 * But the zero bit would be set, so the rest of the
                 * counter would again be ignored.
                 *
                 * On return from the IRQ, the counter may have the zero
                 * bit be 0 and the counter the same as the return from
                 * the NMI/SMI. If the state machine was so unlucky to
                 * see that, it still doesn't matter, since all
                 * RCU read-side critical sections on this CPU would
                 * have already completed.
                 */
                per_cpu(dynticks_progress_counter, cpu)++;
                /*
                 * The following memory barrier ensures that any
                 * rcu_read_lock() primitives in the irq handler
                 * are seen by other CPUs to follow the above
                 * increment to dynticks_progress_counter. This is
                 * required in order for other CPUs to correctly
                 * determine when it is safe to advance the RCU
                 * grace-period state machine.
                 */
                smp_mb(); /* see above block comment. */
                /*
                 * Since we can't determine the dynamic tick mode from
                 * the dynticks_progress_counter after this routine,
                 * we use a second flag to acknowledge that we came
                 * from an idle state with ticks stopped.
                 */
                per_cpu(rcu_update_flag, cpu)++;
                /*
                 * If we take an NMI/SMI now, they will also increment
                 * the rcu_update_flag, and will not update the
                 * dynticks_progress_counter on exit. That is for
                 * this IRQ to do.
                 */
        }
}

/**
 * rcu_irq_exit - Called from exiting Hard irq context.
 *
 * If the CPU was idle with dynamic ticks active, update the
 * dynticks_progress_counter to put let the RCU handling be
 * aware that the CPU is going back to idle with no ticks.
 */
void rcu_irq_exit(void)
{
        int cpu = smp_processor_id();

        /*
         * rcu_update_flag is set if we interrupted the CPU
         * when it was idle with ticks stopped.
         * Once this occurs, we keep track of interrupt nesting
         * because a NMI/SMI could also come in, and we still
         * only want the IRQ that started the increment of the
         * dynticks_progress_counter to be the one that modifies
         * it on exit.
         */
        if (per_cpu(rcu_update_flag, cpu)) {
                if (--per_cpu(rcu_update_flag, cpu))
                        return;

                /* This must match the interrupt nesting */
                WARN_ON(in_interrupt());

                /*
                 * If an NMI/SMI happens now we are still
                 * protected by the dynticks_progress_counter being odd.
                 */

                /*
                 * The following memory barrier ensures that any
                 * rcu_read_unlock() primitives in the irq handler
                 * are seen by other CPUs to preceed the following
                 * increment to dynticks_progress_counter. This
                 * is required in order for other CPUs to determine
                 * when it is safe to advance the RCU grace-period
                 * state machine.
                 */
                smp_mb(); /* see above block comment. */
                per_cpu(dynticks_progress_counter, cpu)++;
                WARN_ON(per_cpu(dynticks_progress_counter, cpu) & 0x1);
        }
}

static void dyntick_save_progress_counter(int cpu)
{
        per_cpu(rcu_dyntick_snapshot, cpu) =
                per_cpu(dynticks_progress_counter, cpu);
}

static inline int
rcu_try_flip_waitack_needed(int cpu)
{
        long curr;
        long snap;

        curr = per_cpu(dynticks_progress_counter, cpu);
        snap = per_cpu(rcu_dyntick_snapshot, cpu);
        smp_mb(); /* force ordering with cpu entering/leaving dynticks. */

        /*
         * If the CPU remained in dynticks mode for the entire time
         * and didn't take any interrupts, NMIs, SMIs, or whatever,
         * then it cannot be in the middle of an rcu_read_lock(), so
         * the next rcu_read_lock() it executes must use the new value
         * of the counter.  So we can safely pretend that this CPU
         * already acknowledged the counter.
         */

        if ((curr == snap) && ((curr & 0x1) == 0))
                return 0;

        /*
         * If the CPU passed through or entered a dynticks idle phase with
         * no active irq handlers, then, as above, we can safely pretend
         * that this CPU already acknowledged the counter.
         */

        if ((curr - snap) > 2 || (snap & 0x1) == 0)
                return 0;

        /* We need this CPU to explicitly acknowledge the counter flip. */

        return 1;
}

static inline int
rcu_try_flip_waitmb_needed(int cpu)
{
        long curr;
        long snap;

        curr = per_cpu(dynticks_progress_counter, cpu);
        snap = per_cpu(rcu_dyntick_snapshot, cpu);
        smp_mb(); /* force ordering with cpu entering/leaving dynticks. */

        /*
         * If the CPU remained in dynticks mode for the entire time
         * and didn't take any interrupts, NMIs, SMIs, or whatever,
         * then it cannot have executed an RCU read-side critical section
         * during that time, so there is no need for it to execute a
         * memory barrier.
         */

        if ((curr == snap) && ((curr & 0x1) == 0))
                return 0;

        /*
         * If the CPU either entered or exited an outermost interrupt,
         * SMI, NMI, or whatever handler, then we know that it executed
         * a memory barrier when doing so.  So we don't need another one.
         */
        if (curr != snap)
                return 0;

        /* We need the CPU to execute a memory barrier. */

        return 1;
}

#else /* !CONFIG_NO_HZ */

# define dyntick_save_progress_counter(cpu)     do { } while (0)
# define rcu_try_flip_waitack_needed(cpu)       (1)
# define rcu_try_flip_waitmb_needed(cpu)        (1)

#endif /* CONFIG_NO_HZ */

/*
 * Get here when RCU is idle.  Decide whether we need to
 * move out of idle state, and return non-zero if so.
 * "Straightforward" approach for the moment, might later
 * use callback-list lengths, grace-period duration, or
 * some such to determine when to exit idle state.
 * Might also need a pre-idle test that does not acquire
 * the lock, but let's get the simple case working first...
 */

static int
rcu_try_flip_idle(void)
{
        int cpu;

        RCU_TRACE_ME(rcupreempt_trace_try_flip_i1);
        if (!rcu_pending(smp_processor_id())) {
                RCU_TRACE_ME(rcupreempt_trace_try_flip_ie1);
                return 0;
        }

        /*
         * Do the flip.
         */

        RCU_TRACE_ME(rcupreempt_trace_try_flip_g1);
        rcu_ctrlblk.completed++;  /* stands in for rcu_try_flip_g2 */

        /*
         * Need a memory barrier so that other CPUs see the new
         * counter value before they see the subsequent change of all
         * the rcu_flip_flag instances to rcu_flipped.
         */

        smp_mb();       /* see above block comment. */

        /* Now ask each CPU for acknowledgement of the flip. */

        for_each_cpu_mask(cpu, rcu_cpu_online_map) {
                per_cpu(rcu_flip_flag, cpu) = rcu_flipped;
                dyntick_save_progress_counter(cpu);
        }

        return 1;
}

/*
 * Wait for CPUs to acknowledge the flip.
 */

static int
rcu_try_flip_waitack(void)
{
        int cpu;

        RCU_TRACE_ME(rcupreempt_trace_try_flip_a1);
        for_each_cpu_mask(cpu, rcu_cpu_online_map)
                if (rcu_try_flip_waitack_needed(cpu) &&
                    per_cpu(rcu_flip_flag, cpu) != rcu_flip_seen) {
                        RCU_TRACE_ME(rcupreempt_trace_try_flip_ae1);
                        return 0;
                }

        /*
         * Make sure our checks above don't bleed into subsequent
         * waiting for the sum of the counters to reach zero.
         */

        smp_mb();       /* see above block comment. */
        RCU_TRACE_ME(rcupreempt_trace_try_flip_a2);
        return 1;
}

/*
 * Wait for collective ``last'' counter to reach zero,
 * then tell all CPUs to do an end-of-grace-period memory barrier.
 */

static int
rcu_try_flip_waitzero(void)
{
        int cpu;
        int lastidx = !(rcu_ctrlblk.completed & 0x1);
        int sum = 0;

        /* Check to see if the sum of the "last" counters is zero. */

        RCU_TRACE_ME(rcupreempt_trace_try_flip_z1);
        for_each_cpu_mask(cpu, rcu_cpu_online_map)
                sum += RCU_DATA_CPU(cpu)->rcu_flipctr[lastidx];
        if (sum != 0) {
                RCU_TRACE_ME(rcupreempt_trace_try_flip_ze1);
                return 0;
        }

        /*
         * This ensures that the other CPUs see the call for
         * memory barriers -after- the sum to zero has been
         * detected here
         */
        smp_mb();  /*  ^^^^^^^^^^^^ */

        /* Call for a memory barrier from each CPU. */
        for_each_cpu_mask(cpu, rcu_cpu_online_map) {
                per_cpu(rcu_mb_flag, cpu) = rcu_mb_needed;
                dyntick_save_progress_counter(cpu);
        }

        RCU_TRACE_ME(rcupreempt_trace_try_flip_z2);
        return 1;
}

/*
 * Wait for all CPUs to do their end-of-grace-period memory barrier.
 * Return 0 once all CPUs have done so.
 */

static int
rcu_try_flip_waitmb(void)
{
        int cpu;

        RCU_TRACE_ME(rcupreempt_trace_try_flip_m1);
        for_each_cpu_mask(cpu, rcu_cpu_online_map)
                if (rcu_try_flip_waitmb_needed(cpu) &&
                    per_cpu(rcu_mb_flag, cpu) != rcu_mb_done) {
                        RCU_TRACE_ME(rcupreempt_trace_try_flip_me1);
                        return 0;
                }

        smp_mb(); /* Ensure that the above checks precede any following flip. */
        RCU_TRACE_ME(rcupreempt_trace_try_flip_m2);
        return 1;
}

/*
 * Attempt a single flip of the counters.  Remember, a single flip does
 * -not- constitute a grace period.  Instead, the interval between
 * at least GP_STAGES consecutive flips is a grace period.
 *
 * If anyone is nuts enough to run this CONFIG_PREEMPT_RCU implementation
 * on a large SMP, they might want to use a hierarchical organization of
 * the per-CPU-counter pairs.
 */
static void rcu_try_flip(void)
{
        unsigned long flags;

        RCU_TRACE_ME(rcupreempt_trace_try_flip_1);
        if (unlikely(!spin_trylock_irqsave(&rcu_ctrlblk.fliplock, flags))) {
                RCU_TRACE_ME(rcupreempt_trace_try_flip_e1);
                return;
        }

        /*
         * Take the next transition(s) through the RCU grace-period
         * flip-counter state machine.
         */

        switch (rcu_ctrlblk.rcu_try_flip_state) {
        case rcu_try_flip_idle_state:
                if (rcu_try_flip_idle())
                        rcu_ctrlblk.rcu_try_flip_state =
                                rcu_try_flip_waitack_state;
                break;
        case rcu_try_flip_waitack_state:
                if (rcu_try_flip_waitack())
                        rcu_ctrlblk.rcu_try_flip_state =
                                rcu_try_flip_waitzero_state;
                break;
        case rcu_try_flip_waitzero_state:
                if (rcu_try_flip_waitzero())
                        rcu_ctrlblk.rcu_try_flip_state =
                                rcu_try_flip_waitmb_state;
                break;
        case rcu_try_flip_waitmb_state:
                if (rcu_try_flip_waitmb())
                        rcu_ctrlblk.rcu_try_flip_state =
                                rcu_try_flip_idle_state;
        }
        spin_unlock_irqrestore(&rcu_ctrlblk.fliplock, flags);
}

/*
 * Check to see if this CPU needs to do a memory barrier in order to
 * ensure that any prior RCU read-side critical sections have committed
 * their counter manipulations and critical-section memory references
 * before declaring the grace period to be completed.
 */
static void rcu_check_mb(int cpu)
{
        if (per_cpu(rcu_mb_flag, cpu) == rcu_mb_needed) {
                smp_mb();  /* Ensure RCU read-side accesses are visible. */
                per_cpu(rcu_mb_flag, cpu) = rcu_mb_done;
        }
}

void rcu_check_callbacks(int cpu, int user)
{
        unsigned long flags;
        struct rcu_data *rdp = RCU_DATA_CPU(cpu);

        rcu_check_mb(cpu);
        if (rcu_ctrlblk.completed == rdp->completed)
                rcu_try_flip();
        spin_lock_irqsave(&rdp->lock, flags);
        RCU_TRACE_RDP(rcupreempt_trace_check_callbacks, rdp);
        __rcu_advance_callbacks(rdp);
        if (rdp->donelist == NULL) {
                spin_unlock_irqrestore(&rdp->lock, flags);
        } else {
                spin_unlock_irqrestore(&rdp->lock, flags);
                raise_softirq(RCU_SOFTIRQ);
        }
}

/*
 * Needed by dynticks, to make sure all RCU processing has finished
 * when we go idle:
 */
void rcu_advance_callbacks(int cpu, int user)
{
        unsigned long flags;
        struct rcu_data *rdp = RCU_DATA_CPU(cpu);

        if (rcu_ctrlblk.completed == rdp->completed) {
                rcu_try_flip();
                if (rcu_ctrlblk.completed == rdp->completed)
                        return;
        }
        spin_lock_irqsave(&rdp->lock, flags);
        RCU_TRACE_RDP(rcupreempt_trace_check_callbacks, rdp);
        __rcu_advance_callbacks(rdp);
        spin_unlock_irqrestore(&rdp->lock, flags);
}

#ifdef CONFIG_HOTPLUG_CPU
#define rcu_offline_cpu_enqueue(srclist, srctail, dstlist, dsttail) do { \
                *dsttail = srclist; \
                if (srclist != NULL) { \
                        dsttail = srctail; \
                        srclist = NULL; \
                        srctail = &srclist;\
                } \
        } while (0)

void rcu_offline_cpu(int cpu)
{
        int i;
        struct rcu_head *list = NULL;
        unsigned long flags;
        struct rcu_data *rdp = RCU_DATA_CPU(cpu);
        struct rcu_head **tail = &list;

        /*
         * Remove all callbacks from the newly dead CPU, retaining order.
         * Otherwise rcu_barrier() will fail
         */

        spin_lock_irqsave(&rdp->lock, flags);
        rcu_offline_cpu_enqueue(rdp->donelist, rdp->donetail, list, tail);
        for (i = GP_STAGES - 1; i >= 0; i--)
                rcu_offline_cpu_enqueue(rdp->waitlist[i], rdp->waittail[i],
                                                list, tail);
        rcu_offline_cpu_enqueue(rdp->nextlist, rdp->nexttail, list, tail);
        spin_unlock_irqrestore(&rdp->lock, flags);
        rdp->waitlistcount = 0;

        /* Disengage the newly dead CPU from the grace-period computation. */

        spin_lock_irqsave(&rcu_ctrlblk.fliplock, flags);
        rcu_check_mb(cpu);
        if (per_cpu(rcu_flip_flag, cpu) == rcu_flipped) {
                smp_mb();  /* Subsequent counter accesses must see new value */
                per_cpu(rcu_flip_flag, cpu) = rcu_flip_seen;
                smp_mb();  /* Subsequent RCU read-side critical sections */
                           /*  seen -after- acknowledgement. */
        }

        RCU_DATA_ME()->rcu_flipctr[0] += RCU_DATA_CPU(cpu)->rcu_flipctr[0];
        RCU_DATA_ME()->rcu_flipctr[1] += RCU_DATA_CPU(cpu)->rcu_flipctr[1];

        RCU_DATA_CPU(cpu)->rcu_flipctr[0] = 0;
        RCU_DATA_CPU(cpu)->rcu_flipctr[1] = 0;

        cpu_clear(cpu, rcu_cpu_online_map);

        spin_unlock_irqrestore(&rcu_ctrlblk.fliplock, flags);

        /*
         * Place the removed callbacks on the current CPU's queue.
         * Make them all start a new grace period: simple approach,
         * in theory could starve a given set of callbacks, but
         * you would need to be doing some serious CPU hotplugging
         * to make this happen.  If this becomes a problem, adding
         * a synchronize_rcu() to the hotplug path would be a simple
         * fix.
         */

        local_irq_save(flags);
        rdp = RCU_DATA_ME();
        spin_lock(&rdp->lock);
        *rdp->nexttail = list;
        if (list)
                rdp->nexttail = tail;
        spin_unlock_irqrestore(&rdp->lock, flags);
}

void __devinit rcu_online_cpu(int cpu)
{
        unsigned long flags;

        spin_lock_irqsave(&rcu_ctrlblk.fliplock, flags);
        cpu_set(cpu, rcu_cpu_online_map);
        spin_unlock_irqrestore(&rcu_ctrlblk.fliplock, flags);
}

#else /* #ifdef CONFIG_HOTPLUG_CPU */

void rcu_offline_cpu(int cpu)
{
}

void __devinit rcu_online_cpu(int cpu)
{
}

#endif /* #else #ifdef CONFIG_HOTPLUG_CPU */

static void rcu_process_callbacks(struct softirq_action *unused)
{
        unsigned long flags;
        struct rcu_head *next, *list;
        struct rcu_data *rdp;

        local_irq_save(flags);
        rdp = RCU_DATA_ME();
        spin_lock(&rdp->lock);
        list = rdp->donelist;
        if (list == NULL) {
                spin_unlock_irqrestore(&rdp->lock, flags);
                return;
        }
        rdp->donelist = NULL;
        rdp->donetail = &rdp->donelist;
        RCU_TRACE_RDP(rcupreempt_trace_done_remove, rdp);
        spin_unlock_irqrestore(&rdp->lock, flags);
        while (list) {
                next = list->next;
                list->func(list);
                list = next;
                RCU_TRACE_ME(rcupreempt_trace_invoke);
        }
}

void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
{
        unsigned long flags;
        struct rcu_data *rdp;

        head->func = func;
        head->next = NULL;
        local_irq_save(flags);
        rdp = RCU_DATA_ME();
        spin_lock(&rdp->lock);
        __rcu_advance_callbacks(rdp);
        *rdp->nexttail = head;
        rdp->nexttail = &head->next;
        RCU_TRACE_RDP(rcupreempt_trace_next_add, rdp);
        spin_unlock(&rdp->lock);
        local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(call_rcu);

/*
 * Wait until all currently running preempt_disable() code segments
 * (including hardware-irq-disable segments) complete.  Note that
 * in -rt this does -not- necessarily result in all currently executing
 * interrupt -handlers- having completed.
 */
void __synchronize_sched(void)
{
        cpumask_t oldmask;
        int cpu;

        if (sched_getaffinity(0, &oldmask) < 0)
                oldmask = cpu_possible_map;
        for_each_online_cpu(cpu) {
                sched_setaffinity(0, cpumask_of_cpu(cpu));
                schedule();
        }
        sched_setaffinity(0, oldmask);
}
EXPORT_SYMBOL_GPL(__synchronize_sched);

/*
 * 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.  Assumes that notifiers would take care of handling any
 * outstanding requests from the RCU core.
 *
 * This function is part of the RCU implementation; it is -not-
 * an exported member of the RCU API.
 */
int rcu_needs_cpu(int cpu)
{
        struct rcu_data *rdp = RCU_DATA_CPU(cpu);

        return (rdp->donelist != NULL ||
                !!rdp->waitlistcount ||
                rdp->nextlist != NULL);
}

int rcu_pending(int cpu)
{
        struct rcu_data *rdp = RCU_DATA_CPU(cpu);

        /* The CPU has at least one callback queued somewhere. */

        if (rdp->donelist != NULL ||
            !!rdp->waitlistcount ||
            rdp->nextlist != NULL)
                return 1;

        /* The RCU core needs an acknowledgement from this CPU. */

        if ((per_cpu(rcu_flip_flag, cpu) == rcu_flipped) ||
            (per_cpu(rcu_mb_flag, cpu) == rcu_mb_needed))
                return 1;

        /* This CPU has fallen behind the global grace-period number. */

        if (rdp->completed != rcu_ctrlblk.completed)
                return 1;

        /* Nothing needed from this CPU. */

        return 0;
}

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_UP_CANCELED:
        case CPU_UP_CANCELED_FROZEN:
        case CPU_DEAD:
        case CPU_DEAD_FROZEN:
                rcu_offline_cpu(cpu);
                break;
        default:
                break;
        }
        return NOTIFY_OK;
}

static struct notifier_block __cpuinitdata rcu_nb = {
        .notifier_call = rcu_cpu_notify,
};

void __init __rcu_init(void)
{
        int cpu;
        int i;
        struct rcu_data *rdp;

        printk(KERN_NOTICE "Preemptible RCU implementation.\n");
        for_each_possible_cpu(cpu) {
                rdp = RCU_DATA_CPU(cpu);
                spin_lock_init(&rdp->lock);
                rdp->completed = 0;
                rdp->waitlistcount = 0;
                rdp->nextlist = NULL;
                rdp->nexttail = &rdp->nextlist;
                for (i = 0; i < GP_STAGES; i++) {
                        rdp->waitlist[i] = NULL;
                        rdp->waittail[i] = &rdp->waitlist[i];
                }
                rdp->donelist = NULL;
                rdp->donetail = &rdp->donelist;
                rdp->rcu_flipctr[0] = 0;
                rdp->rcu_flipctr[1] = 0;
        }
        register_cpu_notifier(&rcu_nb);

        /*
         * We don't need protection against CPU-Hotplug here
         * since
         * a) If a CPU comes online while we are iterating over the
         *    cpu_online_map below, we would only end up making a
         *    duplicate call to rcu_online_cpu() which sets the corresponding
         *    CPU's mask in the rcu_cpu_online_map.
         *
         * b) A CPU cannot go offline at this point in time since the user
         *    does not have access to the sysfs interface, nor do we
         *    suspend the system.
         */
        for_each_online_cpu(cpu)
                rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE, (void *)(long) cpu);

        open_softirq(RCU_SOFTIRQ, rcu_process_callbacks, NULL);
}

/*
 * Deprecated, use synchronize_rcu() or synchronize_sched() instead.
 */
void synchronize_kernel(void)
{
        synchronize_rcu();
}

#ifdef CONFIG_RCU_TRACE
long *rcupreempt_flipctr(int cpu)
{
        return &RCU_DATA_CPU(cpu)->rcu_flipctr[0];
}
EXPORT_SYMBOL_GPL(rcupreempt_flipctr);

int rcupreempt_flip_flag(int cpu)
{
        return per_cpu(rcu_flip_flag, cpu);
}
EXPORT_SYMBOL_GPL(rcupreempt_flip_flag);

int rcupreempt_mb_flag(int cpu)
{
        return per_cpu(rcu_mb_flag, cpu);
}
EXPORT_SYMBOL_GPL(rcupreempt_mb_flag);

char *rcupreempt_try_flip_state_name(void)
{
        return rcu_try_flip_state_names[rcu_ctrlblk.rcu_try_flip_state];
}
EXPORT_SYMBOL_GPL(rcupreempt_try_flip_state_name);

struct rcupreempt_trace *rcupreempt_trace_cpu(int cpu)
{
        struct rcu_data *rdp = RCU_DATA_CPU(cpu);

        return &rdp->trace;
}
EXPORT_SYMBOL_GPL(rcupreempt_trace_cpu);

#endif /* #ifdef RCU_TRACE */