[PATCH] add rcu_barrier() synchronization point

[mirror_ubuntu-artful-kernel.git] / kernel / rcupdate.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 (C) IBM Corporation, 2001
 *
 * Authors: Dipankar Sarma <dipankar@in.ibm.com>
 *          Manfred Spraul <manfred@colorfullife.com>
 * 
 * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
 * Papers:
 * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
 * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
 *
 * For detailed explanation of Read-Copy Update mechanism see -
 *              http://lse.sourceforge.net/locking/rcupdate.html
 *
 */
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/spinlock.h>
#include <linux/smp.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/rcuref.h>
#include <linux/cpu.h>

/* Definition for rcupdate control block. */
struct rcu_ctrlblk rcu_ctrlblk = 
        { .cur = -300, .completed = -300 };
struct rcu_ctrlblk rcu_bh_ctrlblk =
        { .cur = -300, .completed = -300 };

/* Bookkeeping of the progress of the grace period */
struct rcu_state {
        spinlock_t      lock; /* Guard this struct and writes to rcu_ctrlblk */
        cpumask_t       cpumask; /* CPUs that need to switch in order    */
                                      /* for current batch to proceed.        */
};

static struct rcu_state rcu_state ____cacheline_maxaligned_in_smp =
          {.lock = SPIN_LOCK_UNLOCKED, .cpumask = CPU_MASK_NONE };
static struct rcu_state rcu_bh_state ____cacheline_maxaligned_in_smp =
          {.lock = SPIN_LOCK_UNLOCKED, .cpumask = CPU_MASK_NONE };

DEFINE_PER_CPU(struct rcu_data, rcu_data) = { 0L };
DEFINE_PER_CPU(struct rcu_data, rcu_bh_data) = { 0L };

/* Fake initialization required by compiler */
static DEFINE_PER_CPU(struct tasklet_struct, rcu_tasklet) = {NULL};
static int maxbatch = 10000;

#ifndef __HAVE_ARCH_CMPXCHG
/*
 * We use an array of spinlocks for the rcurefs -- similar to ones in sparc
 * 32 bit atomic_t implementations, and a hash function similar to that
 * for our refcounting needs.
 * Can't help multiprocessors which donot have cmpxchg :(
 */

spinlock_t __rcuref_hash[RCUREF_HASH_SIZE] = {
        [0 ... (RCUREF_HASH_SIZE-1)] = SPIN_LOCK_UNLOCKED
};
#endif

/**
 * call_rcu - Queue an RCU callback for invocation after a grace period.
 * @head: structure to be used for queueing the RCU updates.
 * @func: actual update function to be invoked after the grace period
 *
 * The update function will be invoked some time after a full grace
 * period elapses, in other words after all currently executing RCU
 * read-side critical sections have completed.  RCU read-side critical
 * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 * and may be nested.
 */
void fastcall 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 = &__get_cpu_var(rcu_data);
        *rdp->nxttail = head;
        rdp->nxttail = &head->next;

        if (unlikely(++rdp->count > 10000))
                set_need_resched();

        local_irq_restore(flags);
}

static atomic_t rcu_barrier_cpu_count;
static struct semaphore rcu_barrier_sema;
static struct completion rcu_barrier_completion;

/**
 * call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
 * @head: structure to be used for queueing the RCU updates.
 * @func: actual update function to be invoked after the grace period
 *
 * The update function will be invoked some time after a full grace
 * period elapses, in other words after all currently executing RCU
 * read-side critical sections have completed. call_rcu_bh() assumes
 * that the read-side critical sections end on completion of a softirq
 * handler. This means that read-side critical sections in process
 * context must not be interrupted by softirqs. This interface is to be
 * used when most of the read-side critical sections are in softirq context.
 * RCU read-side critical sections are delimited by rcu_read_lock() and
 * rcu_read_unlock(), * if in interrupt context or rcu_read_lock_bh()
 * and rcu_read_unlock_bh(), if in process context. These may be nested.
 */
void fastcall call_rcu_bh(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 = &__get_cpu_var(rcu_bh_data);
        *rdp->nxttail = head;
        rdp->nxttail = &head->next;
        rdp->count++;
/*
 *  Should we directly call rcu_do_batch() here ?
 *  if (unlikely(rdp->count > 10000))
 *      rcu_do_batch(rdp);
 */
        local_irq_restore(flags);
}

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

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 *notused)
{
        int cpu = smp_processor_id();
        struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
        struct rcu_head *head;

        head = &rdp->barrier;
        atomic_inc(&rcu_barrier_cpu_count);
        call_rcu(head, rcu_barrier_callback);
}

/**
 * rcu_barrier - Wait until all the in-flight RCUs are complete.
 */
void rcu_barrier(void)
{
        BUG_ON(in_interrupt());
        /* Take cpucontrol semaphore to protect against CPU hotplug */
        down(&rcu_barrier_sema);
        init_completion(&rcu_barrier_completion);
        atomic_set(&rcu_barrier_cpu_count, 0);
        on_each_cpu(rcu_barrier_func, NULL, 0, 1);
        wait_for_completion(&rcu_barrier_completion);
        up(&rcu_barrier_sema);
}
EXPORT_SYMBOL_GPL(rcu_barrier);

/*
 * Invoke the completed RCU callbacks. They are expected to be in
 * a per-cpu list.
 */
static void rcu_do_batch(struct rcu_data *rdp)
{
        struct rcu_head *next, *list;
        int count = 0;

        list = rdp->donelist;
        while (list) {
                next = rdp->donelist = list->next;
                list->func(list);
                list = next;
                rdp->count--;
                if (++count >= maxbatch)
                        break;
        }
        if (!rdp->donelist)
                rdp->donetail = &rdp->donelist;
        else
                tasklet_schedule(&per_cpu(rcu_tasklet, rdp->cpu));
}

/*
 * Grace period handling:
 * The grace period handling consists out of two steps:
 * - A new grace period is started.
 *   This is done by rcu_start_batch. The start is not broadcasted to
 *   all cpus, they must pick this up by comparing rcp->cur with
 *   rdp->quiescbatch. All cpus are recorded  in the
 *   rcu_state.cpumask bitmap.
 * - All cpus must go through a quiescent state.
 *   Since the start of the grace period is not broadcasted, at least two
 *   calls to rcu_check_quiescent_state are required:
 *   The first call just notices that a new grace period is running. The
 *   following calls check if there was a quiescent state since the beginning
 *   of the grace period. If so, it updates rcu_state.cpumask. If
 *   the bitmap is empty, then the grace period is completed.
 *   rcu_check_quiescent_state calls rcu_start_batch(0) to start the next grace
 *   period (if necessary).
 */
/*
 * Register a new batch of callbacks, and start it up if there is currently no
 * active batch and the batch to be registered has not already occurred.
 * Caller must hold rcu_state.lock.
 */
static void rcu_start_batch(struct rcu_ctrlblk *rcp, struct rcu_state *rsp,
                                int next_pending)
{
        if (next_pending)
                rcp->next_pending = 1;

        if (rcp->next_pending &&
                        rcp->completed == rcp->cur) {
                /* Can't change, since spin lock held. */
                cpus_andnot(rsp->cpumask, cpu_online_map, nohz_cpu_mask);

                rcp->next_pending = 0;
                /* next_pending == 0 must be visible in __rcu_process_callbacks()
                 * before it can see new value of cur.
                 */
                smp_wmb();
                rcp->cur++;
        }
}

/*
 * cpu went through a quiescent state since the beginning of the grace period.
 * Clear it from the cpu mask and complete the grace period if it was the last
 * cpu. Start another grace period if someone has further entries pending
 */
static void cpu_quiet(int cpu, struct rcu_ctrlblk *rcp, struct rcu_state *rsp)
{
        cpu_clear(cpu, rsp->cpumask);
        if (cpus_empty(rsp->cpumask)) {
                /* batch completed ! */
                rcp->completed = rcp->cur;
                rcu_start_batch(rcp, rsp, 0);
        }
}

/*
 * Check if the cpu has gone through a quiescent state (say context
 * switch). If so and if it already hasn't done so in this RCU
 * quiescent cycle, then indicate that it has done so.
 */
static void rcu_check_quiescent_state(struct rcu_ctrlblk *rcp,
                        struct rcu_state *rsp, struct rcu_data *rdp)
{
        if (rdp->quiescbatch != rcp->cur) {
                /* start new grace period: */
                rdp->qs_pending = 1;
                rdp->passed_quiesc = 0;
                rdp->quiescbatch = rcp->cur;
                return;
        }

        /* Grace period already completed for this cpu?
         * qs_pending is checked instead of the actual bitmap to avoid
         * cacheline trashing.
         */
        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;
        rdp->qs_pending = 0;

        spin_lock(&rsp->lock);
        /*
         * rdp->quiescbatch/rcp->cur and the cpu bitmap can come out of sync
         * during cpu startup. Ignore the quiescent state.
         */
        if (likely(rdp->quiescbatch == rcp->cur))
                cpu_quiet(rdp->cpu, rcp, rsp);

        spin_unlock(&rsp->lock);
}


#ifdef CONFIG_HOTPLUG_CPU

/* warning! helper for rcu_offline_cpu. do not use elsewhere without reviewing
 * locking requirements, the list it's pulling from has to belong to a cpu
 * which is dead and hence not processing interrupts.
 */
static void rcu_move_batch(struct rcu_data *this_rdp, struct rcu_head *list,
                                struct rcu_head **tail)
{
        local_irq_disable();
        *this_rdp->nxttail = list;
        if (list)
                this_rdp->nxttail = tail;
        local_irq_enable();
}

static void __rcu_offline_cpu(struct rcu_data *this_rdp,
        struct rcu_ctrlblk *rcp, struct rcu_state *rsp, struct rcu_data *rdp)
{
        /* if the cpu going offline owns the grace period
         * we can block indefinitely waiting for it, so flush
         * it here
         */
        spin_lock_bh(&rsp->lock);
        if (rcp->cur != rcp->completed)
                cpu_quiet(rdp->cpu, rcp, rsp);
        spin_unlock_bh(&rsp->lock);
        rcu_move_batch(this_rdp, rdp->curlist, rdp->curtail);
        rcu_move_batch(this_rdp, rdp->nxtlist, rdp->nxttail);

}
static void rcu_offline_cpu(int cpu)
{
        struct rcu_data *this_rdp = &get_cpu_var(rcu_data);
        struct rcu_data *this_bh_rdp = &get_cpu_var(rcu_bh_data);

        __rcu_offline_cpu(this_rdp, &rcu_ctrlblk, &rcu_state,
                                        &per_cpu(rcu_data, cpu));
        __rcu_offline_cpu(this_bh_rdp, &rcu_bh_ctrlblk, &rcu_bh_state,
                                        &per_cpu(rcu_bh_data, cpu));
        put_cpu_var(rcu_data);
        put_cpu_var(rcu_bh_data);
        tasklet_kill_immediate(&per_cpu(rcu_tasklet, cpu), cpu);
}

#else

static void rcu_offline_cpu(int cpu)
{
}

#endif

/*
 * This does the RCU processing work from tasklet context. 
 */
static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
                        struct rcu_state *rsp, struct rcu_data *rdp)
{
        if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch)) {
                *rdp->donetail = rdp->curlist;
                rdp->donetail = rdp->curtail;
                rdp->curlist = NULL;
                rdp->curtail = &rdp->curlist;
        }

        local_irq_disable();
        if (rdp->nxtlist && !rdp->curlist) {
                rdp->curlist = rdp->nxtlist;
                rdp->curtail = rdp->nxttail;
                rdp->nxtlist = NULL;
                rdp->nxttail = &rdp->nxtlist;
                local_irq_enable();

                /*
                 * start the next batch of callbacks
                 */

                /* determine batch number */
                rdp->batch = rcp->cur + 1;
                /* see the comment and corresponding wmb() in
                 * the rcu_start_batch()
                 */
                smp_rmb();

                if (!rcp->next_pending) {
                        /* and start it/schedule start if it's a new batch */
                        spin_lock(&rsp->lock);
                        rcu_start_batch(rcp, rsp, 1);
                        spin_unlock(&rsp->lock);
                }
        } else {
                local_irq_enable();
        }
        rcu_check_quiescent_state(rcp, rsp, rdp);
        if (rdp->donelist)
                rcu_do_batch(rdp);
}

static void rcu_process_callbacks(unsigned long unused)
{
        __rcu_process_callbacks(&rcu_ctrlblk, &rcu_state,
                                &__get_cpu_var(rcu_data));
        __rcu_process_callbacks(&rcu_bh_ctrlblk, &rcu_bh_state,
                                &__get_cpu_var(rcu_bh_data));
}

void rcu_check_callbacks(int cpu, int user)
{
        if (user || 
            (idle_cpu(cpu) && !in_softirq() && 
                                hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
                rcu_qsctr_inc(cpu);
                rcu_bh_qsctr_inc(cpu);
        } else if (!in_softirq())
                rcu_bh_qsctr_inc(cpu);
        tasklet_schedule(&per_cpu(rcu_tasklet, cpu));
}

static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
                                                struct rcu_data *rdp)
{
        memset(rdp, 0, sizeof(*rdp));
        rdp->curtail = &rdp->curlist;
        rdp->nxttail = &rdp->nxtlist;
        rdp->donetail = &rdp->donelist;
        rdp->quiescbatch = rcp->completed;
        rdp->qs_pending = 0;
        rdp->cpu = cpu;
}

static void __devinit rcu_online_cpu(int cpu)
{
        struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
        struct rcu_data *bh_rdp = &per_cpu(rcu_bh_data, cpu);

        rcu_init_percpu_data(cpu, &rcu_ctrlblk, rdp);
        rcu_init_percpu_data(cpu, &rcu_bh_ctrlblk, bh_rdp);
        tasklet_init(&per_cpu(rcu_tasklet, cpu), rcu_process_callbacks, 0UL);
}

static int __devinit rcu_cpu_notify(struct notifier_block *self, 
                                unsigned long action, void *hcpu)
{
        long cpu = (long)hcpu;
        switch (action) {
        case CPU_UP_PREPARE:
                rcu_online_cpu(cpu);
                break;
        case CPU_DEAD:
                rcu_offline_cpu(cpu);
                break;
        default:
                break;
        }
        return NOTIFY_OK;
}

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

/*
 * Initializes rcu mechanism.  Assumed to be called early.
 * That is before local timer(SMP) or jiffie timer (uniproc) is setup.
 * Note that rcu_qsctr and friends are implicitly
 * initialized due to the choice of ``0'' for RCU_CTR_INVALID.
 */
void __init rcu_init(void)
{
        sema_init(&rcu_barrier_sema, 1);
        rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE,
                        (void *)(long)smp_processor_id());
        /* Register notifier for non-boot CPUs */
        register_cpu_notifier(&rcu_nb);
}

struct rcu_synchronize {
        struct rcu_head head;
        struct completion completion;
};

/* Because of FASTCALL declaration of complete, we use this wrapper */
static void wakeme_after_rcu(struct rcu_head  *head)
{
        struct rcu_synchronize *rcu;

        rcu = container_of(head, struct rcu_synchronize, head);
        complete(&rcu->completion);
}

/**
 * synchronize_rcu - wait until a grace period has elapsed.
 *
 * Control will return to the caller some time after a full grace
 * period has elapsed, in other words after all currently executing RCU
 * read-side critical sections have completed.  RCU read-side critical
 * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 * and may be nested.
 *
 * If your read-side code is not protected by rcu_read_lock(), do -not-
 * use synchronize_rcu().
 */
void synchronize_rcu(void)
{
        struct rcu_synchronize rcu;

        init_completion(&rcu.completion);
        /* Will wake me after RCU finished */
        call_rcu(&rcu.head, wakeme_after_rcu);

        /* Wait for it */
        wait_for_completion(&rcu.completion);
}

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

module_param(maxbatch, int, 0);
EXPORT_SYMBOL_GPL(rcu_batches_completed);
EXPORT_SYMBOL(call_rcu);  /* WARNING: GPL-only in April 2006. */
EXPORT_SYMBOL(call_rcu_bh);  /* WARNING: GPL-only in April 2006. */
EXPORT_SYMBOL_GPL(synchronize_rcu);
EXPORT_SYMBOL(synchronize_kernel);  /* WARNING: GPL-only in April 2006. */