source: mainline/kernel/generic/src/synch/rcu.c@ e8471b9

/*
 * Copyright (c) 2012 Adam Hraska
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * - Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 * - The name of the author may not be used to endorse or promote products
 *   derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
 
/** @addtogroup sync
 * @{
 */

/**
 * @file
 * @brief Preemptible read-copy update. Usable from interrupt handlers.
 */
 
#include <synch/rcu.h>
#include <synch/condvar.h>
#include <synch/semaphore.h>
#include <synch/spinlock.h>
#include <proc/thread.h>
#include <cpu/cpu_mask.h>
#include <cpu.h>
#include <smp/smp_call.h>
#include <compiler/barrier.h>
#include <atomic.h>
#include <arch.h>
#include <macros.h>

/* 
 * Number of milliseconds to give to preexisting readers to finish 
 * when non-expedited grace period detection is in progress.
 */
#define DETECT_SLEEP_MS    5
/* 
 * Max number of pending callbacks in the local cpu's queue before 
 * aggressively expediting the current grace period
 */
#define EXPEDITE_THRESHOLD 1000
/* Half the number of values a uint32 can hold. */
#define UINT32_MAX_HALF    2147483648U


/*c Global RCU data. */
typedef struct rcu_data {
        /** Detector uses so signal reclaimers that a grace period ended. */
        condvar_t gp_ended;
        /** Reclaimers notify the detector when they request more grace periods.*/
        condvar_t req_gp_changed;
        /** Reclaimers use to notify the detector to accelerate GP detection. */
        condvar_t expedite_now;
        /** 
         * The detector waits on this semaphore for any readers delaying the GP.
         * 
         * Each of the cpus with readers that are delaying the current GP 
         * must up() this sema once they reach a quiescent state. If there 
         * are any readers in cur_preempted (ie preempted preexisting) and 
         * they are already delaying GP detection, the last to unlock its
         * reader section must up() this sema once.
         */
        semaphore_t remaining_readers;
        
        /** Protects the 4 fields below. */
        SPINLOCK_DECLARE(gp_lock);
        /** Number of grace period ends the detector was requested to announce. */
        size_t req_gp_end_cnt;
        /** Number of consecutive grace periods to detect quickly and aggressively.*/
        size_t req_expedited_cnt;
        /** 
         * The current grace period number. Increases monotonically. 
         * Lock gp_lock or preempt_lock to get a current value.
         */
        rcu_gp_t cur_gp;
        /**
         * The number of the most recently completed grace period. 
         * At most one behind cur_gp. If equal to cur_gp, a grace 
         * period detection is not in progress and the detector
         * is idle.
         */
        rcu_gp_t completed_gp;
        
        /** Protect the following 3 fields. */
        IRQ_SPINLOCK_DECLARE(preempt_lock);
        /** Preexisting readers that have been preempted. */
        list_t cur_preempted;
        /** Reader that have been preempted and might delay the next grace period.*/
        list_t next_preempted;
        /** 
         * The detector is waiting for the last preempted reader 
         * in cur_preempted to announce that it exited its reader 
         * section by up()ing remaining_readers.
         */
        bool preempt_blocking_det;
        
        /** 
         * Number of cpus with readers that are delaying the current GP.
         * They will up() remaining_readers.
         */
        atomic_t delaying_cpu_cnt;
        
        /** Interruptable attached detector thread pointer. */
        thread_t *detector_thr;
        
        /* Some statistics. */
        size_t stat_expedited_cnt;
        size_t stat_delayed_cnt;
        size_t stat_preempt_blocking_cnt;
        /* Does not contain self/local calls. */
        size_t stat_smp_call_cnt;
} rcu_data_t;


static rcu_data_t rcu;

static void start_detector(void);
static void start_reclaimers(void);
static void rcu_read_unlock_impl(size_t *pnesting_cnt);
static void synch_complete(rcu_item_t *rcu_item);
static void check_qs(void);
static void record_qs(void);
static void signal_read_unlock(void);
static bool arriving_cbs_empty(void);
static bool next_cbs_empty(void);
static bool cur_cbs_empty(void);
static bool all_cbs_empty(void);
static void reclaimer(void *arg);
static bool wait_for_pending_cbs(void);
static bool advance_cbs(void);
static void exec_completed_cbs(rcu_gp_t last_completed_gp);
static void exec_cbs(rcu_item_t **phead);
static void req_detection(size_t req_cnt);
static bool wait_for_cur_cbs_gp_end(bool expedite, rcu_gp_t *last_completed_gp);
static bool cv_wait_for_gp(rcu_gp_t wait_on_gp);
static void detector(void *);
static bool wait_for_detect_req(void);
static void start_new_gp(void);
static void end_cur_gp(void);
static bool wait_for_readers(void);
static void rm_quiescent_cpus(cpu_mask_t *cpu_mask);
static bool gp_sleep(void);
static void interrupt_delaying_cpus(cpu_mask_t *cpu_mask);
static void sample_local_cpu(void *);
static bool wait_for_delaying_cpus(void);
static bool wait_for_preempt_reader(void);



/** Initializes global RCU structures. */
void rcu_init(void)
{
        condvar_initialize(&rcu.gp_ended);
        condvar_initialize(&rcu.req_gp_changed);
        condvar_initialize(&rcu.expedite_now);
        semaphore_initialize(&rcu.remaining_readers, 0);
        
        spinlock_initialize(&rcu.gp_lock, "rcu.gp_lock");
        rcu.req_gp_end_cnt = 0;
        rcu.req_expedited_cnt = 0;
        rcu.cur_gp = 0;
        rcu.completed_gp = 0;
        
        irq_spinlock_initialize(&rcu.preempt_lock, "rcu.preempt_lock");
        list_initialize(&rcu.cur_preempted);
        list_initialize(&rcu.next_preempted);
        rcu.preempt_blocking_det = false;
        
        atomic_set(&rcu.delaying_cpu_cnt, 0);
        
        rcu.detector_thr = 0;
        
        rcu.stat_expedited_cnt = 0;
        rcu.stat_delayed_cnt = 0;
        rcu.stat_preempt_blocking_cnt = 0;
        rcu.stat_smp_call_cnt = 0;
}

/** Initializes per-CPU RCU data. If on the boot cpu inits global data too.*/
void rcu_cpu_init(void)
{
        if (config.cpu_active == 1) {
                rcu_init();
        }
        
        CPU->rcu.last_seen_gp = 0;
        
        CPU->rcu.pnesting_cnt = &CPU->rcu.tmp_nesting_cnt;
        CPU->rcu.tmp_nesting_cnt = 0;
        
        CPU->rcu.cur_cbs = 0;
        CPU->rcu.next_cbs = 0;
        CPU->rcu.arriving_cbs = 0;
        CPU->rcu.parriving_cbs_tail = &CPU->rcu.arriving_cbs;
        
        CPU->rcu.arriving_cbs_cnt = 0;

        CPU->rcu.cur_cbs_gp = 0;
        CPU->rcu.next_cbs_gp = 0;
        
        CPU->rcu.is_delaying_gp = false;
        
        semaphore_initialize(&CPU->rcu.arrived_flag, 0);
        CPU->rcu.reclaimer_thr = 0;
        
        CPU->rcu.stat_max_cbs = 0;
        CPU->rcu.stat_avg_cbs = 0;
        CPU->rcu.stat_missed_gps = 0;
}

/** Completes RCU init. Creates and runs the detector and reclaimer threads.*/
void rcu_kinit_init(void)
{
        start_detector();
        start_reclaimers();
}

/** Initializes any per-thread RCU structures. */
void rcu_thread_init(thread_t *thread)
{
        thread->rcu.nesting_cnt = 0;
        thread->rcu.was_preempted = false;
        link_initialize(&thread->rcu.preempt_link);
}

/** Called from scheduler() when exiting the current thread. 
 * 
 * Preemption or interrupts are disabled and the scheduler() already
 * switched away from the current thread, calling rcu_after_thread_ran().
 */
void rcu_thread_exiting(void)
{
        ASSERT(THREAD != 0);
        ASSERT(THREAD->state == Exiting);
        ASSERT(PREEMPTION_DISABLED || interrupts_disabled());
        /* 
         * The scheduler() must have already switched to a temporary
         * nesting counter for interrupt handlers (we could be idle)
         * so that interrupt handlers do not modify the exiting thread's
         * reader section nesting count while we examine/process it.
         */
        ASSERT(&CPU->rcu.tmp_nesting_cnt == CPU->rcu.pnesting_cnt);
        
        /* 
         * The thread forgot to exit its reader critical secion. 
         * It is a bug, but rather than letting the entire system lock up
         * forcefully leave the reader section. The thread is not holding 
         * any references anyway since it is exiting so it is safe.
         */
        if (0 < THREAD->rcu.nesting_cnt) {
                THREAD->rcu.nesting_cnt = 1;
                rcu_read_unlock_impl(&THREAD->rcu.nesting_cnt);
        }
}

/** Cleans up global RCU resources and stops dispatching callbacks. 
 * 
 * Call when shutting down the kernel. Outstanding callbacks will
 * not be processed. Instead they will linger forever.
 */
void rcu_stop(void)
{
        /* todo: stop accepting new callbacks instead of just letting them linger?*/
        
        /* Stop and wait for reclaimers. */
        for (unsigned int cpu_id = 0; cpu_id < config.cpu_active; ++cpu_id) {
                ASSERT(cpus[cpu_id].rcu.reclaimer_thr != 0);
        
                if (cpus[cpu_id].rcu.reclaimer_thr) {
                        thread_interrupt(cpus[cpu_id].rcu.reclaimer_thr);
                        thread_join(cpus[cpu_id].rcu.reclaimer_thr);
                        thread_detach(cpus[cpu_id].rcu.reclaimer_thr);
                        cpus[cpu_id].rcu.reclaimer_thr = 0;
                }
        }

        /* Stop the detector and wait. */
        if (rcu.detector_thr) {
                thread_interrupt(rcu.detector_thr);
                thread_join(rcu.detector_thr);
                thread_detach(rcu.detector_thr);
                rcu.detector_thr = 0;
        }
}

/** Starts the detector thread. */
static void start_detector(void)
{
        rcu.detector_thr = 
                thread_create(detector, 0, TASK, THREAD_FLAG_NONE, "rcu-det");
        
        if (!rcu.detector_thr) 
                panic("Failed to create RCU detector thread.");
        
        thread_ready(rcu.detector_thr);
}

/** Creates and runs cpu-bound reclaimer threads. */
static void start_reclaimers(void)
{
        for (unsigned int cpu_id = 0; cpu_id < config.cpu_count; ++cpu_id) {
                char name[THREAD_NAME_BUFLEN] = {0};
                
                snprintf(name, THREAD_NAME_BUFLEN - 1, "rcu-rec/%u", cpu_id);
                
                cpus[cpu_id].rcu.reclaimer_thr = 
                        thread_create(reclaimer, 0, TASK, THREAD_FLAG_NONE, name);

                if (!cpus[cpu_id].rcu.reclaimer_thr) 
                        panic("Failed to create RCU reclaimer thread on cpu%u.", cpu_id);

                thread_wire(cpus[cpu_id].rcu.reclaimer_thr, &cpus[cpu_id]);
                thread_ready(cpus[cpu_id].rcu.reclaimer_thr);
        }
}

/** Returns the number of elapsed grace periods since boot. */
uint64_t rcu_completed_gps(void)
{
        spinlock_lock(&rcu.gp_lock);
        uint64_t completed = rcu.completed_gp;
        spinlock_unlock(&rcu.gp_lock);
        
        return completed;
}

/** Delimits the start of an RCU reader critical section. 
 * 
 * Reader sections may be nested and are preemptable. You must not
 * however block/sleep within reader sections.
 */
void rcu_read_lock(void)
{
        ASSERT(CPU);
        preemption_disable();

        check_qs();
        ++(*CPU->rcu.pnesting_cnt);

        preemption_enable();
}

/** Delimits the end of an RCU reader critical section. */
void rcu_read_unlock(void)
{
        ASSERT(CPU);
        preemption_disable();
        
        rcu_read_unlock_impl(CPU->rcu.pnesting_cnt);
        
        preemption_enable();
}

/** Unlocks the local reader section using the given nesting count. 
 * 
 * Preemption or interrupts must be disabled. 
 * 
 * @param pnesting_cnt Either &CPU->rcu.tmp_nesting_cnt or 
 *           THREAD->rcu.nesting_cnt.
 */
static void rcu_read_unlock_impl(size_t *pnesting_cnt)
{
        ASSERT(PREEMPTION_DISABLED || interrupts_disabled());
        
        if (0 == --(*pnesting_cnt)) {
                record_qs();
                
                /* 
                 * The thread was preempted while in a critical section or 
                 * the detector is eagerly waiting for this cpu's reader 
                 * to finish. 
                 * 
                 * Note that THREAD may be 0 in scheduler() and not just during boot.
                 */
                if ((THREAD && THREAD->rcu.was_preempted) || CPU->rcu.is_delaying_gp) {
                        /* Rechecks with disabled interrupts. */
                        signal_read_unlock();
                }
        }
}

/** Records a QS if not in a reader critical section. */
static void check_qs(void)
{
        ASSERT(PREEMPTION_DISABLED || interrupts_disabled());
        
        if (0 == *CPU->rcu.pnesting_cnt)
                record_qs();
}

/** Unconditionally records a quiescent state for the local cpu. */
static void record_qs(void)
{
        ASSERT(PREEMPTION_DISABLED || interrupts_disabled());
        
        /* 
         * A new GP was started since the last time we passed a QS. 
         * Notify the detector we have reached a new QS.
         */
        if (CPU->rcu.last_seen_gp != rcu.cur_gp) {
                rcu_gp_t cur_gp = ACCESS_ONCE(rcu.cur_gp);
                /* 
                 * Contain memory accesses within a reader critical section. 
                 * If we are in rcu_lock() it also makes changes prior to the
                 * start of the GP visible in the reader section.
                 */
                memory_barrier();
                /*
                 * Acknowledge we passed a QS since the beginning of rcu.cur_gp.
                 * Cache coherency will lazily transport the value to the
                 * detector while it sleeps in gp_sleep(). 
                 * 
                 * Note that there is a theoretical possibility that we
                 * overwrite a more recent/greater last_seen_gp here with 
                 * an older/smaller value. If this cpu is interrupted here
                 * while in rcu_lock() reader sections in the interrupt handler 
                 * will update last_seen_gp to the same value as is currently 
                 * in local cur_gp. However, if the cpu continues processing 
                 * interrupts and the detector starts a new GP immediately, 
                 * local interrupt handlers may update last_seen_gp again (ie 
                 * properly ack the new GP) with a value greater than local cur_gp. 
                 * Resetting last_seen_gp to a previous value here is however 
                 * benign and we only have to remember that this reader may end up 
                 * in cur_preempted even after the GP ends. That is why we
                 * append next_preempted to cur_preempted rather than overwriting 
                 * it as if cur_preempted were empty.
                 */
                CPU->rcu.last_seen_gp = cur_gp;
        }
}

/** If necessary, signals the detector that we exited a reader section. */
static void signal_read_unlock(void)
{
        ASSERT(PREEMPTION_DISABLED || interrupts_disabled());
        
        /*
         * We have to disable interrupts in order to make checking
         * and resetting was_preempted and is_delaying_gp atomic 
         * with respect to local interrupt handlers. Otherwise
         * an interrupt could beat us to calling semaphore_up()
         * before we reset the appropriate flag.
         */
        ipl_t ipl = interrupts_disable();
        
        /* 
         * If the detector is eagerly waiting for this cpu's reader to unlock,
         * notify it that the reader did so.
         */
        if (CPU->rcu.is_delaying_gp) {
                CPU->rcu.is_delaying_gp = false;
                semaphore_up(&rcu.remaining_readers);
        }
        
        /*
         * This reader was preempted while in a reader section.
         * We might be holding up the current GP. Notify the
         * detector if so.
         */
        if (THREAD && THREAD->rcu.was_preempted) {
                ASSERT(link_used(&THREAD->rcu.preempt_link));
                THREAD->rcu.was_preempted = false;

                irq_spinlock_lock(&rcu.preempt_lock, false);
                
                bool prev_empty = list_empty(&rcu.cur_preempted);
                list_remove(&THREAD->rcu.preempt_link);
                bool now_empty = list_empty(&rcu.cur_preempted);
                
                /* This was the last reader in cur_preempted. */
                bool last_removed = now_empty && !prev_empty;
                
                /* 
                 * Preempted readers are blocking the detector and 
                 * this was the last reader blocking the current GP. 
                 */
                if (last_removed && rcu.preempt_blocking_det) {
                        rcu.preempt_blocking_det = false;
                        semaphore_up(&rcu.remaining_readers);
                }
                
                irq_spinlock_unlock(&rcu.preempt_lock, false);
        }
        interrupts_restore(ipl);
}

typedef struct synch_item {
        waitq_t wq;
        rcu_item_t rcu_item;
} synch_item_t;

/** Blocks until all preexisting readers exit their critical sections. */
void rcu_synchronize(void)
{
        /* Calling from a reader section will deadlock. */
        ASSERT(THREAD == 0 || 0 == THREAD->rcu.nesting_cnt);
        
        synch_item_t completion; 

        waitq_initialize(&completion.wq);
        rcu_call(&completion.rcu_item, synch_complete);
        waitq_sleep(&completion.wq);
        waitq_complete_wakeup(&completion.wq);
}

/** rcu_synchronize's callback. */
static void synch_complete(rcu_item_t *rcu_item)
{
        synch_item_t *completion = member_to_inst(rcu_item, synch_item_t, rcu_item);
        ASSERT(completion);
        waitq_wakeup(&completion->wq, WAKEUP_FIRST);
}

/** Adds a callback to invoke after all preexisting readers finish. 
 * 
 * May be called from within interrupt handlers or RCU reader sections.
 * 
 * @param rcu_item Used by RCU to track the call. Must remain
 *         until the user callback function is entered.
 * @param func User callback function that will be invoked once a full
 *         grace period elapsed, ie at a time when all preexisting
 *         readers have finished. The callback should be short and must
 *         not block. If you must sleep, enqueue your work in the system
 *         work queue from the callback (ie workq_global_enqueue()).
 */
void rcu_call(rcu_item_t *rcu_item, rcu_func_t func)
{
        _rcu_call(false, rcu_item, func);
}

/** rcu_call() implementation. See rcu_call() for comments. */
void _rcu_call(bool expedite, rcu_item_t *rcu_item, rcu_func_t func)
{
        ASSERT(rcu_item);
        
        rcu_item->func = func;
        rcu_item->next = 0;
        
        preemption_disable();
        
        ipl_t ipl = interrupts_disable();

        *CPU->rcu.parriving_cbs_tail = rcu_item;
        CPU->rcu.parriving_cbs_tail = &rcu_item->next;
        
        size_t cnt = ++CPU->rcu.arriving_cbs_cnt;
        interrupts_restore(ipl);
        
        if (expedite) {
                CPU->rcu.expedite_arriving = true;
        }
        
        /* Added first callback - notify the reclaimer. */
        if (cnt == 1 && !semaphore_count_get(&CPU->rcu.arrived_flag)) {
                semaphore_up(&CPU->rcu.arrived_flag);
        }
        
        preemption_enable();
}

static bool cur_cbs_empty(void)
{
        ASSERT(THREAD && THREAD->wired);
        return 0 == CPU->rcu.cur_cbs;
}

static bool next_cbs_empty(void)
{
        ASSERT(THREAD && THREAD->wired);
        return 0 == CPU->rcu.next_cbs;
}

/** Disable interrupts to get an up-to-date result. */
static bool arriving_cbs_empty(void)
{
        ASSERT(THREAD && THREAD->wired);
        /* 
         * Accessing with interrupts enabled may at worst lead to 
         * a false negative if we race with a local interrupt handler.
         */
        return 0 == CPU->rcu.arriving_cbs;
}

static bool all_cbs_empty(void)
{
        return cur_cbs_empty() && next_cbs_empty() && arriving_cbs_empty();
}

/** Reclaimer thread dispatches locally queued callbacks once a GP ends. */
static void reclaimer(void *arg)
{
        ASSERT(THREAD && THREAD->wired);

        rcu_gp_t last_compl_gp = 0;
        bool ok = true;
        
        while (ok && wait_for_pending_cbs()) {
                exec_completed_cbs(last_compl_gp);
                
                bool expedite = advance_cbs();
                
                ok = wait_for_cur_cbs_gp_end(expedite, &last_compl_gp);
        }
}

/** Waits until there are callbacks waiting to be dispatched. */
static bool wait_for_pending_cbs(void)
{
        if (!all_cbs_empty()) 
                return true;

        bool ok = true;
        
        while (arriving_cbs_empty() && ok) {
                ok = semaphore_down_interruptable(&CPU->rcu.arrived_flag);
        }
        
        return ok;
}

static void upd_stat_missed_gp(rcu_gp_t compl)
{
        if (CPU->rcu.cur_cbs_gp < compl) {
                CPU->rcu.stat_missed_gps += (size_t)(compl - CPU->rcu.cur_cbs_gp);
        }
}

/** Executes all callbacks for the given completed grace period. */
static void exec_completed_cbs(rcu_gp_t last_completed_gp)
{
        upd_stat_missed_gp(last_completed_gp);
        
        if (CPU->rcu.cur_cbs_gp <= last_completed_gp) {
                exec_cbs(&CPU->rcu.cur_cbs);
        }
        
        if (CPU->rcu.next_cbs_gp <= last_completed_gp) {
                exec_cbs(&CPU->rcu.next_cbs);   
        }
}

/** Executes callbacks in the single-linked list. The list is left empty. */
static void exec_cbs(rcu_item_t **phead)
{
        rcu_item_t *rcu_item = *phead;

        while (rcu_item) {
                /* func() may free rcu_item. Get a local copy. */
                rcu_item_t *next = rcu_item->next;
                rcu_func_t func = rcu_item->func;
                
                func(rcu_item);
                
                rcu_item = next;
        }
        
        *phead = 0;
}

static void upd_stat_cb_cnts(size_t arriving_cnt)
{
        CPU->rcu.stat_max_cbs = max(arriving_cnt, CPU->rcu.stat_max_cbs);
        if (0 < arriving_cnt) {
                CPU->rcu.stat_avg_cbs = 
                        (99 * CPU->rcu.stat_avg_cbs + 1 * arriving_cnt) / 100;
        }
}


/** Prepares another batch of callbacks to dispatch at the nest grace period.
 * 
 * @return True if the next batch of callbacks must be expedited quickly.
 */
static bool advance_cbs(void)
{
        /* Move next_cbs to cur_cbs. */
        CPU->rcu.cur_cbs = CPU->rcu.next_cbs;
        CPU->rcu.cur_cbs_gp = CPU->rcu.next_cbs_gp;
        
        /* Move arriving_cbs to next_cbs. Empties arriving_cbs. */
        ipl_t ipl = interrupts_disable();

        /* 
         * Too many callbacks queued. Better speed up the detection
         * or risk exhausting all system memory.
         */
        bool expedite = (EXPEDITE_THRESHOLD < CPU->rcu.arriving_cbs_cnt)
                || CPU->rcu.expedite_arriving;  

        /* Update statistics. */
        upd_stat_cb_cnts(CPU->rcu.arriving_cbs_cnt);
                
        CPU->rcu.expedite_arriving = false;
        CPU->rcu.next_cbs = CPU->rcu.arriving_cbs;
        CPU->rcu.arriving_cbs = 0;
        CPU->rcu.parriving_cbs_tail = &CPU->rcu.arriving_cbs;
        CPU->rcu.arriving_cbs_cnt = 0;
        
        interrupts_restore(ipl);
        
        /* 
         * Make changes prior to queuing next_cbs visible to readers. 
         * See comment in wait_for_readers().
         */
        memory_barrier(); /* MB A, B */

        /* At the end of next_cbs_gp, exec next_cbs. Determine what GP that is. */
        
        if (!next_cbs_empty()) {
                spinlock_lock(&rcu.gp_lock);
        
                /* Exec next_cbs at the end of the next GP. */
                CPU->rcu.next_cbs_gp = rcu.cur_gp + 1;
                
                /* 
                 * There are no callbacks to invoke before next_cbs. Instruct
                 * wait_for_cur_cbs_gp() to notify us of the nearest GP end.
                 * That could be sooner than next_cbs_gp (if the current GP 
                 * had not yet completed), so we'll create a shorter batch
                 * of callbacks next time around.
                 */
                if (cur_cbs_empty()) {
                        CPU->rcu.cur_cbs_gp = rcu.completed_gp + 1;
                } 
                
                spinlock_unlock(&rcu.gp_lock);
        } else {
                CPU->rcu.next_cbs_gp = CPU->rcu.cur_cbs_gp;
        }
        
        ASSERT(CPU->rcu.cur_cbs_gp <= CPU->rcu.next_cbs_gp);
        
        return expedite;        
}

/** Waits for the grace period associated with callbacks cub_cbs to elapse. 
 * 
 * @param expedite Instructs the detector to aggressively speed up grace 
 *            period detection without any delay.
 * @param completed_gp Returns the most recent completed grace period 
 *            number.
 * @return false if the thread was interrupted and should stop.
 */
static bool wait_for_cur_cbs_gp_end(bool expedite, rcu_gp_t *completed_gp)
{
        /* 
         * Use a possibly outdated version of completed_gp to bypass checking
         * with the lock.
         * 
         * Note that loading and storing rcu.completed_gp is not atomic 
         * (it is 64bit wide). Reading a clobbered value that is less than 
         * rcu.completed_gp is harmless - we'll recheck with a lock. The 
         * only way to read a clobbered value that is greater than the actual 
         * value is if the detector increases the higher-order word first and 
         * then decreases the lower-order word (or we see stores in that order), 
         * eg when incrementing from 2^32 - 1 to 2^32. The loaded value 
         * suddenly jumps by 2^32. It would take hours for such an increase 
         * to occur so it is safe to discard the value. We allow increases 
         * of up to half the maximum to generously accommodate for loading an
         * outdated lower word.
         */
        rcu_gp_t compl_gp = ACCESS_ONCE(rcu.completed_gp);
        if (CPU->rcu.cur_cbs_gp <= compl_gp 
                && compl_gp <= CPU->rcu.cur_cbs_gp + UINT32_MAX_HALF) {
                *completed_gp = compl_gp;
                return true;
        }
        
        spinlock_lock(&rcu.gp_lock);
        
        if (CPU->rcu.cur_cbs_gp <= rcu.completed_gp) {
                *completed_gp = rcu.completed_gp;
                spinlock_unlock(&rcu.gp_lock);
                return true;
        }
        
        ASSERT(CPU->rcu.cur_cbs_gp <= CPU->rcu.next_cbs_gp);
        ASSERT(rcu.cur_gp <= CPU->rcu.cur_cbs_gp);
        
        /* 
         * Notify the detector of how many GP ends we intend to wait for, so 
         * it can avoid going to sleep unnecessarily. Optimistically assume
         * new callbacks will arrive while we're waiting; hence +1.
         */
        size_t remaining_gp_ends = (size_t) (CPU->rcu.next_cbs_gp - rcu.cur_gp);
        req_detection(remaining_gp_ends + (arriving_cbs_empty() ? 0 : 1));
        
        /* 
         * Ask the detector to speed up GP detection if there are too many 
         * pending callbacks and other reclaimers have not already done so.
         */
        if (expedite) {
                if(0 == rcu.req_expedited_cnt) 
                        condvar_signal(&rcu.expedite_now);
                
                /* 
                 * Expedite only cub_cbs. If there really is a surge of callbacks 
                 * the arriving batch will expedite the GP for the huge number
                 * of callbacks currently in next_cbs
                 */
                rcu.req_expedited_cnt = 1;
        }

        /* Wait for cur_cbs_gp to end. */
        bool interrupted = cv_wait_for_gp(CPU->rcu.cur_cbs_gp);

        *completed_gp = rcu.completed_gp;
        spinlock_unlock(&rcu.gp_lock);  
        
        return !interrupted;
}

/** Requests the detector to detect at least req_cnt consecutive grace periods.*/
static void req_detection(size_t req_cnt)
{
        if (rcu.req_gp_end_cnt < req_cnt) {
                bool detector_idle = (0 == rcu.req_gp_end_cnt);
                rcu.req_gp_end_cnt = req_cnt;

                //printf("reqs:%d,idle:%d ", req_cnt, detector_idle);
                
                if (detector_idle) {
                        ASSERT(rcu.cur_gp == rcu.completed_gp);
                        condvar_signal(&rcu.req_gp_changed);
                }
        } else {
                //printf("myreqs:%d,detr:%d ", req_cnt, rcu.req_gp_end_cnt);
        }
}

/** Waits for an announcement of the end of the grace period wait_on_gp. */
static bool cv_wait_for_gp(rcu_gp_t wait_on_gp)
{
        ASSERT(spinlock_locked(&rcu.gp_lock));
        
        bool interrupted = false;
        
        /* Wait until wait_on_gp ends. */
        while (rcu.completed_gp < wait_on_gp && !interrupted) {
                int ret = _condvar_wait_timeout_spinlock(&rcu.gp_ended, &rcu.gp_lock, 
                        SYNCH_NO_TIMEOUT, SYNCH_FLAGS_INTERRUPTIBLE);
                interrupted = (ret == ESYNCH_INTERRUPTED);
        }
        
        ASSERT(wait_on_gp <= rcu.completed_gp);
        
        return interrupted;
}

/** The detector thread detects and notifies reclaimers of grace period ends. */
static void detector(void *arg)
{
        spinlock_lock(&rcu.gp_lock);
        
        while (wait_for_detect_req()) {
                /* 
                 * Announce new GP started. Readers start lazily acknowledging that
                 * they passed a QS.
                 */
                start_new_gp();
                
                spinlock_unlock(&rcu.gp_lock);
                
                if (!wait_for_readers()) 
                        goto unlocked_out;
                
                spinlock_lock(&rcu.gp_lock);

                /* Notify reclaimers that they may now invoke queued callbacks. */
                end_cur_gp();
        }
        
        spinlock_unlock(&rcu.gp_lock);
        
unlocked_out:
        return;
}

/** Waits for a request from a reclaimer thread to detect a grace period. */
static bool wait_for_detect_req(void)
{
        ASSERT(spinlock_locked(&rcu.gp_lock));
        
        bool interrupted = false;
        
        while (0 == rcu.req_gp_end_cnt && !interrupted) {
                int ret = _condvar_wait_timeout_spinlock(&rcu.req_gp_changed, 
                        &rcu.gp_lock, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_INTERRUPTIBLE);
                
                interrupted = (ret == ESYNCH_INTERRUPTED);
        }
        
        return !interrupted;
}

/** Announces the start of a new grace period for preexisting readers to ack. */
static void start_new_gp(void)
{
        ASSERT(spinlock_locked(&rcu.gp_lock));
        
        irq_spinlock_lock(&rcu.preempt_lock, true);
        
        /* Start a new GP. Announce to readers that a quiescent state is needed. */
        ++rcu.cur_gp;
        
        /* 
         * Readers preempted before the start of this GP (next_preempted)
         * are preexisting readers now that a GP started and will hold up 
         * the current GP until they exit their reader sections.
         * 
         * Preempted readers from the previous GP have finished so 
         * cur_preempted is empty, but see comment in record_qs(). 
         */
        list_concat(&rcu.cur_preempted, &rcu.next_preempted);
        
        irq_spinlock_unlock(&rcu.preempt_lock, true);
}

static void end_cur_gp(void)
{
        ASSERT(spinlock_locked(&rcu.gp_lock));
        
        rcu.completed_gp = rcu.cur_gp;
        --rcu.req_gp_end_cnt;
        
        condvar_broadcast(&rcu.gp_ended);
}

/** Waits for readers that started before the current GP started to finish. */
static bool wait_for_readers(void)
{
        DEFINE_CPU_MASK(reading_cpus);
        
        /* All running cpus have potential readers. */
        cpu_mask_active(reading_cpus);

        /*
         * Ensure the announcement of the start of a new GP (ie up-to-date 
         * cur_gp) propagates to cpus that are just coming out of idle 
         * mode before we sample their idle state flag.
         * 
         * Cpus guarantee that after they set CPU->idle = true they will not
         * execute any RCU reader sections without first setting idle to
         * false and issuing a memory barrier. Therefore, if rm_quiescent_cpus()
         * later on sees an idle cpu, but the cpu is just exiting its idle mode,
         * the cpu must not have yet executed its memory barrier (otherwise
         * it would pair up with this mem barrier and we would see idle == false).
         * That memory barrier will pair up with the one below and ensure
         * that a reader on the now-non-idle cpu will see the most current
         * cur_gp. As a result, such a reader will never attempt to semaphore_up(
         * pending_readers) during this GP, which allows the detector to
         * ignore that cpu (the detector thinks it is idle). Moreover, any
         * changes made by RCU updaters will have propagated to readers
         * on the previously idle cpu -- again thanks to issuing a memory
         * barrier after returning from idle mode.
         * 
         * idle -> non-idle cpu      | detector      | reclaimer
         * ------------------------------------------------------
         * rcu reader 1              |               | rcu_call()
         * MB X                      |               |
         * idle = true               |               | rcu_call() 
         * (no rcu readers allowed ) |               | MB A in advance_cbs() 
         * MB Y                      | (...)         | (...)
         * (no rcu readers allowed)  |               | MB B in advance_cbs() 
         * idle = false              | ++cur_gp      |
         * (no rcu readers allowed)  | MB C          |
         * MB Z                      | signal gp_end |
         * rcu reader 2              |               | exec_cur_cbs()
         * 
         * 
         * MB Y orders visibility of changes to idle for detector's sake.
         * 
         * MB Z pairs up with MB C. The cpu making a transition from idle 
         * will see the most current value of cur_gp and will not attempt
         * to notify the detector even if preempted during this GP.
         * 
         * MB Z pairs up with MB A from the previous batch. Updaters' changes
         * are visible to reader 2 even when the detector thinks the cpu is idle 
         * but it is not anymore.
         * 
         * MB X pairs up with MB B. Late mem accesses of reader 1 are contained
         * and visible before idling and before any callbacks are executed 
         * by reclaimers.
         * 
         * In summary, the detector does not know of or wait for reader 2, but
         * it does not have to since it is a new reader that will not access
         * data from previous GPs and will see any changes.
         */
        memory_barrier(); /* MB C */
        
        /* 
         * Give readers time to pass through a QS. Also, batch arriving 
         * callbacks in order to amortize detection overhead.
         */
        if (!gp_sleep())
                return false;
        
        /* Non-intrusively determine which cpus have yet to pass a QS. */
        rm_quiescent_cpus(reading_cpus);
        
        /* Actively interrupt cpus delaying the current GP and demand a QS. */
        interrupt_delaying_cpus(reading_cpus);
        
        /* Wait for the interrupted cpus to notify us that they reached a QS. */
        if (!wait_for_delaying_cpus())
                return false;
        /*
         * All cpus recorded a QS or are still idle. Any new readers will be added
         * to next_preempt if preempted, ie the number of readers in cur_preempted
         * monotonically descreases.
         */
        
        /* Wait for the last reader in cur_preempted to notify us it is done. */
        if (!wait_for_preempt_reader())
                return false;
        
        return true;
}

/** Remove those cpus from the mask that have already passed a quiescent
 * state since the start of the current grace period.
 */
static void rm_quiescent_cpus(cpu_mask_t *cpu_mask)
{
        cpu_mask_for_each(*cpu_mask, cpu_id) {
                /* 
                 * The cpu already checked for and passed through a quiescent 
                 * state since the beginning of this GP.
                 * 
                 * rcu.cur_gp is modified by local detector thread only. 
                 * Therefore, it is up-to-date even without a lock. 
                 */
                bool cpu_acked_gp = (cpus[cpu_id].rcu.last_seen_gp == rcu.cur_gp);
                
                /*
                 * Either the cpu is idle or it is exiting away from idle mode
                 * and already sees the most current rcu.cur_gp. See comment
                 * in wait_for_readers().
                 */
                bool cpu_idle = cpus[cpu_id].idle;
                
                if (cpu_acked_gp || cpu_idle) {
                        cpu_mask_reset(cpu_mask, cpu_id);
                }
        }
}

/** Sleeps a while if the current grace period is not to be expedited. */
static bool gp_sleep(void)
{
        spinlock_lock(&rcu.gp_lock);

        int ret = 0;
        while (0 == rcu.req_expedited_cnt && 0 == ret) {
                /* minor bug: sleeps for the same duration if woken up spuriously. */
                ret = _condvar_wait_timeout_spinlock(&rcu.expedite_now, &rcu.gp_lock,
                        DETECT_SLEEP_MS * 1000, SYNCH_FLAGS_INTERRUPTIBLE);
        }
        
        if (0 < rcu.req_expedited_cnt) {
                --rcu.req_expedited_cnt;
                /* Update statistic. */
                ++rcu.stat_expedited_cnt;
        }
        
        spinlock_unlock(&rcu.gp_lock);
        
        return (ret != ESYNCH_INTERRUPTED);
}

/** Actively interrupts and checks the offending cpus for quiescent states. */
static void interrupt_delaying_cpus(cpu_mask_t *cpu_mask)
{
        const size_t max_conconcurrent_calls = 16;
        smp_call_t call[max_conconcurrent_calls];
        size_t outstanding_calls = 0;
        
        atomic_set(&rcu.delaying_cpu_cnt, 0);
        
        cpu_mask_for_each(*cpu_mask, cpu_id) {
                smp_call_async(cpu_id, sample_local_cpu, 0, &call[outstanding_calls]);
                ++outstanding_calls;

                /* Update statistic. */
                if (CPU->id != cpu_id)
                        ++rcu.stat_smp_call_cnt;
                
                if (outstanding_calls == max_conconcurrent_calls) {
                        for (size_t k = 0; k < outstanding_calls; ++k) {
                                smp_call_wait(&call[k]);
                        }
                        
                        outstanding_calls = 0;
                }
        }
        
        for (size_t k = 0; k < outstanding_calls; ++k) {
                smp_call_wait(&call[k]);
        }
}

/** Invoked on a cpu delaying grace period detection. 
 * 
 * Induces a quiescent state for the cpu or it instructs remaining 
 * readers to notify the detector once they finish.
 */
static void sample_local_cpu(void *arg)
{
        ASSERT(interrupts_disabled());
        ASSERT(!CPU->rcu.is_delaying_gp);
        
        /* Cpu did not pass a quiescent state yet. */
        if (CPU->rcu.last_seen_gp != rcu.cur_gp) {
                /* Interrupted a reader in a reader critical section. */
                if (0 < (*CPU->rcu.pnesting_cnt)) {
                        ASSERT(!CPU->idle);
                        /* Note to notify the detector from rcu_read_unlock(). */
                        CPU->rcu.is_delaying_gp = true;
                        atomic_inc(&rcu.delaying_cpu_cnt);
                } else {
                        /* 
                         * The cpu did not enter any rcu reader sections since 
                         * the start of the current GP. Record a quiescent state.
                         * 
                         * Or, we interrupted rcu_read_unlock_impl() right before
                         * it recorded a QS. Record a QS for it. The memory barrier 
                         * contains the reader section's mem accesses before 
                         * updating last_seen_gp.
                         * 
                         * Or, we interrupted rcu_read_lock() right after it recorded
                         * a QS for the previous GP but before it got a chance to
                         * increment its nesting count. The memory barrier again
                         * stops the CS code from spilling out of the CS.
                         */
                        memory_barrier();
                        CPU->rcu.last_seen_gp = rcu.cur_gp;
                }
        } else {
                /* 
                 * This cpu already acknowledged that it had passed through 
                 * a quiescent state since the start of cur_gp. 
                 */
        }
        
        /* 
         * smp_call() makes sure any changes propagate back to the caller.
         * In particular, it makes the most current last_seen_gp visible
         * to the detector.
         */
}

/** Waits for cpus delaying the current grace period if there are any. */
static bool wait_for_delaying_cpus(void)
{
        int delaying_cpu_cnt = atomic_get(&rcu.delaying_cpu_cnt);

        for (int i = 0; i < delaying_cpu_cnt; ++i){
                if (!semaphore_down_interruptable(&rcu.remaining_readers))
                        return false;
        }
        
        /* Update statistic. */
        rcu.stat_delayed_cnt += delaying_cpu_cnt;
        
        return true;
}

/** Waits for any preempted readers blocking this grace period to finish.*/
static bool wait_for_preempt_reader(void)
{
        irq_spinlock_lock(&rcu.preempt_lock, true);

        bool reader_exists = !list_empty(&rcu.cur_preempted);
        rcu.preempt_blocking_det = reader_exists;
        
        irq_spinlock_unlock(&rcu.preempt_lock, true);
        
        if (reader_exists) {
                /* Update statistic. */
                ++rcu.stat_preempt_blocking_cnt;
                
                return semaphore_down_interruptable(&rcu.remaining_readers);
        }       
        
        return true;
}

/** Called by the scheduler() when switching away from the current thread. */
void rcu_after_thread_ran(void)
{
        ASSERT(interrupts_disabled());
        ASSERT(CPU->rcu.pnesting_cnt == &THREAD->rcu.nesting_cnt);
        
        /* Preempted a reader critical section for the first time. */
        if (0 < THREAD->rcu.nesting_cnt && !THREAD->rcu.was_preempted) {
                THREAD->rcu.was_preempted = true;
                
                irq_spinlock_lock(&rcu.preempt_lock, false);
                
                if (CPU->rcu.last_seen_gp != rcu.cur_gp) {
                        /* The reader started before the GP started - we must wait for it.*/
                        list_append(&THREAD->rcu.preempt_link, &rcu.cur_preempted);
                } else {
                        /* 
                         * The reader started after the GP started and this cpu
                         * already noted a quiescent state. We might block the next GP.
                         */
                        list_append(&THREAD->rcu.preempt_link, &rcu.next_preempted);
                }

                irq_spinlock_unlock(&rcu.preempt_lock, false);
        }
        
        /* 
         * The preempted reader has been noted globally. There are therefore
         * no readers running on this cpu so this is a quiescent state.
         */
        record_qs();

        /* 
         * This cpu is holding up the current GP. Let the detector know 
         * it has just passed a quiescent state. 
         * 
         * The detector waits separately for preempted readers, so we have 
         * to notify the detector even if we have just preempted a reader.
         */
        if (CPU->rcu.is_delaying_gp) {
                CPU->rcu.is_delaying_gp = false;
                semaphore_up(&rcu.remaining_readers);
        }
        
        /* 
         * After this point THREAD is 0 and stays 0 until the scheduler()
         * switches to a new thread. Use a temporary nesting counter for readers
         * in handlers of interrupts that are raised while idle in the scheduler.
         */
        CPU->rcu.pnesting_cnt = &CPU->rcu.tmp_nesting_cnt;

        /* 
         * Forcefully associate the detector with the highest priority
         * even if preempted due to its time slice running out.
         * 
         * todo: Replace with strict scheduler priority classes.
         */
        if (THREAD == rcu.detector_thr) {
                THREAD->priority = -1;
        } 
}

/** Called by the scheduler() when switching to a newly scheduled thread. */
void rcu_before_thread_runs(void)
{
        ASSERT(PREEMPTION_DISABLED || interrupts_disabled());
        ASSERT(&CPU->rcu.tmp_nesting_cnt == CPU->rcu.pnesting_cnt);
        
        CPU->rcu.pnesting_cnt = &THREAD->rcu.nesting_cnt;
}


/** Prints RCU run-time statistics. */
void rcu_print_stat(void)
{
        /* Don't take locks. Worst case is we get out-dated values. */
        printf("Configuration: expedite_threshold=%d, detect_sleep=%dms\n",
                EXPEDITE_THRESHOLD, DETECT_SLEEP_MS);
        printf("Completed GPs: %" PRIu64 "\n", rcu.completed_gp);
        printf("Expedited GPs: %zu\n", rcu.stat_expedited_cnt);
        printf("Delayed GPs:   %zu (cpus w/ still running readers after gp sleep)\n", 
                rcu.stat_delayed_cnt);
        printf("Preempt blocked GPs: %zu (waited for preempted readers; "
                "running or not)\n", rcu.stat_preempt_blocking_cnt);
        printf("Smp calls:     %zu\n", rcu.stat_smp_call_cnt);
        
        printf("Max callbacks per GP:\n");
        for (unsigned i = 0; i < config.cpu_count; ++i) {
                printf(" %zu", cpus[i].rcu.stat_max_cbs);
        }

        printf("\nAvg callbacks per GP (nonempty batches only):\n");
        for (unsigned i = 0; i < config.cpu_count; ++i) {
                printf(" %zu", cpus[i].rcu.stat_avg_cbs);
        }
        
        printf("\nMissed GP notifications:\n");
        for (unsigned i = 0; i < config.cpu_count; ++i) {
                printf(" %zu", cpus[i].rcu.stat_missed_gps);
        }
        printf("\n");
}

/** @}
 */