Changeset 181a746 in mainline for kernel/generic/src/synch/rcu.c

Timestamp:

2012-07-10T17:50:29Z (12 years ago)

Author:

Adam Hraska <adam.hraska+hos@…>

Branches:

lfn, master, serial, ticket/834-toolchain-update, topic/msim-upgrade, topic/simplify-dev-export

Children:

5b6c033

Parents:

22b5924

Message:

rcu: Added preemptible RCU's core API implementation.

File:

• kernel/generic/src/synch/rcu.c (modified) (1 diff)

kernel/generic/src/synch/rcu.c

@@ -34 +34 @@
 /**
  * @file
- * @brief RCU.
+ * @brief Preemptible read-copy update. Usable from interrupt handlers.
  */
  
 #include <synch/rcu.h>
-
-
-/** Delimits the start of an RCU reader critical section.
- *
+#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)
 {
-        /* todo: implement */
-}
-
-
-/** Delimits the end of an RCU reader critical section.
- *
- */
+        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)
 {
-        /* todo: implement */
-}
-
-
-/** Blocks until all preexisting readers exit their critical sections.
- *
- */
+        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)
 {
-        /* todo: implement */
-}
-
-
-/** Adds a callback to invoke after all preexisting readers finish.
- *
- */
-void rcu_call(struct rcu_head *head, rcu_cb_func_t func)
-{
-        /* todo: implement */
-}
-
+        /* 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, rcu_gp_t cb_gp)
+{
+        if (compl > cb_gp) {
+                CPU->rcu.stat_missed_gps += (size_t)(compl - cb_gp);
+        }
+}
+
+/** Executes all callbacks for the given completed grace period. */
+static void exec_completed_cbs(rcu_gp_t last_completed_gp)
+{
+        if (CPU->rcu.cur_cbs_gp <= last_completed_gp) {
+                upd_stat_missed_gp(last_completed_gp, CPU->rcu.cur_cbs_gp);
+                exec_cbs(&CPU->rcu.cur_cbs);
+        }
+        
+        if (CPU->rcu.next_cbs_gp <= last_completed_gp) {
+                upd_stat_missed_gp(last_completed_gp, CPU->rcu.next_cbs_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_append_list(&rcu.next_preempted, &rcu.cur_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_delayed_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");
+}
 
 /** @}
  */
- 
-