subr_epoch.c

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/*-
 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
 *
 * Copyright (c) 2018, Matthew Macy <mmacy@freebsd.org>
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. 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.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``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 OR CONTRIBUTORS 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.
 *
 */
 
#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");
 
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/counter.h>
#include <sys/epoch.h>
#include <sys/gtaskqueue.h>
#include <sys/kernel.h>
#include <sys/limits.h>
#include <sys/lock.h>
#include <sys/malloc.h>
#include <sys/mutex.h>
#include <sys/pcpu.h>
#include <sys/proc.h>
#include <sys/sched.h>
#include <sys/sx.h>
#include <sys/smp.h>
#include <sys/sysctl.h>
#include <sys/turnstile.h>
#ifdef EPOCH_TRACE
#include <machine/stdarg.h>
#include <sys/stack.h>
#include <sys/tree.h>
#endif
#include <vm/vm.h>
#include <vm/vm_extern.h>
#include <vm/vm_kern.h>
#include <vm/uma.h>
 
#include <ck_epoch.h>
 
#ifdef __amd64__
#define EPOCH_ALIGN CACHE_LINE_SIZE*2
#else
#define EPOCH_ALIGN CACHE_LINE_SIZE
#endif
 
TAILQ_HEAD (epoch_tdlist, epoch_tracker);
typedef struct epoch_record {
        ck_epoch_record_t er_record;
        struct epoch_context er_drain_ctx;
        struct epoch *er_parent;
        volatile struct epoch_tdlist er_tdlist;
        volatile uint32_t er_gen;
        uint32_t er_cpuid;
#ifdef INVARIANTS
        /* Used to verify record ownership for non-preemptible epochs. */
        struct thread *er_td;
#endif
} __aligned(EPOCH_ALIGN)     *epoch_record_t;
 
struct epoch {
        struct ck_epoch e_epoch __aligned(EPOCH_ALIGN);
        epoch_record_t e_pcpu_record;
        int     e_in_use;
        int     e_flags;
        struct sx e_drain_sx;
        struct mtx e_drain_mtx;
        volatile int e_drain_count;
        const char *e_name;
};
 
/* arbitrary --- needs benchmarking */
#define MAX_ADAPTIVE_SPIN 100
#define MAX_EPOCHS 64
 
CTASSERT(sizeof(ck_epoch_entry_t) == sizeof(struct epoch_context));
SYSCTL_NODE(_kern, OID_AUTO, epoch, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
    "epoch information");
SYSCTL_NODE(_kern_epoch, OID_AUTO, stats, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
    "epoch stats");
 
/* Stats. */
static counter_u64_t block_count;
 
SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, nblocked, CTLFLAG_RW,
    &block_count, "# of times a thread was in an epoch when epoch_wait was called");
static counter_u64_t migrate_count;
 
SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, migrations, CTLFLAG_RW,
    &migrate_count, "# of times thread was migrated to another CPU in epoch_wait");
static counter_u64_t turnstile_count;
 
SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, ncontended, CTLFLAG_RW,
    &turnstile_count, "# of times a thread was blocked on a lock in an epoch during an epoch_wait");
static counter_u64_t switch_count;
 
SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, switches, CTLFLAG_RW,
    &switch_count, "# of times a thread voluntarily context switched in epoch_wait");
static counter_u64_t epoch_call_count;
 
SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, epoch_calls, CTLFLAG_RW,
    &epoch_call_count, "# of times a callback was deferred");
static counter_u64_t epoch_call_task_count;
 
SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, epoch_call_tasks, CTLFLAG_RW,
    &epoch_call_task_count, "# of times a callback task was run");
 
TAILQ_HEAD (threadlist, thread);
 
CK_STACK_CONTAINER(struct ck_epoch_entry, stack_entry,
    ck_epoch_entry_container)
 
static struct epoch epoch_array[MAX_EPOCHS];
 
DPCPU_DEFINE(struct grouptask, epoch_cb_task);
DPCPU_DEFINE(int, epoch_cb_count);
 
static __read_mostly int inited;
__read_mostly epoch_t global_epoch;
__read_mostly epoch_t global_epoch_preempt;
 
static void epoch_call_task(void *context __unused);
static  uma_zone_t pcpu_zone_record;
 
static struct sx epoch_sx;
 
#define EPOCH_LOCK() sx_xlock(&epoch_sx)
#define EPOCH_UNLOCK() sx_xunlock(&epoch_sx)
 
static epoch_record_t
epoch_currecord(epoch_t epoch)
{
 
        return (zpcpu_get(epoch->e_pcpu_record));
}
 
#ifdef EPOCH_TRACE
struct stackentry {
        RB_ENTRY(stackentry) se_node;
        struct stack se_stack;
};
 
static int
stackentry_compare(struct stackentry *a, struct stackentry *b)
{
 
        if (a->se_stack.depth > b->se_stack.depth)
                return (1);
        if (a->se_stack.depth < b->se_stack.depth)
                return (-1);
        for (int i = 0; i < a->se_stack.depth; i++) {
                if (a->se_stack.pcs[i] > b->se_stack.pcs[i])
                        return (1);
                if (a->se_stack.pcs[i] < b->se_stack.pcs[i])
                        return (-1);
        }
 
        return (0);
}
 
RB_HEAD(stacktree, stackentry) epoch_stacks = RB_INITIALIZER(&epoch_stacks);
RB_GENERATE_STATIC(stacktree, stackentry, se_node, stackentry_compare);
 
static struct mtx epoch_stacks_lock;
MTX_SYSINIT(epochstacks, &epoch_stacks_lock, "epoch_stacks", MTX_DEF);
 
static bool epoch_trace_stack_print = true;
SYSCTL_BOOL(_kern_epoch, OID_AUTO, trace_stack_print, CTLFLAG_RWTUN,
    &epoch_trace_stack_print, 0, "Print stack traces on epoch reports");
 
static void epoch_trace_report(const char *fmt, ...) __printflike(1, 2);
static inline void
epoch_trace_report(const char *fmt, ...)
{
        va_list ap;
        struct stackentry se, *new;
 
        stack_zero(&se.se_stack);       /* XXX: is it really needed? */
        stack_save(&se.se_stack);
 
        /* Tree is never reduced - go lockless. */
        if (RB_FIND(stacktree, &epoch_stacks, &se) != NULL)
                return;
 
        new = malloc(sizeof(*new), M_STACK, M_NOWAIT);
        if (new != NULL) {
                bcopy(&se.se_stack, &new->se_stack, sizeof(struct stack));
 
                mtx_lock(&epoch_stacks_lock);
                new = RB_INSERT(stacktree, &epoch_stacks, new);
                mtx_unlock(&epoch_stacks_lock);
                if (new != NULL)
                        free(new, M_STACK);
        }
 
        va_start(ap, fmt);
        (void)vprintf(fmt, ap);
        va_end(ap);
        if (epoch_trace_stack_print)
                stack_print_ddb(&se.se_stack);
}
 
static inline void
epoch_trace_enter(struct thread *td, epoch_t epoch, epoch_tracker_t et,
    const char *file, int line)
{
        epoch_tracker_t iet;
 
        SLIST_FOREACH(iet, &td->td_epochs, et_tlink) {
                if (iet->et_epoch != epoch)
                        continue;
                epoch_trace_report("Recursively entering epoch %s "
                    "at %s:%d, previously entered at %s:%d\n",
                    epoch->e_name, file, line,
                    iet->et_file, iet->et_line);
        }
        et->et_epoch = epoch;
        et->et_file = file;
        et->et_line = line;
        et->et_flags = 0;
        SLIST_INSERT_HEAD(&td->td_epochs, et, et_tlink);
}
 
static inline void
epoch_trace_exit(struct thread *td, epoch_t epoch, epoch_tracker_t et,
    const char *file, int line)
{
 
        if (SLIST_FIRST(&td->td_epochs) != et) {
                epoch_trace_report("Exiting epoch %s in a not nested order "
                    "at %s:%d. Most recently entered %s at %s:%d\n",
                    epoch->e_name,
                    file, line,
                    SLIST_FIRST(&td->td_epochs)->et_epoch->e_name,
                    SLIST_FIRST(&td->td_epochs)->et_file,
                    SLIST_FIRST(&td->td_epochs)->et_line);
                /* This will panic if et is not anywhere on td_epochs. */
                SLIST_REMOVE(&td->td_epochs, et, epoch_tracker, et_tlink);
        } else
                SLIST_REMOVE_HEAD(&td->td_epochs, et_tlink);
        if (et->et_flags & ET_REPORT_EXIT)
                printf("Td %p exiting epoch %s at %s:%d\n", td, epoch->e_name,
                    file, line);
}
 
/* Used by assertions that check thread state before going to sleep. */
void
epoch_trace_list(struct thread *td)
{
        epoch_tracker_t iet;
 
        SLIST_FOREACH(iet, &td->td_epochs, et_tlink)
                printf("Epoch %s entered at %s:%d\n", iet->et_epoch->e_name,
                    iet->et_file, iet->et_line);
}
 
void
epoch_where_report(epoch_t epoch)
{
        epoch_record_t er;
        struct epoch_tracker *tdwait;
 
        MPASS(epoch != NULL);
        MPASS((epoch->e_flags & EPOCH_PREEMPT) != 0);
        MPASS(!THREAD_CAN_SLEEP());
        critical_enter();
        er = epoch_currecord(epoch);
        TAILQ_FOREACH(tdwait, &er->er_tdlist, et_link)
                if (tdwait->et_td == curthread)
                        break;
        critical_exit();
        if (tdwait != NULL) {
                tdwait->et_flags |= ET_REPORT_EXIT;
                printf("Td %p entered epoch %s at %s:%d\n", curthread,
                    epoch->e_name, tdwait->et_file, tdwait->et_line);
        }
}
#endif /* EPOCH_TRACE */
 
static void
epoch_init(void *arg __unused)
{
        int cpu;
 
        block_count = counter_u64_alloc(M_WAITOK);
        migrate_count = counter_u64_alloc(M_WAITOK);
        turnstile_count = counter_u64_alloc(M_WAITOK);
        switch_count = counter_u64_alloc(M_WAITOK);
        epoch_call_count = counter_u64_alloc(M_WAITOK);
        epoch_call_task_count = counter_u64_alloc(M_WAITOK);
 
        pcpu_zone_record = uma_zcreate("epoch_record pcpu",
            sizeof(struct epoch_record), NULL, NULL, NULL, NULL,
            UMA_ALIGN_PTR, UMA_ZONE_PCPU);
        CPU_FOREACH(cpu) {
                GROUPTASK_INIT(DPCPU_ID_PTR(cpu, epoch_cb_task), 0,
                    epoch_call_task, NULL);
                taskqgroup_attach_cpu(qgroup_softirq,
                    DPCPU_ID_PTR(cpu, epoch_cb_task), NULL, cpu, NULL, NULL,
                    "epoch call task");
        }
#ifdef EPOCH_TRACE
        SLIST_INIT(&thread0.td_epochs);
#endif
        sx_init(&epoch_sx, "epoch-sx");
        inited = 1;
        global_epoch = epoch_alloc("Global", 0);
        global_epoch_preempt = epoch_alloc("Global preemptible", EPOCH_PREEMPT);
}
SYSINIT(epoch, SI_SUB_EPOCH, SI_ORDER_FIRST, epoch_init, NULL);
 
#if !defined(EARLY_AP_STARTUP)
static void
epoch_init_smp(void *dummy __unused)
{
        inited = 2;
}
SYSINIT(epoch_smp, SI_SUB_SMP + 1, SI_ORDER_FIRST, epoch_init_smp, NULL);
#endif
 
static void
epoch_ctor(epoch_t epoch)
{
        epoch_record_t er;
        int cpu;
 
        epoch->e_pcpu_record = uma_zalloc_pcpu(pcpu_zone_record, M_WAITOK);
        CPU_FOREACH(cpu) {
                er = zpcpu_get_cpu(epoch->e_pcpu_record, cpu);
                bzero(er, sizeof(*er));
                ck_epoch_register(&epoch->e_epoch, &er->er_record, NULL);
                TAILQ_INIT((struct threadlist *)(uintptr_t)&er->er_tdlist);
                er->er_cpuid = cpu;
                er->er_parent = epoch;
        }
}
 
static void
epoch_adjust_prio(struct thread *td, u_char prio)
{
 
        thread_lock(td);
        sched_prio(td, prio);
        thread_unlock(td);
}
 
epoch_t
epoch_alloc(const char *name, int flags)
{
        epoch_t epoch;
        int i;
 
        MPASS(name != NULL);
 
        if (__predict_false(!inited))
                panic("%s called too early in boot", __func__);
 
        EPOCH_LOCK();
 
        /*
         * Find a free index in the epoch array. If no free index is
         * found, try to use the index after the last one.
         */
        for (i = 0;; i++) {
                /*
                 * If too many epochs are currently allocated,
                 * return NULL.
                 */
                if (i == MAX_EPOCHS) {
                        epoch = NULL;
                        goto done;
                }
                if (epoch_array[i].e_in_use == 0)
                        break;
        }
 
        epoch = epoch_array + i;
        ck_epoch_init(&epoch->e_epoch);
        epoch_ctor(epoch);
        epoch->e_flags = flags;
        epoch->e_name = name;
        sx_init(&epoch->e_drain_sx, "epoch-drain-sx");
        mtx_init(&epoch->e_drain_mtx, "epoch-drain-mtx", NULL, MTX_DEF);
 
        /*
         * Set e_in_use last, because when this field is set the
         * epoch_call_task() function will start scanning this epoch
         * structure.
         */
        atomic_store_rel_int(&epoch->e_in_use, 1);
done:
        EPOCH_UNLOCK();
        return (epoch);
}
 
void
epoch_free(epoch_t epoch)
{
#ifdef INVARIANTS
        int cpu;
#endif
 
        EPOCH_LOCK();
 
        MPASS(epoch->e_in_use != 0);
 
        epoch_drain_callbacks(epoch);
 
        atomic_store_rel_int(&epoch->e_in_use, 0);
        /*
         * Make sure the epoch_call_task() function see e_in_use equal
         * to zero, by calling epoch_wait() on the global_epoch:
         */
        epoch_wait(global_epoch);
#ifdef INVARIANTS
        CPU_FOREACH(cpu) {
                epoch_record_t er;
 
                er = zpcpu_get_cpu(epoch->e_pcpu_record, cpu);
 
                /*
                 * Sanity check: none of the records should be in use anymore.
                 * We drained callbacks above and freeing the pcpu records is
                 * imminent.
                 */
                MPASS(er->er_td == NULL);
                MPASS(TAILQ_EMPTY(&er->er_tdlist));
        }
#endif
        uma_zfree_pcpu(pcpu_zone_record, epoch->e_pcpu_record);
        mtx_destroy(&epoch->e_drain_mtx);
        sx_destroy(&epoch->e_drain_sx);
        memset(epoch, 0, sizeof(*epoch));
 
        EPOCH_UNLOCK();
}
 
#define INIT_CHECK(epoch)                                       \
        do {                                                    \
                if (__predict_false((epoch) == NULL))           \
                        return;                                 \
        } while (0)
 
void
_epoch_enter_preempt(epoch_t epoch, epoch_tracker_t et EPOCH_FILE_LINE)
{
        struct epoch_record *er;
        struct thread *td;
 
        MPASS(cold || epoch != NULL);
        td = curthread;
        MPASS((vm_offset_t)et >= td->td_kstack &&
            (vm_offset_t)et + sizeof(struct epoch_tracker) <=
            td->td_kstack + td->td_kstack_pages * PAGE_SIZE);
 
        INIT_CHECK(epoch);
        MPASS(epoch->e_flags & EPOCH_PREEMPT);
 
#ifdef EPOCH_TRACE
        epoch_trace_enter(td, epoch, et, file, line);
#endif
        et->et_td = td;
        THREAD_NO_SLEEPING();
        critical_enter();
        sched_pin();
        et->et_old_priority = td->td_priority;
        er = epoch_currecord(epoch);
        /* Record-level tracking is reserved for non-preemptible epochs. */
        MPASS(er->er_td == NULL);
        TAILQ_INSERT_TAIL(&er->er_tdlist, et, et_link);
        ck_epoch_begin(&er->er_record, &et->et_section);
        critical_exit();
}
 
void
epoch_enter(epoch_t epoch)
{
        epoch_record_t er;
 
        MPASS(cold || epoch != NULL);
        INIT_CHECK(epoch);
        critical_enter();
        er = epoch_currecord(epoch);
#ifdef INVARIANTS
        if (er->er_record.active == 0) {
                MPASS(er->er_td == NULL);
                er->er_td = curthread;
        } else {
                /* We've recursed, just make sure our accounting isn't wrong. */
                MPASS(er->er_td == curthread);
        }
#endif
        ck_epoch_begin(&er->er_record, NULL);
}
 
void
_epoch_exit_preempt(epoch_t epoch, epoch_tracker_t et EPOCH_FILE_LINE)
{
        struct epoch_record *er;
        struct thread *td;
 
        INIT_CHECK(epoch);
        td = curthread;
        critical_enter();
        sched_unpin();
        THREAD_SLEEPING_OK();
        er = epoch_currecord(epoch);
        MPASS(epoch->e_flags & EPOCH_PREEMPT);
        MPASS(et != NULL);
        MPASS(et->et_td == td);
#ifdef INVARIANTS
        et->et_td = (void*)0xDEADBEEF;
        /* Record-level tracking is reserved for non-preemptible epochs. */
        MPASS(er->er_td == NULL);
#endif
        ck_epoch_end(&er->er_record, &et->et_section);
        TAILQ_REMOVE(&er->er_tdlist, et, et_link);
        er->er_gen++;
        if (__predict_false(et->et_old_priority != td->td_priority))
                epoch_adjust_prio(td, et->et_old_priority);
        critical_exit();
#ifdef EPOCH_TRACE
        epoch_trace_exit(td, epoch, et, file, line);
#endif
}
 
void
epoch_exit(epoch_t epoch)
{
        epoch_record_t er;
 
        INIT_CHECK(epoch);
        er = epoch_currecord(epoch);
        ck_epoch_end(&er->er_record, NULL);
#ifdef INVARIANTS
        MPASS(er->er_td == curthread);
        if (er->er_record.active == 0)
                er->er_td = NULL;
#endif
        critical_exit();
}
 
/*
 * epoch_block_handler_preempt() is a callback from the CK code when another
 * thread is currently in an epoch section.
 */
static void
epoch_block_handler_preempt(struct ck_epoch *global __unused,
    ck_epoch_record_t *cr, void *arg __unused)
{
        epoch_record_t record;
        struct thread *td, *owner, *curwaittd;
        struct epoch_tracker *tdwait;
        struct turnstile *ts;
        struct lock_object *lock;
        int spincount, gen;
        int locksheld __unused;
 
        record = __containerof(cr, struct epoch_record, er_record);
        td = curthread;
        locksheld = td->td_locks;
        spincount = 0;
        counter_u64_add(block_count, 1);
        /*
         * We lost a race and there's no longer any threads
         * on the CPU in an epoch section.
         */
        if (TAILQ_EMPTY(&record->er_tdlist))
                return;
 
        if (record->er_cpuid != curcpu) {
                /*
                 * If the head of the list is running, we can wait for it
                 * to remove itself from the list and thus save us the
                 * overhead of a migration
                 */
                gen = record->er_gen;
                thread_unlock(td);
                /*
                 * We can't actually check if the waiting thread is running
                 * so we simply poll for it to exit before giving up and
                 * migrating.
                 */
                do {
                        cpu_spinwait();
                } while (!TAILQ_EMPTY(&record->er_tdlist) &&
                                 gen == record->er_gen &&
                                 spincount++ < MAX_ADAPTIVE_SPIN);
                thread_lock(td);
                /*
                 * If the generation has changed we can poll again
                 * otherwise we need to migrate.
                 */
                if (gen != record->er_gen)
                        return;
                /*
                 * Being on the same CPU as that of the record on which
                 * we need to wait allows us access to the thread
                 * list associated with that CPU. We can then examine the
                 * oldest thread in the queue and wait on its turnstile
                 * until it resumes and so on until a grace period
                 * elapses.
                 *
                 */
                counter_u64_add(migrate_count, 1);
                sched_bind(td, record->er_cpuid);
                /*
                 * At this point we need to return to the ck code
                 * to scan to see if a grace period has elapsed.
                 * We can't move on to check the thread list, because
                 * in the meantime new threads may have arrived that
                 * in fact belong to a different epoch.
                 */
                return;
        }
        /*
         * Try to find a thread in an epoch section on this CPU
         * waiting on a turnstile. Otherwise find the lowest
         * priority thread (highest prio value) and drop our priority
         * to match to allow it to run.
         */
        TAILQ_FOREACH(tdwait, &record->er_tdlist, et_link) {
                /*
                 * Propagate our priority to any other waiters to prevent us
                 * from starving them. They will have their original priority
                 * restore on exit from epoch_wait().
                 */
                curwaittd = tdwait->et_td;
                if (!TD_IS_INHIBITED(curwaittd) && curwaittd->td_priority > td->td_priority) {
                        critical_enter();
                        thread_unlock(td);
                        thread_lock(curwaittd);
                        sched_prio(curwaittd, td->td_priority);
                        thread_unlock(curwaittd);
                        thread_lock(td);
                        critical_exit();
                }
                if (TD_IS_INHIBITED(curwaittd) && TD_ON_LOCK(curwaittd) &&
                    ((ts = curwaittd->td_blocked) != NULL)) {
                        /*
                         * We unlock td to allow turnstile_wait to reacquire
                         * the thread lock. Before unlocking it we enter a
                         * critical section to prevent preemption after we
                         * reenable interrupts by dropping the thread lock in
                         * order to prevent curwaittd from getting to run.
                         */
                        critical_enter();
                        thread_unlock(td);
 
                        if (turnstile_lock(ts, &lock, &owner)) {
                                if (ts == curwaittd->td_blocked) {
                                        MPASS(TD_IS_INHIBITED(curwaittd) &&
                                            TD_ON_LOCK(curwaittd));
                                        critical_exit();
                                        turnstile_wait(ts, owner,
                                            curwaittd->td_tsqueue);
                                        counter_u64_add(turnstile_count, 1);
                                        thread_lock(td);
                                        return;
                                }
                                turnstile_unlock(ts, lock);
                        }
                        thread_lock(td);
                        critical_exit();
                        KASSERT(td->td_locks == locksheld,
                            ("%d extra locks held", td->td_locks - locksheld));
                }
        }
        /*
         * We didn't find any threads actually blocked on a lock
         * so we have nothing to do except context switch away.
         */
        counter_u64_add(switch_count, 1);
        mi_switch(SW_VOL | SWT_RELINQUISH);
        /*
         * It is important the thread lock is dropped while yielding
         * to allow other threads to acquire the lock pointed to by
         * TDQ_LOCKPTR(td). Currently mi_switch() will unlock the
         * thread lock before returning. Else a deadlock like
         * situation might happen.
         */
        thread_lock(td);
}
 
void
epoch_wait_preempt(epoch_t epoch)
{
        struct thread *td;
        int was_bound;
        int old_cpu;
        int old_pinned;
        u_char old_prio;
        int locks __unused;
 
        MPASS(cold || epoch != NULL);
        INIT_CHECK(epoch);
        td = curthread;
#ifdef INVARIANTS
        locks = curthread->td_locks;
        MPASS(epoch->e_flags & EPOCH_PREEMPT);
        if ((epoch->e_flags & EPOCH_LOCKED) == 0)
                WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL,
                    "epoch_wait() can be long running");
        KASSERT(!in_epoch(epoch), ("epoch_wait_preempt() called in the middle "
            "of an epoch section of the same epoch"));
#endif
        DROP_GIANT();
        thread_lock(td);
 
        old_cpu = PCPU_GET(cpuid);
        old_pinned = td->td_pinned;
        old_prio = td->td_priority;
        was_bound = sched_is_bound(td);
        sched_unbind(td);
        td->td_pinned = 0;
        sched_bind(td, old_cpu);
 
        ck_epoch_synchronize_wait(&epoch->e_epoch, epoch_block_handler_preempt,
            NULL);
 
        /* restore CPU binding, if any */
        if (was_bound != 0) {
                sched_bind(td, old_cpu);
        } else {
                /* get thread back to initial CPU, if any */
                if (old_pinned != 0)
                        sched_bind(td, old_cpu);
                sched_unbind(td);
        }
        /* restore pinned after bind */
        td->td_pinned = old_pinned;
 
        /* restore thread priority */
        sched_prio(td, old_prio);
        thread_unlock(td);
        PICKUP_GIANT();
        KASSERT(td->td_locks == locks,
            ("%d residual locks held", td->td_locks - locks));
}
 
static void
epoch_block_handler(struct ck_epoch *g __unused, ck_epoch_record_t *c __unused,
    void *arg __unused)
{
        cpu_spinwait();
}
 
void
epoch_wait(epoch_t epoch)
{
 
        MPASS(cold || epoch != NULL);
        INIT_CHECK(epoch);
        MPASS(epoch->e_flags == 0);
        critical_enter();
        ck_epoch_synchronize_wait(&epoch->e_epoch, epoch_block_handler, NULL);
        critical_exit();
}
 
void
epoch_call(epoch_t epoch, epoch_callback_t callback, epoch_context_t ctx)
{
        epoch_record_t er;
        ck_epoch_entry_t *cb;
 
        cb = (void *)ctx;
 
        MPASS(callback);
        /* too early in boot to have epoch set up */
        if (__predict_false(epoch == NULL))
                goto boottime;
#if !defined(EARLY_AP_STARTUP)
        if (__predict_false(inited < 2))
                goto boottime;
#endif
 
        critical_enter();
        *DPCPU_PTR(epoch_cb_count) += 1;
        er = epoch_currecord(epoch);
        ck_epoch_call(&er->er_record, cb, (ck_epoch_cb_t *)callback);
        critical_exit();
        return;
boottime:
        callback(ctx);
}
 
static void
epoch_call_task(void *arg __unused)
{
        ck_stack_entry_t *cursor, *head, *next;
        ck_epoch_record_t *record;
        epoch_record_t er;
        epoch_t epoch;
        ck_stack_t cb_stack;
        int i, npending, total;
 
        ck_stack_init(&cb_stack);
        critical_enter();
        epoch_enter(global_epoch);
        for (total = i = 0; i != MAX_EPOCHS; i++) {
                epoch = epoch_array + i;
                if (__predict_false(
                    atomic_load_acq_int(&epoch->e_in_use) == 0))
                        continue;
                er = epoch_currecord(epoch);
                record = &er->er_record;
                if ((npending = record->n_pending) == 0)
                        continue;
                ck_epoch_poll_deferred(record, &cb_stack);
                total += npending - record->n_pending;
        }
        epoch_exit(global_epoch);
        *DPCPU_PTR(epoch_cb_count) -= total;
        critical_exit();
 
        counter_u64_add(epoch_call_count, total);
        counter_u64_add(epoch_call_task_count, 1);
 
        head = ck_stack_batch_pop_npsc(&cb_stack);
        for (cursor = head; cursor != NULL; cursor = next) {
                struct ck_epoch_entry *entry =
                    ck_epoch_entry_container(cursor);
 
                next = CK_STACK_NEXT(cursor);
                entry->function(entry);
        }
}
 
static int
in_epoch_verbose_preempt(epoch_t epoch, int dump_onfail)
{
        epoch_record_t er;
        struct epoch_tracker *tdwait;
        struct thread *td;
 
        MPASS(epoch != NULL);
        MPASS((epoch->e_flags & EPOCH_PREEMPT) != 0);
        td = curthread;
        if (THREAD_CAN_SLEEP())
                return (0);
        critical_enter();
        er = epoch_currecord(epoch);
        TAILQ_FOREACH(tdwait, &er->er_tdlist, et_link)
                if (tdwait->et_td == td) {
                        critical_exit();
                        return (1);
                }
#ifdef INVARIANTS
        if (dump_onfail) {
                MPASS(td->td_pinned);
                printf("cpu: %d id: %d\n", curcpu, td->td_tid);
                TAILQ_FOREACH(tdwait, &er->er_tdlist, et_link)
                        printf("td_tid: %d ", tdwait->et_td->td_tid);
                printf("\n");
        }
#endif
        critical_exit();
        return (0);
}
 
#ifdef INVARIANTS
static void
epoch_assert_nocpu(epoch_t epoch, struct thread *td)
{
        epoch_record_t er;
        int cpu;
        bool crit;
 
        crit = td->td_critnest > 0;
 
        /* Check for a critical section mishap. */
        CPU_FOREACH(cpu) {
                er = zpcpu_get_cpu(epoch->e_pcpu_record, cpu);
                KASSERT(er->er_td != td,
                    ("%s critical section in epoch '%s', from cpu %d",
                    (crit ? "exited" : "re-entered"), epoch->e_name, cpu));
        }
}
#else
#define epoch_assert_nocpu(e, td) do {} while (0)
#endif
 
int
in_epoch_verbose(epoch_t epoch, int dump_onfail)
{
        epoch_record_t er;
        struct thread *td;
 
        if (__predict_false((epoch) == NULL))
                return (0);
        if ((epoch->e_flags & EPOCH_PREEMPT) != 0)
                return (in_epoch_verbose_preempt(epoch, dump_onfail));
 
        /*
         * The thread being in a critical section is a necessary
         * condition to be correctly inside a non-preemptible epoch,
         * so it's definitely not in this epoch.
         */
        td = curthread;
        if (td->td_critnest == 0) {
                epoch_assert_nocpu(epoch, td);
                return (0);
        }
 
        /*
         * The current cpu is in a critical section, so the epoch record will be
         * stable for the rest of this function.  Knowing that the record is not
         * active is sufficient for knowing whether we're in this epoch or not,
         * since it's a pcpu record.
         */
        er = epoch_currecord(epoch);
        if (er->er_record.active == 0) {
                epoch_assert_nocpu(epoch, td);
                return (0);
        }
 
        MPASS(er->er_td == td);
        return (1);
}
 
int
in_epoch(epoch_t epoch)
{
        return (in_epoch_verbose(epoch, 0));
}
 
static void
epoch_drain_cb(struct epoch_context *ctx)
{
        struct epoch *epoch =
            __containerof(ctx, struct epoch_record, er_drain_ctx)->er_parent;
 
        if (atomic_fetchadd_int(&epoch->e_drain_count, -1) == 1) {
                mtx_lock(&epoch->e_drain_mtx);
                wakeup(epoch);
                mtx_unlock(&epoch->e_drain_mtx);
        }
}
 
void
epoch_drain_callbacks(epoch_t epoch)
{
        epoch_record_t er;
        struct thread *td;
        int was_bound;
        int old_pinned;
        int old_cpu;
        int cpu;
 
        WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL,
            "epoch_drain_callbacks() may sleep!");
 
        /* too early in boot to have epoch set up */
        if (__predict_false(epoch == NULL))
                return;
#if !defined(EARLY_AP_STARTUP)
        if (__predict_false(inited < 2))
                return;
#endif
        DROP_GIANT();
 
        sx_xlock(&epoch->e_drain_sx);
        mtx_lock(&epoch->e_drain_mtx);
 
        td = curthread;
        thread_lock(td);
        old_cpu = PCPU_GET(cpuid);
        old_pinned = td->td_pinned;
        was_bound = sched_is_bound(td);
        sched_unbind(td);
        td->td_pinned = 0;
 
        CPU_FOREACH(cpu)
                epoch->e_drain_count++;
        CPU_FOREACH(cpu) {
                er = zpcpu_get_cpu(epoch->e_pcpu_record, cpu);
                sched_bind(td, cpu);
                epoch_call(epoch, &epoch_drain_cb, &er->er_drain_ctx);
        }
 
        /* restore CPU binding, if any */
        if (was_bound != 0) {
                sched_bind(td, old_cpu);
        } else {
                /* get thread back to initial CPU, if any */
                if (old_pinned != 0)
                        sched_bind(td, old_cpu);
                sched_unbind(td);
        }
        /* restore pinned after bind */
        td->td_pinned = old_pinned;
 
        thread_unlock(td);
 
        while (epoch->e_drain_count != 0)
                msleep(epoch, &epoch->e_drain_mtx, PZERO, "EDRAIN", 0);
 
        mtx_unlock(&epoch->e_drain_mtx);
        sx_xunlock(&epoch->e_drain_sx);
 
        PICKUP_GIANT();
}