sys/kern/subr_epoch.c

   1 /*-
   2  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
   3  *
   4  * Copyright (c) 2018, Matthew Macy <mmacy@freebsd.org>
   5  *
   6  * Redistribution and use in source and binary forms, with or without
   7  * modification, are permitted provided that the following conditions
   8  * are met:
   9  * 1. Redistributions of source code must retain the above copyright
  10  *    notice, this list of conditions and the following disclaimer.
  11  * 2. Redistributions in binary form must reproduce the above copyright
  12  *    notice, this list of conditions and the following disclaimer in the
  13  *    documentation and/or other materials provided with the distribution.
  14  *
  15  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  16  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  17  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  18  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  19  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  20  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  21  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  22  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  23  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  24  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  25  * SUCH DAMAGE.
  26  *
  27  */
  28
  29 #include <sys/cdefs.h>
  30 __FBSDID("$FreeBSD$");
  31
  32 #include <sys/param.h>
  33 #include <sys/systm.h>
  34 #include <sys/counter.h>
  35 #include <sys/epoch.h>
  36 #include <sys/gtaskqueue.h>
  37 #include <sys/kernel.h>
  38 #include <sys/limits.h>
  39 #include <sys/lock.h>
  40 #include <sys/malloc.h>
  41 #include <sys/mutex.h>
  42 #include <sys/pcpu.h>
  43 #include <sys/proc.h>
  44 #include <sys/sched.h>
  45 #include <sys/sx.h>
  46 #include <sys/smp.h>
  47 #include <sys/sysctl.h>
  48 #include <sys/turnstile.h>
  49 #ifdef EPOCH_TRACE
  50 #include <machine/stdarg.h>
  51 #include <sys/stack.h>
  52 #include <sys/tree.h>
  53 #endif
  54 #include <vm/vm.h>
  55 #include <vm/vm_extern.h>
  56 #include <vm/vm_kern.h>
  57 #include <vm/uma.h>
  58
  59 #include <ck_epoch.h>
  60
  61 #ifdef __amd64__
  62 #define EPOCH_ALIGN CACHE_LINE_SIZE*2
  63 #else
  64 #define EPOCH_ALIGN CACHE_LINE_SIZE
  65 #endif
  66
  67 TAILQ_HEAD (epoch_tdlist, epoch_tracker);
  68 typedef struct epoch_record {
  69         ck_epoch_record_t er_record;
  70         struct epoch_context er_drain_ctx;
  71         struct epoch *er_parent;
  72         volatile struct epoch_tdlist er_tdlist;
  73         volatile uint32_t er_gen;
  74         uint32_t er_cpuid;
  75 #ifdef INVARIANTS
  76         /* Used to verify record ownership for non-preemptible epochs. */
  77         struct thread *er_td;
  78 #endif
  79 } __aligned(EPOCH_ALIGN)     *epoch_record_t;
  80
  81 struct epoch {
  82         struct ck_epoch e_epoch __aligned(EPOCH_ALIGN);
  83         epoch_record_t e_pcpu_record;
  84         int     e_in_use;
  85         int     e_flags;
  86         struct sx e_drain_sx;
  87         struct mtx e_drain_mtx;
  88         volatile int e_drain_count;
  89         const char *e_name;
  90 };
  91
  92 /* arbitrary --- needs benchmarking */
  93 #define MAX_ADAPTIVE_SPIN 100
  94 #define MAX_EPOCHS 64
  95
  96 CTASSERT(sizeof(ck_epoch_entry_t) == sizeof(struct epoch_context));
  97 SYSCTL_NODE(_kern, OID_AUTO, epoch, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
  98     "epoch information");
  99 SYSCTL_NODE(_kern_epoch, OID_AUTO, stats, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
 100     "epoch stats");
 101
 102 /* Stats. */
 103 static counter_u64_t block_count;
 104
 105 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, nblocked, CTLFLAG_RW,
 106     &block_count, "# of times a thread was in an epoch when epoch_wait was called");
 107 static counter_u64_t migrate_count;
 108
 109 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, migrations, CTLFLAG_RW,
 110     &migrate_count, "# of times thread was migrated to another CPU in epoch_wait");
 111 static counter_u64_t turnstile_count;
 112
 113 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, ncontended, CTLFLAG_RW,
 114     &turnstile_count, "# of times a thread was blocked on a lock in an epoch during an epoch_wait");
 115 static counter_u64_t switch_count;
 116
 117 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, switches, CTLFLAG_RW,
 118     &switch_count, "# of times a thread voluntarily context switched in epoch_wait");
 119 static counter_u64_t epoch_call_count;
 120
 121 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, epoch_calls, CTLFLAG_RW,
 122     &epoch_call_count, "# of times a callback was deferred");
 123 static counter_u64_t epoch_call_task_count;
 124
 125 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, epoch_call_tasks, CTLFLAG_RW,
 126     &epoch_call_task_count, "# of times a callback task was run");
 127
 128 TAILQ_HEAD (threadlist, thread);
 129
 130 CK_STACK_CONTAINER(struct ck_epoch_entry, stack_entry,
 131     ck_epoch_entry_container)
 132
 133 static struct epoch epoch_array[MAX_EPOCHS];
 134
 135 DPCPU_DEFINE(struct grouptask, epoch_cb_task);
 136 DPCPU_DEFINE(int, epoch_cb_count);
 137
 138 static __read_mostly int inited;
 139 __read_mostly epoch_t global_epoch;
 140 __read_mostly epoch_t global_epoch_preempt;
 141
 142 static void epoch_call_task(void *context __unused);
 143 static  uma_zone_t pcpu_zone_record;
 144
 145 static struct sx epoch_sx;
 146
 147 #define EPOCH_LOCK() sx_xlock(&epoch_sx)
 148 #define EPOCH_UNLOCK() sx_xunlock(&epoch_sx)
 149
 150 static epoch_record_t
 151 epoch_currecord(epoch_t epoch)
 152 {
 153
 154         return (zpcpu_get(epoch->e_pcpu_record));
 155 }
 156
 157 #ifdef EPOCH_TRACE
 158 struct stackentry {
 159         RB_ENTRY(stackentry) se_node;
 160         struct stack se_stack;
 161 };
 162
 163 static int
 164 stackentry_compare(struct stackentry *a, struct stackentry *b)
 165 {
 166
 167         if (a->se_stack.depth > b->se_stack.depth)
 168                 return (1);
 169         if (a->se_stack.depth < b->se_stack.depth)
 170                 return (-1);
 171         for (int i = 0; i < a->se_stack.depth; i++) {
 172                 if (a->se_stack.pcs[i] > b->se_stack.pcs[i])
 173                         return (1);
 174                 if (a->se_stack.pcs[i] < b->se_stack.pcs[i])
 175                         return (-1);
 176         }
 177
 178         return (0);
 179 }
 180
 181 RB_HEAD(stacktree, stackentry) epoch_stacks = RB_INITIALIZER(&epoch_stacks);
 182 RB_GENERATE_STATIC(stacktree, stackentry, se_node, stackentry_compare);
 183
 184 static struct mtx epoch_stacks_lock;
 185 MTX_SYSINIT(epochstacks, &epoch_stacks_lock, "epoch_stacks", MTX_DEF);
 186
 187 static bool epoch_trace_stack_print = true;
 188 SYSCTL_BOOL(_kern_epoch, OID_AUTO, trace_stack_print, CTLFLAG_RWTUN,
 189     &epoch_trace_stack_print, 0, "Print stack traces on epoch reports");
 190
 191 static void epoch_trace_report(const char *fmt, ...) __printflike(1, 2);
 192 static inline void
 193 epoch_trace_report(const char *fmt, ...)
 194 {
 195         va_list ap;
 196         struct stackentry se, *new;
 197
 198         stack_save(&se.se_stack);
 199
 200         /* Tree is never reduced - go lockless. */
 201         if (RB_FIND(stacktree, &epoch_stacks, &se) != NULL)
 202                 return;
 203
 204         new = malloc(sizeof(*new), M_STACK, M_NOWAIT);
 205         if (new != NULL) {
 206                 bcopy(&se.se_stack, &new->se_stack, sizeof(struct stack));
 207
 208                 mtx_lock(&epoch_stacks_lock);
 209                 new = RB_INSERT(stacktree, &epoch_stacks, new);
 210                 mtx_unlock(&epoch_stacks_lock);
 211                 if (new != NULL)
 212                         free(new, M_STACK);
 213         }
 214
 215         va_start(ap, fmt);
 216         (void)vprintf(fmt, ap);
 217         va_end(ap);
 218         if (epoch_trace_stack_print)
 219                 stack_print_ddb(&se.se_stack);
 220 }
 221
 222 static inline void
 223 epoch_trace_enter(struct thread *td, epoch_t epoch, epoch_tracker_t et,
 224     const char *file, int line)
 225 {
 226         epoch_tracker_t iet;
 227
 228         SLIST_FOREACH(iet, &td->td_epochs, et_tlink) {
 229                 if (iet->et_epoch != epoch)
 230                         continue;
 231                 epoch_trace_report("Recursively entering epoch %s "
 232                     "at %s:%d, previously entered at %s:%d\n",
 233                     epoch->e_name, file, line,
 234                     iet->et_file, iet->et_line);
 235         }
 236         et->et_epoch = epoch;
 237         et->et_file = file;
 238         et->et_line = line;
 239         et->et_flags = 0;
 240         SLIST_INSERT_HEAD(&td->td_epochs, et, et_tlink);
 241 }
 242
 243 static inline void
 244 epoch_trace_exit(struct thread *td, epoch_t epoch, epoch_tracker_t et,
 245     const char *file, int line)
 246 {
 247
 248         if (SLIST_FIRST(&td->td_epochs) != et) {
 249                 epoch_trace_report("Exiting epoch %s in a not nested order "
 250                     "at %s:%d. Most recently entered %s at %s:%d\n",
 251                     epoch->e_name,
 252                     file, line,
 253                     SLIST_FIRST(&td->td_epochs)->et_epoch->e_name,
 254                     SLIST_FIRST(&td->td_epochs)->et_file,
 255                     SLIST_FIRST(&td->td_epochs)->et_line);
 256                 /* This will panic if et is not anywhere on td_epochs. */
 257                 SLIST_REMOVE(&td->td_epochs, et, epoch_tracker, et_tlink);
 258         } else
 259                 SLIST_REMOVE_HEAD(&td->td_epochs, et_tlink);
 260         if (et->et_flags & ET_REPORT_EXIT)
 261                 printf("Td %p exiting epoch %s at %s:%d\n", td, epoch->e_name,
 262                     file, line);
 263 }
 264
 265 /* Used by assertions that check thread state before going to sleep. */
 266 void
 267 epoch_trace_list(struct thread *td)
 268 {
 269         epoch_tracker_t iet;
 270
 271         SLIST_FOREACH(iet, &td->td_epochs, et_tlink)
 272                 printf("Epoch %s entered at %s:%d\n", iet->et_epoch->e_name,
 273                     iet->et_file, iet->et_line);
 274 }
 275
 276 void
 277 epoch_where_report(epoch_t epoch)
 278 {
 279         epoch_record_t er;
 280         struct epoch_tracker *tdwait;
 281
 282         MPASS(epoch != NULL);
 283         MPASS((epoch->e_flags & EPOCH_PREEMPT) != 0);
 284         MPASS(!THREAD_CAN_SLEEP());
 285         critical_enter();
 286         er = epoch_currecord(epoch);
 287         TAILQ_FOREACH(tdwait, &er->er_tdlist, et_link)
 288                 if (tdwait->et_td == curthread)
 289                         break;
 290         critical_exit();
 291         if (tdwait != NULL) {
 292                 tdwait->et_flags |= ET_REPORT_EXIT;
 293                 printf("Td %p entered epoch %s at %s:%d\n", curthread,
 294                     epoch->e_name, tdwait->et_file, tdwait->et_line);
 295         }
 296 }
 297 #endif /* EPOCH_TRACE */
 298
 299 static void
 300 epoch_init(void *arg __unused)
 301 {
 302         int cpu;
 303
 304         block_count = counter_u64_alloc(M_WAITOK);
 305         migrate_count = counter_u64_alloc(M_WAITOK);
 306         turnstile_count = counter_u64_alloc(M_WAITOK);
 307         switch_count = counter_u64_alloc(M_WAITOK);
 308         epoch_call_count = counter_u64_alloc(M_WAITOK);
 309         epoch_call_task_count = counter_u64_alloc(M_WAITOK);
 310
 311         pcpu_zone_record = uma_zcreate("epoch_record pcpu",
 312             sizeof(struct epoch_record), NULL, NULL, NULL, NULL,
 313             UMA_ALIGN_PTR, UMA_ZONE_PCPU);
 314         CPU_FOREACH(cpu) {
 315                 GROUPTASK_INIT(DPCPU_ID_PTR(cpu, epoch_cb_task), 0,
 316                     epoch_call_task, NULL);
 317                 taskqgroup_attach_cpu(qgroup_softirq,
 318                     DPCPU_ID_PTR(cpu, epoch_cb_task), NULL, cpu, NULL, NULL,
 319                     "epoch call task");
 320         }
 321 #ifdef EPOCH_TRACE
 322         SLIST_INIT(&thread0.td_epochs);
 323 #endif
 324         sx_init(&epoch_sx, "epoch-sx");
 325         inited = 1;
 326         global_epoch = epoch_alloc("Global", 0);
 327         global_epoch_preempt = epoch_alloc("Global preemptible", EPOCH_PREEMPT);
 328 }
 329 SYSINIT(epoch, SI_SUB_EPOCH, SI_ORDER_FIRST, epoch_init, NULL);
 330
 331 #if !defined(EARLY_AP_STARTUP)
 332 static void
 333 epoch_init_smp(void *dummy __unused)
 334 {
 335         inited = 2;
 336 }
 337 SYSINIT(epoch_smp, SI_SUB_SMP + 1, SI_ORDER_FIRST, epoch_init_smp, NULL);
 338 #endif
 339
 340 static void
 341 epoch_ctor(epoch_t epoch)
 342 {
 343         epoch_record_t er;
 344         int cpu;
 345
 346         epoch->e_pcpu_record = uma_zalloc_pcpu(pcpu_zone_record, M_WAITOK);
 347         CPU_FOREACH(cpu) {
 348                 er = zpcpu_get_cpu(epoch->e_pcpu_record, cpu);
 349                 bzero(er, sizeof(*er));
 350                 ck_epoch_register(&epoch->e_epoch, &er->er_record, NULL);
 351                 TAILQ_INIT((struct threadlist *)(uintptr_t)&er->er_tdlist);
 352                 er->er_cpuid = cpu;
 353                 er->er_parent = epoch;
 354         }
 355 }
 356
 357 static void
 358 epoch_adjust_prio(struct thread *td, u_char prio)
 359 {
 360
 361         thread_lock(td);
 362         sched_prio(td, prio);
 363         thread_unlock(td);
 364 }
 365
 366 epoch_t
 367 epoch_alloc(const char *name, int flags)
 368 {
 369         epoch_t epoch;
 370         int i;
 371
 372         MPASS(name != NULL);
 373
 374         if (__predict_false(!inited))
 375                 panic("%s called too early in boot", __func__);
 376
 377         EPOCH_LOCK();
 378
 379         /*
 380          * Find a free index in the epoch array. If no free index is
 381          * found, try to use the index after the last one.
 382          */
 383         for (i = 0;; i++) {
 384                 /*
 385                  * If too many epochs are currently allocated,
 386                  * return NULL.
 387                  */
 388                 if (i == MAX_EPOCHS) {
 389                         epoch = NULL;
 390                         goto done;
 391                 }
 392                 if (epoch_array[i].e_in_use == 0)
 393                         break;
 394         }
 395
 396         epoch = epoch_array + i;
 397         ck_epoch_init(&epoch->e_epoch);
 398         epoch_ctor(epoch);
 399         epoch->e_flags = flags;
 400         epoch->e_name = name;
 401         sx_init(&epoch->e_drain_sx, "epoch-drain-sx");
 402         mtx_init(&epoch->e_drain_mtx, "epoch-drain-mtx", NULL, MTX_DEF);
 403
 404         /*
 405          * Set e_in_use last, because when this field is set the
 406          * epoch_call_task() function will start scanning this epoch
 407          * structure.
 408          */
 409         atomic_store_rel_int(&epoch->e_in_use, 1);
 410 done:
 411         EPOCH_UNLOCK();
 412         return (epoch);
 413 }
 414
 415 void
 416 epoch_free(epoch_t epoch)
 417 {
 418 #ifdef INVARIANTS
 419         int cpu;
 420 #endif
 421
 422         EPOCH_LOCK();
 423
 424         MPASS(epoch->e_in_use != 0);
 425
 426         epoch_drain_callbacks(epoch);
 427
 428         atomic_store_rel_int(&epoch->e_in_use, 0);
 429         /*
 430          * Make sure the epoch_call_task() function see e_in_use equal
 431          * to zero, by calling epoch_wait() on the global_epoch:
 432          */
 433         epoch_wait(global_epoch);
 434 #ifdef INVARIANTS
 435         CPU_FOREACH(cpu) {
 436                 epoch_record_t er;
 437
 438                 er = zpcpu_get_cpu(epoch->e_pcpu_record, cpu);
 439
 440                 /*
 441                  * Sanity check: none of the records should be in use anymore.
 442                  * We drained callbacks above and freeing the pcpu records is
 443                  * imminent.
 444                  */
 445                 MPASS(er->er_td == NULL);
 446                 MPASS(TAILQ_EMPTY(&er->er_tdlist));
 447         }
 448 #endif
 449         uma_zfree_pcpu(pcpu_zone_record, epoch->e_pcpu_record);
 450         mtx_destroy(&epoch->e_drain_mtx);
 451         sx_destroy(&epoch->e_drain_sx);
 452         memset(epoch, 0, sizeof(*epoch));
 453
 454         EPOCH_UNLOCK();
 455 }
 456
 457 #define INIT_CHECK(epoch)                                       \
 458         do {                                                    \
 459                 if (__predict_false((epoch) == NULL))           \
 460                         return;                                 \
 461         } while (0)
 462
 463 void
 464 _epoch_enter_preempt(epoch_t epoch, epoch_tracker_t et EPOCH_FILE_LINE)
 465 {
 466         struct epoch_record *er;
 467         struct thread *td;
 468
 469         MPASS(cold || epoch != NULL);
 470         td = curthread;
 471         MPASS((vm_offset_t)et >= td->td_kstack &&
 472             (vm_offset_t)et + sizeof(struct epoch_tracker) <=
 473             td->td_kstack + td->td_kstack_pages * PAGE_SIZE);
 474
 475         INIT_CHECK(epoch);
 476         MPASS(epoch->e_flags & EPOCH_PREEMPT);
 477
 478 #ifdef EPOCH_TRACE
 479         epoch_trace_enter(td, epoch, et, file, line);
 480 #endif
 481         et->et_td = td;
 482         THREAD_NO_SLEEPING();
 483         critical_enter();
 484         sched_pin();
 485         et->et_old_priority = td->td_priority;
 486         er = epoch_currecord(epoch);
 487         /* Record-level tracking is reserved for non-preemptible epochs. */
 488         MPASS(er->er_td == NULL);
 489         TAILQ_INSERT_TAIL(&er->er_tdlist, et, et_link);
 490         ck_epoch_begin(&er->er_record, &et->et_section);
 491         critical_exit();
 492 }
 493
 494 void
 495 epoch_enter(epoch_t epoch)
 496 {
 497         epoch_record_t er;
 498
 499         MPASS(cold || epoch != NULL);
 500         INIT_CHECK(epoch);
 501         critical_enter();
 502         er = epoch_currecord(epoch);
 503 #ifdef INVARIANTS
 504         if (er->er_record.active == 0) {
 505                 MPASS(er->er_td == NULL);
 506                 er->er_td = curthread;
 507         } else {
 508                 /* We've recursed, just make sure our accounting isn't wrong. */
 509                 MPASS(er->er_td == curthread);
 510         }
 511 #endif
 512         ck_epoch_begin(&er->er_record, NULL);
 513 }
 514
 515 void
 516 _epoch_exit_preempt(epoch_t epoch, epoch_tracker_t et EPOCH_FILE_LINE)
 517 {
 518         struct epoch_record *er;
 519         struct thread *td;
 520
 521         INIT_CHECK(epoch);
 522         td = curthread;
 523         critical_enter();
 524         sched_unpin();
 525         THREAD_SLEEPING_OK();
 526         er = epoch_currecord(epoch);
 527         MPASS(epoch->e_flags & EPOCH_PREEMPT);
 528         MPASS(et != NULL);
 529         MPASS(et->et_td == td);
 530 #ifdef INVARIANTS
 531         et->et_td = (void*)0xDEADBEEF;
 532         /* Record-level tracking is reserved for non-preemptible epochs. */
 533         MPASS(er->er_td == NULL);
 534 #endif
 535         ck_epoch_end(&er->er_record, &et->et_section);
 536         TAILQ_REMOVE(&er->er_tdlist, et, et_link);
 537         er->er_gen++;
 538         if (__predict_false(et->et_old_priority != td->td_priority))
 539                 epoch_adjust_prio(td, et->et_old_priority);
 540         critical_exit();
 541 #ifdef EPOCH_TRACE
 542         epoch_trace_exit(td, epoch, et, file, line);
 543 #endif
 544 }
 545
 546 void
 547 epoch_exit(epoch_t epoch)
 548 {
 549         epoch_record_t er;
 550
 551         INIT_CHECK(epoch);
 552         er = epoch_currecord(epoch);
 553         ck_epoch_end(&er->er_record, NULL);
 554 #ifdef INVARIANTS
 555         MPASS(er->er_td == curthread);
 556         if (er->er_record.active == 0)
 557                 er->er_td = NULL;
 558 #endif
 559         critical_exit();
 560 }
 561
 562 /*
 563  * epoch_block_handler_preempt() is a callback from the CK code when another
 564  * thread is currently in an epoch section.
 565  */
 566 static void
 567 epoch_block_handler_preempt(struct ck_epoch *global __unused,
 568     ck_epoch_record_t *cr, void *arg __unused)
 569 {
 570         epoch_record_t record;
 571         struct thread *td, *owner, *curwaittd;
 572         struct epoch_tracker *tdwait;
 573         struct turnstile *ts;
 574         struct lock_object *lock;
 575         int spincount, gen;
 576         int locksheld __unused;
 577
 578         record = __containerof(cr, struct epoch_record, er_record);
 579         td = curthread;
 580         locksheld = td->td_locks;
 581         spincount = 0;
 582         counter_u64_add(block_count, 1);
 583         /*
 584          * We lost a race and there's no longer any threads
 585          * on the CPU in an epoch section.
 586          */
 587         if (TAILQ_EMPTY(&record->er_tdlist))
 588                 return;
 589
 590         if (record->er_cpuid != curcpu) {
 591                 /*
 592                  * If the head of the list is running, we can wait for it
 593                  * to remove itself from the list and thus save us the
 594                  * overhead of a migration
 595                  */
 596                 gen = record->er_gen;
 597                 thread_unlock(td);
 598                 /*
 599                  * We can't actually check if the waiting thread is running
 600                  * so we simply poll for it to exit before giving up and
 601                  * migrating.
 602                  */
 603                 do {
 604                         cpu_spinwait();
 605                 } while (!TAILQ_EMPTY(&record->er_tdlist) &&
 606                                  gen == record->er_gen &&
 607                                  spincount++ < MAX_ADAPTIVE_SPIN);
 608                 thread_lock(td);
 609                 /*
 610                  * If the generation has changed we can poll again
 611                  * otherwise we need to migrate.
 612                  */
 613                 if (gen != record->er_gen)
 614                         return;
 615                 /*
 616                  * Being on the same CPU as that of the record on which
 617                  * we need to wait allows us access to the thread
 618                  * list associated with that CPU. We can then examine the
 619                  * oldest thread in the queue and wait on its turnstile
 620                  * until it resumes and so on until a grace period
 621                  * elapses.
 622                  *
 623                  */
 624                 counter_u64_add(migrate_count, 1);
 625                 sched_bind(td, record->er_cpuid);
 626                 /*
 627                  * At this point we need to return to the ck code
 628                  * to scan to see if a grace period has elapsed.
 629                  * We can't move on to check the thread list, because
 630                  * in the meantime new threads may have arrived that
 631                  * in fact belong to a different epoch.
 632                  */
 633                 return;
 634         }
 635         /*
 636          * Try to find a thread in an epoch section on this CPU
 637          * waiting on a turnstile. Otherwise find the lowest
 638          * priority thread (highest prio value) and drop our priority
 639          * to match to allow it to run.
 640          */
 641         TAILQ_FOREACH(tdwait, &record->er_tdlist, et_link) {
 642                 /*
 643                  * Propagate our priority to any other waiters to prevent us
 644                  * from starving them. They will have their original priority
 645                  * restore on exit from epoch_wait().
 646                  */
 647                 curwaittd = tdwait->et_td;
 648                 if (!TD_IS_INHIBITED(curwaittd) && curwaittd->td_priority > td->td_priority) {
 649                         critical_enter();
 650                         thread_unlock(td);
 651                         thread_lock(curwaittd);
 652                         sched_prio(curwaittd, td->td_priority);
 653                         thread_unlock(curwaittd);
 654                         thread_lock(td);
 655                         critical_exit();
 656                 }
 657                 if (TD_IS_INHIBITED(curwaittd) && TD_ON_LOCK(curwaittd) &&
 658                     ((ts = curwaittd->td_blocked) != NULL)) {
 659                         /*
 660                          * We unlock td to allow turnstile_wait to reacquire
 661                          * the thread lock. Before unlocking it we enter a
 662                          * critical section to prevent preemption after we
 663                          * reenable interrupts by dropping the thread lock in
 664                          * order to prevent curwaittd from getting to run.
 665                          */
 666                         critical_enter();
 667                         thread_unlock(td);
 668
 669                         if (turnstile_lock(ts, &lock, &owner)) {
 670                                 if (ts == curwaittd->td_blocked) {
 671                                         MPASS(TD_IS_INHIBITED(curwaittd) &&
 672                                             TD_ON_LOCK(curwaittd));
 673                                         critical_exit();
 674                                         turnstile_wait(ts, owner,
 675                                             curwaittd->td_tsqueue);
 676                                         counter_u64_add(turnstile_count, 1);
 677                                         thread_lock(td);
 678                                         return;
 679                                 }
 680                                 turnstile_unlock(ts, lock);
 681                         }
 682                         thread_lock(td);
 683                         critical_exit();
 684                         KASSERT(td->td_locks == locksheld,
 685                             ("%d extra locks held", td->td_locks - locksheld));
 686                 }
 687         }
 688         /*
 689          * We didn't find any threads actually blocked on a lock
 690          * so we have nothing to do except context switch away.
 691          */
 692         counter_u64_add(switch_count, 1);
 693         mi_switch(SW_VOL | SWT_RELINQUISH);
 694         /*
 695          * It is important the thread lock is dropped while yielding
 696          * to allow other threads to acquire the lock pointed to by
 697          * TDQ_LOCKPTR(td). Currently mi_switch() will unlock the
 698          * thread lock before returning. Else a deadlock like
 699          * situation might happen.
 700          */
 701         thread_lock(td);
 702 }
 703
 704 void
 705 epoch_wait_preempt(epoch_t epoch)
 706 {
 707         struct thread *td;
 708         int was_bound;
 709         int old_cpu;
 710         int old_pinned;
 711         u_char old_prio;
 712         int locks __unused;
 713
 714         MPASS(cold || epoch != NULL);
 715         INIT_CHECK(epoch);
 716         td = curthread;
 717 #ifdef INVARIANTS
 718         locks = curthread->td_locks;
 719         MPASS(epoch->e_flags & EPOCH_PREEMPT);
 720         if ((epoch->e_flags & EPOCH_LOCKED) == 0)
 721                 WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL,
 722                     "epoch_wait() can be long running");
 723         KASSERT(!in_epoch(epoch), ("epoch_wait_preempt() called in the middle "
 724             "of an epoch section of the same epoch"));
 725 #endif
 726         DROP_GIANT();
 727         thread_lock(td);
 728
 729         old_cpu = PCPU_GET(cpuid);
 730         old_pinned = td->td_pinned;
 731         old_prio = td->td_priority;
 732         was_bound = sched_is_bound(td);
 733         sched_unbind(td);
 734         td->td_pinned = 0;
 735         sched_bind(td, old_cpu);
 736
 737         ck_epoch_synchronize_wait(&epoch->e_epoch, epoch_block_handler_preempt,
 738             NULL);
 739
 740         /* restore CPU binding, if any */
 741         if (was_bound != 0) {
 742                 sched_bind(td, old_cpu);
 743         } else {
 744                 /* get thread back to initial CPU, if any */
 745                 if (old_pinned != 0)
 746                         sched_bind(td, old_cpu);
 747                 sched_unbind(td);
 748         }
 749         /* restore pinned after bind */
 750         td->td_pinned = old_pinned;
 751
 752         /* restore thread priority */
 753         sched_prio(td, old_prio);
 754         thread_unlock(td);
 755         PICKUP_GIANT();
 756         KASSERT(td->td_locks == locks,
 757             ("%d residual locks held", td->td_locks - locks));
 758 }
 759
 760 static void
 761 epoch_block_handler(struct ck_epoch *g __unused, ck_epoch_record_t *c __unused,
 762     void *arg __unused)
 763 {
 764         cpu_spinwait();
 765 }
 766
 767 void
 768 epoch_wait(epoch_t epoch)
 769 {
 770
 771         MPASS(cold || epoch != NULL);
 772         INIT_CHECK(epoch);
 773         MPASS(epoch->e_flags == 0);
 774         critical_enter();
 775         ck_epoch_synchronize_wait(&epoch->e_epoch, epoch_block_handler, NULL);
 776         critical_exit();
 777 }
 778
 779 void
 780 epoch_call(epoch_t epoch, epoch_callback_t callback, epoch_context_t ctx)
 781 {
 782         epoch_record_t er;
 783         ck_epoch_entry_t *cb;
 784
 785         cb = (void *)ctx;
 786
 787         MPASS(callback);
 788         /* too early in boot to have epoch set up */
 789         if (__predict_false(epoch == NULL))
 790                 goto boottime;
 791 #if !defined(EARLY_AP_STARTUP)
 792         if (__predict_false(inited < 2))
 793                 goto boottime;
 794 #endif
 795
 796         critical_enter();
 797         *DPCPU_PTR(epoch_cb_count) += 1;
 798         er = epoch_currecord(epoch);
 799         ck_epoch_call(&er->er_record, cb, (ck_epoch_cb_t *)callback);
 800         critical_exit();
 801         return;
 802 boottime:
 803         callback(ctx);
 804 }
 805
 806 static void
 807 epoch_call_task(void *arg __unused)
 808 {
 809         ck_stack_entry_t *cursor, *head, *next;
 810         ck_epoch_record_t *record;
 811         epoch_record_t er;
 812         epoch_t epoch;
 813         ck_stack_t cb_stack;
 814         int i, npending, total;
 815
 816         ck_stack_init(&cb_stack);
 817         critical_enter();
 818         epoch_enter(global_epoch);
 819         for (total = i = 0; i != MAX_EPOCHS; i++) {
 820                 epoch = epoch_array + i;
 821                 if (__predict_false(
 822                     atomic_load_acq_int(&epoch->e_in_use) == 0))
 823                         continue;
 824                 er = epoch_currecord(epoch);
 825                 record = &er->er_record;
 826                 if ((npending = record->n_pending) == 0)
 827                         continue;
 828                 ck_epoch_poll_deferred(record, &cb_stack);
 829                 total += npending - record->n_pending;
 830         }
 831         epoch_exit(global_epoch);
 832         *DPCPU_PTR(epoch_cb_count) -= total;
 833         critical_exit();
 834
 835         counter_u64_add(epoch_call_count, total);
 836         counter_u64_add(epoch_call_task_count, 1);
 837
 838         head = ck_stack_batch_pop_npsc(&cb_stack);
 839         for (cursor = head; cursor != NULL; cursor = next) {
 840                 struct ck_epoch_entry *entry =
 841                     ck_epoch_entry_container(cursor);
 842
 843                 next = CK_STACK_NEXT(cursor);
 844                 entry->function(entry);
 845         }
 846 }
 847
 848 static int
 849 in_epoch_verbose_preempt(epoch_t epoch, int dump_onfail)
 850 {
 851         epoch_record_t er;
 852         struct epoch_tracker *tdwait;
 853         struct thread *td;
 854
 855         MPASS(epoch != NULL);
 856         MPASS((epoch->e_flags & EPOCH_PREEMPT) != 0);
 857         td = curthread;
 858         if (THREAD_CAN_SLEEP())
 859                 return (0);
 860         critical_enter();
 861         er = epoch_currecord(epoch);
 862         TAILQ_FOREACH(tdwait, &er->er_tdlist, et_link)
 863                 if (tdwait->et_td == td) {
 864                         critical_exit();
 865                         return (1);
 866                 }
 867 #ifdef INVARIANTS
 868         if (dump_onfail) {
 869                 MPASS(td->td_pinned);
 870                 printf("cpu: %d id: %d\n", curcpu, td->td_tid);
 871                 TAILQ_FOREACH(tdwait, &er->er_tdlist, et_link)
 872                         printf("td_tid: %d ", tdwait->et_td->td_tid);
 873                 printf("\n");
 874         }
 875 #endif
 876         critical_exit();
 877         return (0);
 878 }
 879
 880 #ifdef INVARIANTS
 881 static void
 882 epoch_assert_nocpu(epoch_t epoch, struct thread *td)
 883 {
 884         epoch_record_t er;
 885         int cpu;
 886         bool crit;
 887
 888         crit = td->td_critnest > 0;
 889
 890         /* Check for a critical section mishap. */
 891         CPU_FOREACH(cpu) {
 892                 er = zpcpu_get_cpu(epoch->e_pcpu_record, cpu);
 893                 KASSERT(er->er_td != td,
 894                     ("%s critical section in epoch '%s', from cpu %d",
 895                     (crit ? "exited" : "re-entered"), epoch->e_name, cpu));
 896         }
 897 }
 898 #else
 899 #define epoch_assert_nocpu(e, td) do {} while (0)
 900 #endif
 901
 902 int
 903 in_epoch_verbose(epoch_t epoch, int dump_onfail)
 904 {
 905         epoch_record_t er;
 906         struct thread *td;
 907
 908         if (__predict_false((epoch) == NULL))
 909                 return (0);
 910         if ((epoch->e_flags & EPOCH_PREEMPT) != 0)
 911                 return (in_epoch_verbose_preempt(epoch, dump_onfail));
 912
 913         /*
 914          * The thread being in a critical section is a necessary
 915          * condition to be correctly inside a non-preemptible epoch,
 916          * so it's definitely not in this epoch.
 917          */
 918         td = curthread;
 919         if (td->td_critnest == 0) {
 920                 epoch_assert_nocpu(epoch, td);
 921                 return (0);
 922         }
 923
 924         /*
 925          * The current cpu is in a critical section, so the epoch record will be
 926          * stable for the rest of this function.  Knowing that the record is not
 927          * active is sufficient for knowing whether we're in this epoch or not,
 928          * since it's a pcpu record.
 929          */
 930         er = epoch_currecord(epoch);
 931         if (er->er_record.active == 0) {
 932                 epoch_assert_nocpu(epoch, td);
 933                 return (0);
 934         }
 935
 936         MPASS(er->er_td == td);
 937         return (1);
 938 }
 939
 940 int
 941 in_epoch(epoch_t epoch)
 942 {
 943         return (in_epoch_verbose(epoch, 0));
 944 }
 945
 946 static void
 947 epoch_drain_cb(struct epoch_context *ctx)
 948 {
 949         struct epoch *epoch =
 950             __containerof(ctx, struct epoch_record, er_drain_ctx)->er_parent;
 951
 952         if (atomic_fetchadd_int(&epoch->e_drain_count, -1) == 1) {
 953                 mtx_lock(&epoch->e_drain_mtx);
 954                 wakeup(epoch);
 955                 mtx_unlock(&epoch->e_drain_mtx);
 956         }
 957 }
 958
 959 void
 960 epoch_drain_callbacks(epoch_t epoch)
 961 {
 962         epoch_record_t er;
 963         struct thread *td;
 964         int was_bound;
 965         int old_pinned;
 966         int old_cpu;
 967         int cpu;
 968
 969         WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL,
 970             "epoch_drain_callbacks() may sleep!");
 971
 972         /* too early in boot to have epoch set up */
 973         if (__predict_false(epoch == NULL))
 974                 return;
 975 #if !defined(EARLY_AP_STARTUP)
 976         if (__predict_false(inited < 2))
 977                 return;
 978 #endif
 979         DROP_GIANT();
 980
 981         sx_xlock(&epoch->e_drain_sx);
 982         mtx_lock(&epoch->e_drain_mtx);
 983
 984         td = curthread;
 985         thread_lock(td);
 986         old_cpu = PCPU_GET(cpuid);
 987         old_pinned = td->td_pinned;
 988         was_bound = sched_is_bound(td);
 989         sched_unbind(td);
 990         td->td_pinned = 0;
 991
 992         CPU_FOREACH(cpu)
 993                 epoch->e_drain_count++;
 994         CPU_FOREACH(cpu) {
 995                 er = zpcpu_get_cpu(epoch->e_pcpu_record, cpu);
 996                 sched_bind(td, cpu);
 997                 epoch_call(epoch, &epoch_drain_cb, &er->er_drain_ctx);
 998         }
 999
1000         /* restore CPU binding, if any */
1001         if (was_bound != 0) {
1002                 sched_bind(td, old_cpu);
1003         } else {
1004                 /* get thread back to initial CPU, if any */
1005                 if (old_pinned != 0)
1006                         sched_bind(td, old_cpu);
1007                 sched_unbind(td);
1008         }
1009         /* restore pinned after bind */
1010         td->td_pinned = old_pinned;
1011
1012         thread_unlock(td);
1013
1014         while (epoch->e_drain_count != 0)
1015                 msleep(epoch, &epoch->e_drain_mtx, PZERO, "EDRAIN", 0);
1016
1017         mtx_unlock(&epoch->e_drain_mtx);
1018         sx_xunlock(&epoch->e_drain_sx);
1019
1020         PICKUP_GIANT();
1021 }