rts/Capability.c

   1 /* ---------------------------------------------------------------------------
   2  *
   3  * (c) The GHC Team, 2003-2006
   4  *
   5  * Capabilities
   6  *
   7  * A Capability represent the token required to execute STG code,
   8  * and all the state an OS thread/task needs to run Haskell code:
   9  * its STG registers, a pointer to its TSO, a nursery etc. During
  10  * STG execution, a pointer to the capabilitity is kept in a
  11  * register (BaseReg; actually it is a pointer to cap->r).
  12  *
  13  * Only in an THREADED_RTS build will there be multiple capabilities,
  14  * for non-threaded builds there is only one global capability, namely
  15  * MainCapability.
  16  *
  17  * --------------------------------------------------------------------------*/
  18
  19 #include "PosixSource.h"
  20 #include "Rts.h"
  21
  22 #include "Capability.h"
  23 #include "Schedule.h"
  24 #include "Sparks.h"
  25 #include "Trace.h"
  26 #include "sm/GC.h" // for gcWorkerThread()
  27 #include "STM.h"
  28 #include "RtsUtils.h"
  29
  30 // one global capability, this is the Capability for non-threaded
  31 // builds, and for +RTS -N1
  32 Capability MainCapability;
  33
  34 nat n_capabilities = 0;
  35 Capability *capabilities = NULL;
  36
  37 // Holds the Capability which last became free.  This is used so that
  38 // an in-call has a chance of quickly finding a free Capability.
  39 // Maintaining a global free list of Capabilities would require global
  40 // locking, so we don't do that.
  41 Capability *last_free_capability = NULL;
  42
  43 /* GC indicator, in scope for the scheduler, init'ed to false */
  44 volatile StgWord waiting_for_gc = 0;
  45
  46 /* Let foreign code get the current Capability -- assuming there is one!
  47  * This is useful for unsafe foreign calls because they are called with
  48  * the current Capability held, but they are not passed it. For example,
  49  * see see the integer-gmp package which calls allocateLocal() in its
  50  * stgAllocForGMP() function (which gets called by gmp functions).
  51  * */
  52 Capability * rts_unsafeGetMyCapability (void)
  53 {
  54 #if defined(THREADED_RTS)
  55   return myTask()->cap;
  56 #else
  57   return &MainCapability;
  58 #endif
  59 }
  60
  61 #if defined(THREADED_RTS)
  62 STATIC_INLINE rtsBool
  63 globalWorkToDo (void)
  64 {
  65     return sched_state >= SCHED_INTERRUPTING
  66         || recent_activity == ACTIVITY_INACTIVE; // need to check for deadlock
  67 }
  68 #endif
  69
  70 #if defined(THREADED_RTS)
  71 StgClosure *
  72 findSpark (Capability *cap)
  73 {
  74   Capability *robbed;
  75   StgClosurePtr spark;
  76   rtsBool retry;
  77   nat i = 0;
  78
  79   if (!emptyRunQueue(cap) || cap->returning_tasks_hd != NULL) {
  80       // If there are other threads, don't try to run any new
  81       // sparks: sparks might be speculative, we don't want to take
  82       // resources away from the main computation.
  83       return 0;
  84   }
  85
  86   do {
  87       retry = rtsFalse;
  88
  89       // first try to get a spark from our own pool.
  90       // We should be using reclaimSpark(), because it works without
  91       // needing any atomic instructions:
  92       //   spark = reclaimSpark(cap->sparks);
  93       // However, measurements show that this makes at least one benchmark
  94       // slower (prsa) and doesn't affect the others.
  95       spark = tryStealSpark(cap);
  96       if (spark != NULL) {
  97           cap->sparks_converted++;
  98
  99           // Post event for running a spark from capability's own pool.
 100           traceEventRunSpark(cap, cap->r.rCurrentTSO);
 101
 102           return spark;
 103       }
 104       if (!emptySparkPoolCap(cap)) {
 105           retry = rtsTrue;
 106       }
 107
 108       if (n_capabilities == 1) { return NULL; } // makes no sense...
 109
 110       debugTrace(DEBUG_sched,
 111                  "cap %d: Trying to steal work from other capabilities",
 112                  cap->no);
 113
 114       /* visit cap.s 0..n-1 in sequence until a theft succeeds. We could
 115       start at a random place instead of 0 as well.  */
 116       for ( i=0 ; i < n_capabilities ; i++ ) {
 117           robbed = &capabilities[i];
 118           if (cap == robbed)  // ourselves...
 119               continue;
 120
 121           if (emptySparkPoolCap(robbed)) // nothing to steal here
 122               continue;
 123
 124           spark = tryStealSpark(robbed);
 125           if (spark == NULL && !emptySparkPoolCap(robbed)) {
 126               // we conflicted with another thread while trying to steal;
 127               // try again later.
 128               retry = rtsTrue;
 129           }
 130
 131           if (spark != NULL) {
 132               cap->sparks_converted++;
 133
 134               traceEventStealSpark(cap, cap->r.rCurrentTSO, robbed->no);
 135
 136               return spark;
 137           }
 138           // otherwise: no success, try next one
 139       }
 140   } while (retry);
 141
 142   debugTrace(DEBUG_sched, "No sparks stolen");
 143   return NULL;
 144 }
 145
 146 // Returns True if any spark pool is non-empty at this moment in time
 147 // The result is only valid for an instant, of course, so in a sense
 148 // is immediately invalid, and should not be relied upon for
 149 // correctness.
 150 rtsBool
 151 anySparks (void)
 152 {
 153     nat i;
 154
 155     for (i=0; i < n_capabilities; i++) {
 156         if (!emptySparkPoolCap(&capabilities[i])) {
 157             return rtsTrue;
 158         }
 159     }
 160     return rtsFalse;
 161 }
 162 #endif
 163
 164 /* -----------------------------------------------------------------------------
 165  * Manage the returning_tasks lists.
 166  *
 167  * These functions require cap->lock
 168  * -------------------------------------------------------------------------- */
 169
 170 #if defined(THREADED_RTS)
 171 STATIC_INLINE void
 172 newReturningTask (Capability *cap, Task *task)
 173 {
 174     ASSERT_LOCK_HELD(&cap->lock);
 175     ASSERT(task->next == NULL);
 176     if (cap->returning_tasks_hd) {
 177         ASSERT(cap->returning_tasks_tl->next == NULL);
 178         cap->returning_tasks_tl->next = task;
 179     } else {
 180         cap->returning_tasks_hd = task;
 181     }
 182     cap->returning_tasks_tl = task;
 183 }
 184
 185 STATIC_INLINE Task *
 186 popReturningTask (Capability *cap)
 187 {
 188     ASSERT_LOCK_HELD(&cap->lock);
 189     Task *task;
 190     task = cap->returning_tasks_hd;
 191     ASSERT(task);
 192     cap->returning_tasks_hd = task->next;
 193     if (!cap->returning_tasks_hd) {
 194         cap->returning_tasks_tl = NULL;
 195     }
 196     task->next = NULL;
 197     return task;
 198 }
 199 #endif
 200
 201 /* ----------------------------------------------------------------------------
 202  * Initialisation
 203  *
 204  * The Capability is initially marked not free.
 205  * ------------------------------------------------------------------------- */
 206
 207 static void
 208 initCapability( Capability *cap, nat i )
 209 {
 210     nat g;
 211
 212     cap->no = i;
 213     cap->in_haskell        = rtsFalse;
 214
 215     cap->run_queue_hd      = END_TSO_QUEUE;
 216     cap->run_queue_tl      = END_TSO_QUEUE;
 217
 218 #if defined(THREADED_RTS)
 219     initMutex(&cap->lock);
 220     cap->running_task      = NULL; // indicates cap is free
 221     cap->spare_workers     = NULL;
 222     cap->suspended_ccalls  = NULL;
 223     cap->returning_tasks_hd = NULL;
 224     cap->returning_tasks_tl = NULL;
 225     cap->inbox              = (Message*)END_TSO_QUEUE;
 226     cap->sparks_created     = 0;
 227     cap->sparks_converted   = 0;
 228     cap->sparks_pruned      = 0;
 229 #endif
 230
 231     cap->f.stgEagerBlackholeInfo = (W_)&__stg_EAGER_BLACKHOLE_info;
 232     cap->f.stgGCEnter1     = (StgFunPtr)__stg_gc_enter_1;
 233     cap->f.stgGCFun        = (StgFunPtr)__stg_gc_fun;
 234
 235     cap->mut_lists  = stgMallocBytes(sizeof(bdescr *) *
 236                                      RtsFlags.GcFlags.generations,
 237                                      "initCapability");
 238     cap->saved_mut_lists = stgMallocBytes(sizeof(bdescr *) *
 239                                           RtsFlags.GcFlags.generations,
 240                                           "initCapability");
 241
 242     for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
 243         cap->mut_lists[g] = NULL;
 244     }
 245
 246     cap->free_tvar_watch_queues = END_STM_WATCH_QUEUE;
 247     cap->free_invariant_check_queues = END_INVARIANT_CHECK_QUEUE;
 248     cap->free_trec_chunks = END_STM_CHUNK_LIST;
 249     cap->free_trec_headers = NO_TREC;
 250     cap->transaction_tokens = 0;
 251     cap->context_switch = 0;
 252     cap->pinned_object_block = NULL;
 253 }
 254
 255 /* ---------------------------------------------------------------------------
 256  * Function:  initCapabilities()
 257  *
 258  * Purpose:   set up the Capability handling. For the THREADED_RTS build,
 259  *            we keep a table of them, the size of which is
 260  *            controlled by the user via the RTS flag -N.
 261  *
 262  * ------------------------------------------------------------------------- */
 263 void
 264 initCapabilities( void )
 265 {
 266 #if defined(THREADED_RTS)
 267     nat i;
 268
 269 #ifndef REG_Base
 270     // We can't support multiple CPUs if BaseReg is not a register
 271     if (RtsFlags.ParFlags.nNodes > 1) {
 272         errorBelch("warning: multiple CPUs not supported in this build, reverting to 1");
 273         RtsFlags.ParFlags.nNodes = 1;
 274     }
 275 #endif
 276
 277     n_capabilities = RtsFlags.ParFlags.nNodes;
 278
 279     if (n_capabilities == 1) {
 280         capabilities = &MainCapability;
 281         // THREADED_RTS must work on builds that don't have a mutable
 282         // BaseReg (eg. unregisterised), so in this case
 283         // capabilities[0] must coincide with &MainCapability.
 284     } else {
 285         capabilities = stgMallocBytes(n_capabilities * sizeof(Capability),
 286                                       "initCapabilities");
 287     }
 288
 289     for (i = 0; i < n_capabilities; i++) {
 290         initCapability(&capabilities[i], i);
 291     }
 292
 293     debugTrace(DEBUG_sched, "allocated %d capabilities", n_capabilities);
 294
 295 #else /* !THREADED_RTS */
 296
 297     n_capabilities = 1;
 298     capabilities = &MainCapability;
 299     initCapability(&MainCapability, 0);
 300
 301 #endif
 302
 303     // There are no free capabilities to begin with.  We will start
 304     // a worker Task to each Capability, which will quickly put the
 305     // Capability on the free list when it finds nothing to do.
 306     last_free_capability = &capabilities[0];
 307 }
 308
 309 /* ----------------------------------------------------------------------------
 310  * setContextSwitches: cause all capabilities to context switch as
 311  * soon as possible.
 312  * ------------------------------------------------------------------------- */
 313
 314 void setContextSwitches(void)
 315 {
 316     nat i;
 317     for (i=0; i < n_capabilities; i++) {
 318         contextSwitchCapability(&capabilities[i]);
 319     }
 320 }
 321
 322 /* ----------------------------------------------------------------------------
 323  * Give a Capability to a Task.  The task must currently be sleeping
 324  * on its condition variable.
 325  *
 326  * Requires cap->lock (modifies cap->running_task).
 327  *
 328  * When migrating a Task, the migrater must take task->lock before
 329  * modifying task->cap, to synchronise with the waking up Task.
 330  * Additionally, the migrater should own the Capability (when
 331  * migrating the run queue), or cap->lock (when migrating
 332  * returning_workers).
 333  *
 334  * ------------------------------------------------------------------------- */
 335
 336 #if defined(THREADED_RTS)
 337 STATIC_INLINE void
 338 giveCapabilityToTask (Capability *cap USED_IF_DEBUG, Task *task)
 339 {
 340     ASSERT_LOCK_HELD(&cap->lock);
 341     ASSERT(task->cap == cap);
 342     debugTrace(DEBUG_sched, "passing capability %d to %s %p",
 343                cap->no, task->incall->tso ? "bound task" : "worker",
 344                (void *)task->id);
 345     ACQUIRE_LOCK(&task->lock);
 346     task->wakeup = rtsTrue;
 347     // the wakeup flag is needed because signalCondition() doesn't
 348     // flag the condition if the thread is already runniing, but we want
 349     // it to be sticky.
 350     signalCondition(&task->cond);
 351     RELEASE_LOCK(&task->lock);
 352 }
 353 #endif
 354
 355 /* ----------------------------------------------------------------------------
 356  * Function:  releaseCapability(Capability*)
 357  *
 358  * Purpose:   Letting go of a capability. Causes a
 359  *            'returning worker' thread or a 'waiting worker'
 360  *            to wake up, in that order.
 361  * ------------------------------------------------------------------------- */
 362
 363 #if defined(THREADED_RTS)
 364 void
 365 releaseCapability_ (Capability* cap,
 366                     rtsBool always_wakeup)
 367 {
 368     Task *task;
 369
 370     task = cap->running_task;
 371
 372     ASSERT_PARTIAL_CAPABILITY_INVARIANTS(cap,task);
 373
 374     cap->running_task = NULL;
 375
 376     // Check to see whether a worker thread can be given
 377     // the go-ahead to return the result of an external call..
 378     if (cap->returning_tasks_hd != NULL) {
 379         giveCapabilityToTask(cap,cap->returning_tasks_hd);
 380         // The Task pops itself from the queue (see waitForReturnCapability())
 381         return;
 382     }
 383
 384     if (waiting_for_gc == PENDING_GC_SEQ) {
 385       last_free_capability = cap; // needed?
 386       debugTrace(DEBUG_sched, "GC pending, set capability %d free", cap->no);
 387       return;
 388     }
 389
 390
 391     // If the next thread on the run queue is a bound thread,
 392     // give this Capability to the appropriate Task.
 393     if (!emptyRunQueue(cap) && cap->run_queue_hd->bound) {
 394         // Make sure we're not about to try to wake ourselves up
 395         // ASSERT(task != cap->run_queue_hd->bound);
 396         // assertion is false: in schedule() we force a yield after
 397         // ThreadBlocked, but the thread may be back on the run queue
 398         // by now.
 399         task = cap->run_queue_hd->bound->task;
 400         giveCapabilityToTask(cap,task);
 401         return;
 402     }
 403
 404     if (!cap->spare_workers) {
 405         // Create a worker thread if we don't have one.  If the system
 406         // is interrupted, we only create a worker task if there
 407         // are threads that need to be completed.  If the system is
 408         // shutting down, we never create a new worker.
 409         if (sched_state < SCHED_SHUTTING_DOWN || !emptyRunQueue(cap)) {
 410             debugTrace(DEBUG_sched,
 411                        "starting new worker on capability %d", cap->no);
 412             startWorkerTask(cap);
 413             return;
 414         }
 415     }
 416
 417     // If we have an unbound thread on the run queue, or if there's
 418     // anything else to do, give the Capability to a worker thread.
 419     if (always_wakeup ||
 420         !emptyRunQueue(cap) || !emptyInbox(cap) ||
 421         !emptySparkPoolCap(cap) || globalWorkToDo()) {
 422         if (cap->spare_workers) {
 423             giveCapabilityToTask(cap,cap->spare_workers);
 424             // The worker Task pops itself from the queue;
 425             return;
 426         }
 427     }
 428
 429     last_free_capability = cap;
 430     debugTrace(DEBUG_sched, "freeing capability %d", cap->no);
 431 }
 432
 433 void
 434 releaseCapability (Capability* cap USED_IF_THREADS)
 435 {
 436     ACQUIRE_LOCK(&cap->lock);
 437     releaseCapability_(cap, rtsFalse);
 438     RELEASE_LOCK(&cap->lock);
 439 }
 440
 441 void
 442 releaseAndWakeupCapability (Capability* cap USED_IF_THREADS)
 443 {
 444     ACQUIRE_LOCK(&cap->lock);
 445     releaseCapability_(cap, rtsTrue);
 446     RELEASE_LOCK(&cap->lock);
 447 }
 448
 449 static void
 450 releaseCapabilityAndQueueWorker (Capability* cap USED_IF_THREADS)
 451 {
 452     Task *task;
 453
 454     ACQUIRE_LOCK(&cap->lock);
 455
 456     task = cap->running_task;
 457
 458     // If the current task is a worker, save it on the spare_workers
 459     // list of this Capability.  A worker can mark itself as stopped,
 460     // in which case it is not replaced on the spare_worker queue.
 461     // This happens when the system is shutting down (see
 462     // Schedule.c:workerStart()).
 463     if (!isBoundTask(task) && !task->stopped) {
 464         task->next = cap->spare_workers;
 465         cap->spare_workers = task;
 466     }
 467     // Bound tasks just float around attached to their TSOs.
 468
 469     releaseCapability_(cap,rtsFalse);
 470
 471     RELEASE_LOCK(&cap->lock);
 472 }
 473 #endif
 474
 475 /* ----------------------------------------------------------------------------
 476  * waitForReturnCapability( Task *task )
 477  *
 478  * Purpose:  when an OS thread returns from an external call,
 479  * it calls waitForReturnCapability() (via Schedule.resumeThread())
 480  * to wait for permission to enter the RTS & communicate the
 481  * result of the external call back to the Haskell thread that
 482  * made it.
 483  *
 484  * ------------------------------------------------------------------------- */
 485 void
 486 waitForReturnCapability (Capability **pCap, Task *task)
 487 {
 488 #if !defined(THREADED_RTS)
 489
 490     MainCapability.running_task = task;
 491     task->cap = &MainCapability;
 492     *pCap = &MainCapability;
 493
 494 #else
 495     Capability *cap = *pCap;
 496
 497     if (cap == NULL) {
 498         // Try last_free_capability first
 499         cap = last_free_capability;
 500         if (cap->running_task) {
 501             nat i;
 502             // otherwise, search for a free capability
 503             cap = NULL;
 504             for (i = 0; i < n_capabilities; i++) {
 505                 if (!capabilities[i].running_task) {
 506                     cap = &capabilities[i];
 507                     break;
 508                 }
 509             }
 510             if (cap == NULL) {
 511                 // Can't find a free one, use last_free_capability.
 512                 cap = last_free_capability;
 513             }
 514         }
 515
 516         // record the Capability as the one this Task is now assocated with.
 517         task->cap = cap;
 518
 519     } else {
 520         ASSERT(task->cap == cap);
 521     }
 522
 523     ACQUIRE_LOCK(&cap->lock);
 524
 525     debugTrace(DEBUG_sched, "returning; I want capability %d", cap->no);
 526
 527     if (!cap->running_task) {
 528         // It's free; just grab it
 529         cap->running_task = task;
 530         RELEASE_LOCK(&cap->lock);
 531     } else {
 532         newReturningTask(cap,task);
 533         RELEASE_LOCK(&cap->lock);
 534
 535         for (;;) {
 536             ACQUIRE_LOCK(&task->lock);
 537             // task->lock held, cap->lock not held
 538             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 539             cap = task->cap;
 540             task->wakeup = rtsFalse;
 541             RELEASE_LOCK(&task->lock);
 542
 543             // now check whether we should wake up...
 544             ACQUIRE_LOCK(&cap->lock);
 545             if (cap->running_task == NULL) {
 546                 if (cap->returning_tasks_hd != task) {
 547                     giveCapabilityToTask(cap,cap->returning_tasks_hd);
 548                     RELEASE_LOCK(&cap->lock);
 549                     continue;
 550                 }
 551                 cap->running_task = task;
 552                 popReturningTask(cap);
 553                 RELEASE_LOCK(&cap->lock);
 554                 break;
 555             }
 556             RELEASE_LOCK(&cap->lock);
 557         }
 558
 559     }
 560
 561     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 562
 563     debugTrace(DEBUG_sched, "resuming capability %d", cap->no);
 564
 565     *pCap = cap;
 566 #endif
 567 }
 568
 569 #if defined(THREADED_RTS)
 570 /* ----------------------------------------------------------------------------
 571  * yieldCapability
 572  * ------------------------------------------------------------------------- */
 573
 574 void
 575 yieldCapability (Capability** pCap, Task *task)
 576 {
 577     Capability *cap = *pCap;
 578
 579     if (waiting_for_gc == PENDING_GC_PAR) {
 580         traceEventGcStart(cap);
 581         gcWorkerThread(cap);
 582         traceEventGcEnd(cap);
 583         return;
 584     }
 585
 586         debugTrace(DEBUG_sched, "giving up capability %d", cap->no);
 587
 588         // We must now release the capability and wait to be woken up
 589         // again.
 590         task->wakeup = rtsFalse;
 591         releaseCapabilityAndQueueWorker(cap);
 592
 593         for (;;) {
 594             ACQUIRE_LOCK(&task->lock);
 595             // task->lock held, cap->lock not held
 596             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 597             cap = task->cap;
 598             task->wakeup = rtsFalse;
 599             RELEASE_LOCK(&task->lock);
 600
 601             debugTrace(DEBUG_sched, "woken up on capability %d", cap->no);
 602
 603             ACQUIRE_LOCK(&cap->lock);
 604             if (cap->running_task != NULL) {
 605                 debugTrace(DEBUG_sched,
 606                            "capability %d is owned by another task", cap->no);
 607                 RELEASE_LOCK(&cap->lock);
 608                 continue;
 609             }
 610
 611             if (task->incall->tso == NULL) {
 612                 ASSERT(cap->spare_workers != NULL);
 613                 // if we're not at the front of the queue, release it
 614                 // again.  This is unlikely to happen.
 615                 if (cap->spare_workers != task) {
 616                     giveCapabilityToTask(cap,cap->spare_workers);
 617                     RELEASE_LOCK(&cap->lock);
 618                     continue;
 619                 }
 620                 cap->spare_workers = task->next;
 621                 task->next = NULL;
 622             }
 623             cap->running_task = task;
 624             RELEASE_LOCK(&cap->lock);
 625             break;
 626         }
 627
 628         debugTrace(DEBUG_sched, "resuming capability %d", cap->no);
 629         ASSERT(cap->running_task == task);
 630
 631     *pCap = cap;
 632
 633     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 634
 635     return;
 636 }
 637
 638 /* ----------------------------------------------------------------------------
 639  * prodCapability
 640  *
 641  * If a Capability is currently idle, wake up a Task on it.  Used to
 642  * get every Capability into the GC.
 643  * ------------------------------------------------------------------------- */
 644
 645 void
 646 prodCapability (Capability *cap, Task *task)
 647 {
 648     ACQUIRE_LOCK(&cap->lock);
 649     if (!cap->running_task) {
 650         cap->running_task = task;
 651         releaseCapability_(cap,rtsTrue);
 652     }
 653     RELEASE_LOCK(&cap->lock);
 654 }
 655
 656 /* ----------------------------------------------------------------------------
 657  * shutdownCapability
 658  *
 659  * At shutdown time, we want to let everything exit as cleanly as
 660  * possible.  For each capability, we let its run queue drain, and
 661  * allow the workers to stop.
 662  *
 663  * This function should be called when interrupted and
 664  * shutting_down_scheduler = rtsTrue, thus any worker that wakes up
 665  * will exit the scheduler and call taskStop(), and any bound thread
 666  * that wakes up will return to its caller.  Runnable threads are
 667  * killed.
 668  *
 669  * ------------------------------------------------------------------------- */
 670
 671 void
 672 shutdownCapability (Capability *cap, Task *task, rtsBool safe)
 673 {
 674     nat i;
 675
 676     task->cap = cap;
 677
 678     // Loop indefinitely until all the workers have exited and there
 679     // are no Haskell threads left.  We used to bail out after 50
 680     // iterations of this loop, but that occasionally left a worker
 681     // running which caused problems later (the closeMutex() below
 682     // isn't safe, for one thing).
 683
 684     for (i = 0; /* i < 50 */; i++) {
 685         ASSERT(sched_state == SCHED_SHUTTING_DOWN);
 686
 687         debugTrace(DEBUG_sched,
 688                    "shutting down capability %d, attempt %d", cap->no, i);
 689         ACQUIRE_LOCK(&cap->lock);
 690         if (cap->running_task) {
 691             RELEASE_LOCK(&cap->lock);
 692             debugTrace(DEBUG_sched, "not owner, yielding");
 693             yieldThread();
 694             continue;
 695         }
 696         cap->running_task = task;
 697
 698         if (cap->spare_workers) {
 699             // Look for workers that have died without removing
 700             // themselves from the list; this could happen if the OS
 701             // summarily killed the thread, for example.  This
 702             // actually happens on Windows when the system is
 703             // terminating the program, and the RTS is running in a
 704             // DLL.
 705             Task *t, *prev;
 706             prev = NULL;
 707             for (t = cap->spare_workers; t != NULL; t = t->next) {
 708                 if (!osThreadIsAlive(t->id)) {
 709                     debugTrace(DEBUG_sched,
 710                                "worker thread %p has died unexpectedly", (void *)t->id);
 711                         if (!prev) {
 712                             cap->spare_workers = t->next;
 713                         } else {
 714                             prev->next = t->next;
 715                         }
 716                         prev = t;
 717                 }
 718             }
 719         }
 720
 721         if (!emptyRunQueue(cap) || cap->spare_workers) {
 722             debugTrace(DEBUG_sched,
 723                        "runnable threads or workers still alive, yielding");
 724             releaseCapability_(cap,rtsFalse); // this will wake up a worker
 725             RELEASE_LOCK(&cap->lock);
 726             yieldThread();
 727             continue;
 728         }
 729
 730         // If "safe", then busy-wait for any threads currently doing
 731         // foreign calls.  If we're about to unload this DLL, for
 732         // example, we need to be sure that there are no OS threads
 733         // that will try to return to code that has been unloaded.
 734         // We can be a bit more relaxed when this is a standalone
 735         // program that is about to terminate, and let safe=false.
 736         if (cap->suspended_ccalls && safe) {
 737             debugTrace(DEBUG_sched,
 738                        "thread(s) are involved in foreign calls, yielding");
 739             cap->running_task = NULL;
 740             RELEASE_LOCK(&cap->lock);
 741             // The IO manager thread might have been slow to start up,
 742             // so the first attempt to kill it might not have
 743             // succeeded.  Just in case, try again - the kill message
 744             // will only be sent once.
 745             //
 746             // To reproduce this deadlock: run ffi002(threaded1)
 747             // repeatedly on a loaded machine.
 748             ioManagerDie();
 749             yieldThread();
 750             continue;
 751         }
 752
 753         traceEventShutdown(cap);
 754         RELEASE_LOCK(&cap->lock);
 755         break;
 756     }
 757     // we now have the Capability, its run queue and spare workers
 758     // list are both empty.
 759
 760     // ToDo: we can't drop this mutex, because there might still be
 761     // threads performing foreign calls that will eventually try to
 762     // return via resumeThread() and attempt to grab cap->lock.
 763     // closeMutex(&cap->lock);
 764 }
 765
 766 /* ----------------------------------------------------------------------------
 767  * tryGrabCapability
 768  *
 769  * Attempt to gain control of a Capability if it is free.
 770  *
 771  * ------------------------------------------------------------------------- */
 772
 773 rtsBool
 774 tryGrabCapability (Capability *cap, Task *task)
 775 {
 776     if (cap->running_task != NULL) return rtsFalse;
 777     ACQUIRE_LOCK(&cap->lock);
 778     if (cap->running_task != NULL) {
 779         RELEASE_LOCK(&cap->lock);
 780         return rtsFalse;
 781     }
 782     task->cap = cap;
 783     cap->running_task = task;
 784     RELEASE_LOCK(&cap->lock);
 785     return rtsTrue;
 786 }
 787
 788
 789 #endif /* THREADED_RTS */
 790
 791 static void
 792 freeCapability (Capability *cap)
 793 {
 794     stgFree(cap->mut_lists);
 795     stgFree(cap->saved_mut_lists);
 796 #if defined(THREADED_RTS)
 797     freeSparkPool(cap->sparks);
 798 #endif
 799 }
 800
 801 void
 802 freeCapabilities (void)
 803 {
 804 #if defined(THREADED_RTS)
 805     nat i;
 806     for (i=0; i < n_capabilities; i++) {
 807         freeCapability(&capabilities[i]);
 808     }
 809 #else
 810     freeCapability(&MainCapability);
 811 #endif
 812 }
 813
 814 /* ---------------------------------------------------------------------------
 815    Mark everything directly reachable from the Capabilities.  When
 816    using multiple GC threads, each GC thread marks all Capabilities
 817    for which (c `mod` n == 0), for Capability c and thread n.
 818    ------------------------------------------------------------------------ */
 819
 820 void
 821 markSomeCapabilities (evac_fn evac, void *user, nat i0, nat delta,
 822                       rtsBool prune_sparks USED_IF_THREADS)
 823 {
 824     nat i;
 825     Capability *cap;
 826     InCall *incall;
 827
 828     // Each GC thread is responsible for following roots from the
 829     // Capability of the same number.  There will usually be the same
 830     // or fewer Capabilities as GC threads, but just in case there
 831     // are more, we mark every Capability whose number is the GC
 832     // thread's index plus a multiple of the number of GC threads.
 833     for (i = i0; i < n_capabilities; i += delta) {
 834         cap = &capabilities[i];
 835         evac(user, (StgClosure **)(void *)&cap->run_queue_hd);
 836         evac(user, (StgClosure **)(void *)&cap->run_queue_tl);
 837 #if defined(THREADED_RTS)
 838         evac(user, (StgClosure **)(void *)&cap->inbox);
 839 #endif
 840         for (incall = cap->suspended_ccalls; incall != NULL;
 841              incall=incall->next) {
 842             evac(user, (StgClosure **)(void *)&incall->suspended_tso);
 843         }
 844
 845 #if defined(THREADED_RTS)
 846         if (prune_sparks) {
 847             pruneSparkQueue (evac, user, cap);
 848         } else {
 849             traverseSparkQueue (evac, user, cap);
 850         }
 851 #endif
 852     }
 853
 854 #if !defined(THREADED_RTS)
 855     evac(user, (StgClosure **)(void *)&blocked_queue_hd);
 856     evac(user, (StgClosure **)(void *)&blocked_queue_tl);
 857     evac(user, (StgClosure **)(void *)&sleeping_queue);
 858 #endif
 859 }
 860
 861 void
 862 markCapabilities (evac_fn evac, void *user)
 863 {
 864     markSomeCapabilities(evac, user, 0, 1, rtsFalse);
 865 }
 866
 867 /* -----------------------------------------------------------------------------
 868    Messages
 869    -------------------------------------------------------------------------- */
 870