Capability.c

/* ---------------------------------------------------------------------------
 *
 * (c) The GHC Team, 2003-2012
 *
 * Capabilities
 *
 * A Capability represents the token required to execute STG code,
 * and all the state an OS thread/task needs to run Haskell code:
 * its STG registers, a pointer to its TSO, a nursery etc. During
 * STG execution, a pointer to the capabilitity is kept in a
 * register (BaseReg; actually it is a pointer to cap->r).
 *
 * Only in an THREADED_RTS build will there be multiple capabilities,
 * for non-threaded builds there is only one global capability, namely
 * MainCapability.
 *
 * --------------------------------------------------------------------------*/

#include "PosixSource.h"
#include "Rts.h"

#include "Capability.h"
#include "Schedule.h"
#include "Sparks.h"
#include "Trace.h"
#include "sm/GC.h" // for gcWorkerThread()
#include "STM.h"
#include "RtsUtils.h"

#include <string.h>

// one global capability, this is the Capability for non-threaded
// builds, and for +RTS -N1
Capability MainCapability;

nat n_capabilities = 0;
nat enabled_capabilities = 0;
Capability *capabilities = NULL;

// Holds the Capability which last became free.  This is used so that
// an in-call has a chance of quickly finding a free Capability.
// Maintaining a global free list of Capabilities would require global
// locking, so we don't do that.
Capability *last_free_capability = NULL;

/*
 * Indicates that the RTS wants to synchronise all the Capabilities
 * for some reason.  All Capabilities should stop and return to the
 * scheduler.
 */
volatile StgWord pending_sync = 0;

/* Let foreign code get the current Capability -- assuming there is one!
 * This is useful for unsafe foreign calls because they are called with
 * the current Capability held, but they are not passed it. For example,
 * see see the integer-gmp package which calls allocate() in its
 * stgAllocForGMP() function (which gets called by gmp functions).
 * */
Capability * rts_unsafeGetMyCapability (void)
{
#if defined(THREADED_RTS)
  return myTask()->cap;
#else
  return &MainCapability;
#endif
}

#if defined(THREADED_RTS)
STATIC_INLINE rtsBool
globalWorkToDo (void)
{
    return sched_state >= SCHED_INTERRUPTING
        || recent_activity == ACTIVITY_INACTIVE; // need to check for deadlock
}
#endif

#if defined(THREADED_RTS)
StgClosure *
findSpark (Capability *cap)
{
  Capability *robbed;
  StgClosurePtr spark;
  rtsBool retry;
  nat i = 0;

  if (!emptyRunQueue(cap) || cap->returning_tasks_hd != NULL) {
      // If there are other threads, don't try to run any new
      // sparks: sparks might be speculative, we don't want to take
      // resources away from the main computation.
      return 0;
  }

  do {
      retry = rtsFalse;

      // first try to get a spark from our own pool.
      // We should be using reclaimSpark(), because it works without
      // needing any atomic instructions:
      //   spark = reclaimSpark(cap->sparks);
      // However, measurements show that this makes at least one benchmark
      // slower (prsa) and doesn't affect the others.
      spark = tryStealSpark(cap->sparks);
      while (spark != NULL && fizzledSpark(spark)) {
          cap->spark_stats.fizzled++;
          traceEventSparkFizzle(cap);
          spark = tryStealSpark(cap->sparks);
      }
      if (spark != NULL) {
          cap->spark_stats.converted++;

          // Post event for running a spark from capability's own pool.
          traceEventSparkRun(cap);

          return spark;
      }
      if (!emptySparkPoolCap(cap)) {
          retry = rtsTrue;
      }

      if (n_capabilities == 1) { return NULL; } // makes no sense...

      debugTrace(DEBUG_sched,
                 "cap %d: Trying to steal work from other capabilities", 
                 cap->no);

      /* visit cap.s 0..n-1 in sequence until a theft succeeds. We could
      start at a random place instead of 0 as well.  */
      for ( i=0 ; i < n_capabilities ; i++ ) {
          robbed = &capabilities[i];
          if (cap == robbed)  // ourselves...
              continue;

          if (emptySparkPoolCap(robbed)) // nothing to steal here
              continue;

          spark = tryStealSpark(robbed->sparks);
          while (spark != NULL && fizzledSpark(spark)) {
              cap->spark_stats.fizzled++;
              traceEventSparkFizzle(cap);
              spark = tryStealSpark(robbed->sparks);
          }
          if (spark == NULL && !emptySparkPoolCap(robbed)) {
              // we conflicted with another thread while trying to steal;
              // try again later.
              retry = rtsTrue;
          }

          if (spark != NULL) {
              cap->spark_stats.converted++;
              traceEventSparkSteal(cap, robbed->no);
              
              return spark;
          }
          // otherwise: no success, try next one
      }
  } while (retry);

  debugTrace(DEBUG_sched, "No sparks stolen");
  return NULL;
}

// Returns True if any spark pool is non-empty at this moment in time
// The result is only valid for an instant, of course, so in a sense
// is immediately invalid, and should not be relied upon for
// correctness.
rtsBool
anySparks (void)
{
    nat i;

    for (i=0; i < n_capabilities; i++) {
        if (!emptySparkPoolCap(&capabilities[i])) {
            return rtsTrue;
        }
    }
    return rtsFalse;
}
#endif

/* -----------------------------------------------------------------------------
 * Manage the returning_tasks lists.
 *
 * These functions require cap->lock
 * -------------------------------------------------------------------------- */

#if defined(THREADED_RTS)
STATIC_INLINE void
newReturningTask (Capability *cap, Task *task)
{
    ASSERT_LOCK_HELD(&cap->lock);
    ASSERT(task->next == NULL);
    if (cap->returning_tasks_hd) {
	ASSERT(cap->returning_tasks_tl->next == NULL);
	cap->returning_tasks_tl->next = task;
    } else {
	cap->returning_tasks_hd = task;
    }
    cap->returning_tasks_tl = task;
}

STATIC_INLINE Task *
popReturningTask (Capability *cap)
{
    ASSERT_LOCK_HELD(&cap->lock);
    Task *task;
    task = cap->returning_tasks_hd;
    ASSERT(task);
    cap->returning_tasks_hd = task->next;
    if (!cap->returning_tasks_hd) {
	cap->returning_tasks_tl = NULL;
    }
    task->next = NULL;
    return task;
}
#endif

/* ----------------------------------------------------------------------------
 * Initialisation
 *
 * The Capability is initially marked not free.
 * ------------------------------------------------------------------------- */

static void
initCapability( Capability *cap, nat i )
{
    nat g;

    cap->no = i;
    cap->in_haskell        = rtsFalse;
    cap->idle              = 0;
    cap->disabled          = rtsFalse;

    cap->run_queue_hd      = END_TSO_QUEUE;
    cap->run_queue_tl      = END_TSO_QUEUE;

#if defined(THREADED_RTS)
    initMutex(&cap->lock);
    cap->running_task      = NULL; // indicates cap is free
    cap->spare_workers     = NULL;
    cap->n_spare_workers   = 0;
    cap->suspended_ccalls  = NULL;
    cap->returning_tasks_hd = NULL;
    cap->returning_tasks_tl = NULL;
    cap->inbox              = (Message*)END_TSO_QUEUE;
    cap->sparks             = allocSparkPool();
    cap->spark_stats.created    = 0;
    cap->spark_stats.dud        = 0;
    cap->spark_stats.overflowed = 0;
    cap->spark_stats.converted  = 0;
    cap->spark_stats.gcd        = 0;
    cap->spark_stats.fizzled    = 0;
#endif
    cap->total_allocated        = 0;

    cap->f.stgEagerBlackholeInfo = (W_)&__stg_EAGER_BLACKHOLE_info;
    cap->f.stgGCEnter1     = (StgFunPtr)__stg_gc_enter_1;
    cap->f.stgGCFun        = (StgFunPtr)__stg_gc_fun;

    cap->mut_lists  = stgMallocBytes(sizeof(bdescr *) *
				     RtsFlags.GcFlags.generations,
				     "initCapability");
    cap->saved_mut_lists = stgMallocBytes(sizeof(bdescr *) *
                                          RtsFlags.GcFlags.generations,
                                          "initCapability");

    for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
	cap->mut_lists[g] = NULL;
    }

    cap->free_tvar_watch_queues = END_STM_WATCH_QUEUE;
    cap->free_invariant_check_queues = END_INVARIANT_CHECK_QUEUE;
    cap->free_trec_chunks = END_STM_CHUNK_LIST;
    cap->free_trec_headers = NO_TREC;
    cap->transaction_tokens = 0;
    cap->context_switch = 0;
    cap->pinned_object_block = NULL;
    cap->pinned_object_blocks = NULL;

#ifdef PROFILING
    cap->r.rCCCS = CCS_SYSTEM;
#else
    cap->r.rCCCS = NULL;
#endif

    traceCapCreate(cap);
    traceCapsetAssignCap(CAPSET_OSPROCESS_DEFAULT, i);
    traceCapsetAssignCap(CAPSET_CLOCKDOMAIN_DEFAULT, i);
#if defined(THREADED_RTS)
    traceSparkCounters(cap);
#endif
}

/* ---------------------------------------------------------------------------
 * Function:  initCapabilities()
 *
 * Purpose:   set up the Capability handling. For the THREADED_RTS build,
 *            we keep a table of them, the size of which is
 *            controlled by the user via the RTS flag -N.
 *
 * ------------------------------------------------------------------------- */
void
initCapabilities( void )
{
    /* Declare a couple capability sets representing the process and
       clock domain. Each capability will get added to these capsets. */
    traceCapsetCreate(CAPSET_OSPROCESS_DEFAULT, CapsetTypeOsProcess);
    traceCapsetCreate(CAPSET_CLOCKDOMAIN_DEFAULT, CapsetTypeClockdomain);

#if defined(THREADED_RTS)

#ifndef REG_Base
    // We can't support multiple CPUs if BaseReg is not a register
    if (RtsFlags.ParFlags.nNodes > 1) {
	errorBelch("warning: multiple CPUs not supported in this build, reverting to 1");
	RtsFlags.ParFlags.nNodes = 1;
    }
#endif

    n_capabilities = 0;
    moreCapabilities(0, RtsFlags.ParFlags.nNodes);
    n_capabilities = RtsFlags.ParFlags.nNodes;

#else /* !THREADED_RTS */

    n_capabilities = 1;
    capabilities = &MainCapability;
    initCapability(&MainCapability, 0);

#endif

    enabled_capabilities = n_capabilities;

    // There are no free capabilities to begin with.  We will start
    // a worker Task to each Capability, which will quickly put the
    // Capability on the free list when it finds nothing to do.
    last_free_capability = &capabilities[0];
}

Capability *
moreCapabilities (nat from USED_IF_THREADS, nat to USED_IF_THREADS)
{
#if defined(THREADED_RTS)
    nat i;
    Capability *old_capabilities = capabilities;

    if (to == 1) {
        // THREADED_RTS must work on builds that don't have a mutable
        // BaseReg (eg. unregisterised), so in this case
	// capabilities[0] must coincide with &MainCapability.
        capabilities = &MainCapability;
    } else {
        capabilities = stgMallocBytes(to * sizeof(Capability),
                                      "moreCapabilities");

        if (from > 0) {
            memcpy(capabilities, old_capabilities, from * sizeof(Capability));
        }
    }

    for (i = from; i < to; i++) {
	initCapability(&capabilities[i], i);
    }

    last_free_capability = &capabilities[0];

    debugTrace(DEBUG_sched, "allocated %d more capabilities", to - from);

    // Return the old array to free later.
    if (from > 1) {
        return old_capabilities;
    } else {
        return NULL;
    }
#else
    return NULL;
#endif
}

/* ----------------------------------------------------------------------------
 * setContextSwitches: cause all capabilities to context switch as
 * soon as possible.
 * ------------------------------------------------------------------------- */

void contextSwitchAllCapabilities(void)
{
    nat i;
    for (i=0; i < n_capabilities; i++) {
        contextSwitchCapability(&capabilities[i]);
    }
}

void interruptAllCapabilities(void)
{
    nat i;
    for (i=0; i < n_capabilities; i++) {
        interruptCapability(&capabilities[i]);
    }
}

/* ----------------------------------------------------------------------------
 * Give a Capability to a Task.  The task must currently be sleeping
 * on its condition variable.
 *
 * Requires cap->lock (modifies cap->running_task).
 *
 * When migrating a Task, the migrater must take task->lock before
 * modifying task->cap, to synchronise with the waking up Task.
 * Additionally, the migrater should own the Capability (when
 * migrating the run queue), or cap->lock (when migrating
 * returning_workers).
 *
 * ------------------------------------------------------------------------- */

#if defined(THREADED_RTS)
STATIC_INLINE void
giveCapabilityToTask (Capability *cap USED_IF_DEBUG, Task *task)
{
    ASSERT_LOCK_HELD(&cap->lock);
    ASSERT(task->cap == cap);
    debugTrace(DEBUG_sched, "passing capability %d to %s %#" FMT_HexWord64,
               cap->no, task->incall->tso ? "bound task" : "worker",
               serialisableTaskId(task));
    ACQUIRE_LOCK(&task->lock);
    if (task->wakeup == rtsFalse) {
        task->wakeup = rtsTrue;
        // the wakeup flag is needed because signalCondition() doesn't
        // flag the condition if the thread is already runniing, but we want
        // it to be sticky.
        signalCondition(&task->cond);
    }
    RELEASE_LOCK(&task->lock);
}
#endif

/* ----------------------------------------------------------------------------
 * Function:  releaseCapability(Capability*)
 *
 * Purpose:   Letting go of a capability. Causes a
 *            'returning worker' thread or a 'waiting worker'
 *            to wake up, in that order.
 * ------------------------------------------------------------------------- */

#if defined(THREADED_RTS)
void
releaseCapability_ (Capability* cap, 
                    rtsBool always_wakeup)
{
    Task *task;

    task = cap->running_task;

    ASSERT_PARTIAL_CAPABILITY_INVARIANTS(cap,task);

    cap->running_task = NULL;

    // Check to see whether a worker thread can be given
    // the go-ahead to return the result of an external call..
    if (cap->returning_tasks_hd != NULL) {
	giveCapabilityToTask(cap,cap->returning_tasks_hd);
	// The Task pops itself from the queue (see waitForReturnCapability())
	return;
    }

    // If there is a pending sync, then we should just leave the
    // Capability free.  The thread trying to sync will be about to
    // call waitForReturnCapability().
    if (pending_sync != 0 && pending_sync != SYNC_GC_PAR) {
      last_free_capability = cap; // needed?
      debugTrace(DEBUG_sched, "sync pending, set capability %d free", cap->no);
      return;
    } 

    // If the next thread on the run queue is a bound thread,
    // give this Capability to the appropriate Task.
    if (!emptyRunQueue(cap) && peekRunQueue(cap)->bound) {
	// Make sure we're not about to try to wake ourselves up
	// ASSERT(task != cap->run_queue_hd->bound);
        // assertion is false: in schedule() we force a yield after
	// ThreadBlocked, but the thread may be back on the run queue
	// by now.
	task = peekRunQueue(cap)->bound->task;
	giveCapabilityToTask(cap, task);
	return;
    }

    if (!cap->spare_workers) {
	// Create a worker thread if we don't have one.  If the system
	// is interrupted, we only create a worker task if there
	// are threads that need to be completed.  If the system is
	// shutting down, we never create a new worker.
	if (sched_state < SCHED_SHUTTING_DOWN || !emptyRunQueue(cap)) {
	    debugTrace(DEBUG_sched,
		       "starting new worker on capability %d", cap->no);
	    startWorkerTask(cap);
	    return;
	}
    }

    // If we have an unbound thread on the run queue, or if there's
    // anything else to do, give the Capability to a worker thread.
    if (always_wakeup || 
        !emptyRunQueue(cap) || !emptyInbox(cap) ||
        (!cap->disabled && !emptySparkPoolCap(cap)) || globalWorkToDo()) {
	if (cap->spare_workers) {
	    giveCapabilityToTask(cap, cap->spare_workers);
	    // The worker Task pops itself from the queue;
	    return;
	}
    }

#ifdef PROFILING
    cap->r.rCCCS = CCS_IDLE;
#endif
    last_free_capability = cap;
    debugTrace(DEBUG_sched, "freeing capability %d", cap->no);
}

void
releaseCapability (Capability* cap USED_IF_THREADS)
{
    ACQUIRE_LOCK(&cap->lock);
    releaseCapability_(cap, rtsFalse);
    RELEASE_LOCK(&cap->lock);
}

void
releaseAndWakeupCapability (Capability* cap USED_IF_THREADS)
{
    ACQUIRE_LOCK(&cap->lock);
    releaseCapability_(cap, rtsTrue);
    RELEASE_LOCK(&cap->lock);
}

static void
releaseCapabilityAndQueueWorker (Capability* cap USED_IF_THREADS)
{
    Task *task;

    ACQUIRE_LOCK(&cap->lock);

    task = cap->running_task;

    // If the Task is stopped, we shouldn't be yielding, we should
    // be just exiting.
    ASSERT(!task->stopped);

    // If the current task is a worker, save it on the spare_workers
    // list of this Capability.  A worker can mark itself as stopped,
    // in which case it is not replaced on the spare_worker queue.
    // This happens when the system is shutting down (see
    // Schedule.c:workerStart()).
    if (!isBoundTask(task))
    {
        if (cap->n_spare_workers < MAX_SPARE_WORKERS)
        {
            task->next = cap->spare_workers;
            cap->spare_workers = task;
            cap->n_spare_workers++;
        }
        else
        {
            debugTrace(DEBUG_sched, "%d spare workers already, exiting",
                       cap->n_spare_workers);
            releaseCapability_(cap,rtsFalse);
            // hold the lock until after workerTaskStop; c.f. scheduleWorker()
            workerTaskStop(task);
            RELEASE_LOCK(&cap->lock);
            shutdownThread();
        }
    }
    // Bound tasks just float around attached to their TSOs.

    releaseCapability_(cap,rtsFalse);

    RELEASE_LOCK(&cap->lock);
}
#endif

/* ----------------------------------------------------------------------------
 * waitForReturnCapability (Capability **pCap, Task *task)
 *
 * Purpose:  when an OS thread returns from an external call,
 * it calls waitForReturnCapability() (via Schedule.resumeThread())
 * to wait for permission to enter the RTS & communicate the
 * result of the external call back to the Haskell thread that
 * made it.
 *
 * ------------------------------------------------------------------------- */
void
waitForReturnCapability (Capability **pCap, Task *task)
{
#if !defined(THREADED_RTS)

    MainCapability.running_task = task;
    task->cap = &MainCapability;
    *pCap = &MainCapability;

#else
    Capability *cap = *pCap;

    if (cap == NULL) {
	// Try last_free_capability first
	cap = last_free_capability;
	if (cap->running_task) {
	    nat i;
	    // otherwise, search for a free capability
            cap = NULL;
	    for (i = 0; i < n_capabilities; i++) {
		if (!capabilities[i].running_task) {
                    cap = &capabilities[i];
		    break;
		}
	    }
            if (cap == NULL) {
                // Can't find a free one, use last_free_capability.
                cap = last_free_capability;
            }
	}

	// record the Capability as the one this Task is now assocated with.
	task->cap = cap;

    } else {
	ASSERT(task->cap == cap);
    }

    ACQUIRE_LOCK(&cap->lock);

    debugTrace(DEBUG_sched, "returning; I want capability %d", cap->no);

    if (!cap->running_task) {
	// It's free; just grab it
	cap->running_task = task;
	RELEASE_LOCK(&cap->lock);
    } else {
	newReturningTask(cap,task);
	RELEASE_LOCK(&cap->lock);

	for (;;) {
	    ACQUIRE_LOCK(&task->lock);
	    // task->lock held, cap->lock not held
	    if (!task->wakeup) waitCondition(&task->cond, &task->lock);
	    cap = task->cap;
	    task->wakeup = rtsFalse;
	    RELEASE_LOCK(&task->lock);

	    // now check whether we should wake up...
	    ACQUIRE_LOCK(&cap->lock);
	    if (cap->running_task == NULL) {
		if (cap->returning_tasks_hd != task) {
		    giveCapabilityToTask(cap,cap->returning_tasks_hd);
		    RELEASE_LOCK(&cap->lock);
		    continue;
		}
		cap->running_task = task;
		popReturningTask(cap);
		RELEASE_LOCK(&cap->lock);
		break;
	    }
	    RELEASE_LOCK(&cap->lock);
	}

    }

#ifdef PROFILING
    cap->r.rCCCS = CCS_SYSTEM;
#endif

    ASSERT_FULL_CAPABILITY_INVARIANTS(cap, task);

    debugTrace(DEBUG_sched, "resuming capability %d", cap->no);

    *pCap = cap;
#endif
}

#if defined(THREADED_RTS)
/* ----------------------------------------------------------------------------
 * yieldCapability
 * ------------------------------------------------------------------------- */

/* See Note [GC livelock] in Schedule.c for why we have gcAllowed
   and return the rtsBool */
rtsBool /* Did we GC? */
yieldCapability (Capability** pCap, Task *task, rtsBool gcAllowed)
{
    Capability *cap = *pCap;

    if ((pending_sync == SYNC_GC_PAR) && gcAllowed) {
        traceEventGcStart(cap);
        gcWorkerThread(cap);
        traceEventGcEnd(cap);
        traceSparkCounters(cap);
        // See Note [migrated bound threads 2]
        if (task->cap == cap) {
            return rtsTrue;
        }
    }

	debugTrace(DEBUG_sched, "giving up capability %d", cap->no);

	// We must now release the capability and wait to be woken up
	// again.
	task->wakeup = rtsFalse;
	releaseCapabilityAndQueueWorker(cap);

	for (;;) {
	    ACQUIRE_LOCK(&task->lock);
	    // task->lock held, cap->lock not held
	    if (!task->wakeup) waitCondition(&task->cond, &task->lock);
	    cap = task->cap;
	    task->wakeup = rtsFalse;
	    RELEASE_LOCK(&task->lock);

	    debugTrace(DEBUG_sched, "woken up on capability %d", cap->no);

	    ACQUIRE_LOCK(&cap->lock);
	    if (cap->running_task != NULL) {
		debugTrace(DEBUG_sched, 
			   "capability %d is owned by another task", cap->no);
		RELEASE_LOCK(&cap->lock);
		continue;
	    }

            if (task->cap != cap) {
                // see Note [migrated bound threads]
                debugTrace(DEBUG_sched,
                           "task has been migrated to cap %d", task->cap->no);
		RELEASE_LOCK(&cap->lock);
		continue;
	    }

            if (task->incall->tso == NULL) {
		ASSERT(cap->spare_workers != NULL);
		// if we're not at the front of the queue, release it
		// again.  This is unlikely to happen.
		if (cap->spare_workers != task) {
		    giveCapabilityToTask(cap,cap->spare_workers);
		    RELEASE_LOCK(&cap->lock);
		    continue;
		}
		cap->spare_workers = task->next;
		task->next = NULL;
                cap->n_spare_workers--;
            }

            cap->running_task = task;
	    RELEASE_LOCK(&cap->lock);
	    break;
	}

        debugTrace(DEBUG_sched, "resuming capability %d", cap->no);
	ASSERT(cap->running_task == task);

#ifdef PROFILING
        cap->r.rCCCS = CCS_SYSTEM;
#endif

    *pCap = cap;

    ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);

    return rtsFalse;
}

// Note [migrated bound threads]
//
// There's a tricky case where:
//    - cap A is running an unbound thread T1
//    - there is a bound thread T2 at the head of the run queue on cap A
//    - T1 makes a safe foreign call, the task bound to T2 is woken up on cap A
//    - T1 returns quickly grabbing A again (T2 is still waking up on A)
//    - T1 blocks, the scheduler migrates T2 to cap B
//    - the task bound to T2 wakes up on cap B
//
// We take advantage of the following invariant:
//
//  - A bound thread can only be migrated by the holder of the
//    Capability on which the bound thread currently lives.  So, if we
//    hold Capabilty C, and task->cap == C, then task cannot be
//    migrated under our feet.

// Note [migrated bound threads 2]
//
// Second tricky case;
//   - A bound Task becomes a GC thread
//   - scheduleDoGC() migrates the thread belonging to this Task,
//     because the Capability it is on is disabled
//   - after GC, gcWorkerThread() returns, but now we are
//     holding a Capability that is not the same as task->cap
//   - Hence we must check for this case and immediately give up the
//     cap we hold.

/* ----------------------------------------------------------------------------
 * prodCapability
 *
 * If a Capability is currently idle, wake up a Task on it.  Used to 
 * get every Capability into the GC.
 * ------------------------------------------------------------------------- */

void
prodCapability (Capability *cap, Task *task)
{
    ACQUIRE_LOCK(&cap->lock);
    if (!cap->running_task) {
        cap->running_task = task;
        releaseCapability_(cap,rtsTrue);
    }
    RELEASE_LOCK(&cap->lock);
}

/* ----------------------------------------------------------------------------
 * tryGrabCapability
 *
 * Attempt to gain control of a Capability if it is free.
 *
 * ------------------------------------------------------------------------- */

rtsBool
tryGrabCapability (Capability *cap, Task *task)
{
    if (cap->running_task != NULL) return rtsFalse;
    ACQUIRE_LOCK(&cap->lock);
    if (cap->running_task != NULL) {
	RELEASE_LOCK(&cap->lock);
	return rtsFalse;
    }
    task->cap = cap;
    cap->running_task = task;
    RELEASE_LOCK(&cap->lock);
    return rtsTrue;
}


#endif /* THREADED_RTS */

/* ----------------------------------------------------------------------------
 * shutdownCapability
 *
 * At shutdown time, we want to let everything exit as cleanly as
 * possible.  For each capability, we let its run queue drain, and
 * allow the workers to stop.
 *
 * This function should be called when interrupted and
 * sched_state = SCHED_SHUTTING_DOWN, thus any worker that wakes up
 * will exit the scheduler and call taskStop(), and any bound thread
 * that wakes up will return to its caller.  Runnable threads are
 * killed.
 *
 * ------------------------------------------------------------------------- */

void
shutdownCapability (Capability *cap USED_IF_THREADS,
                    Task *task USED_IF_THREADS,
                    rtsBool safe USED_IF_THREADS)
{
#if defined(THREADED_RTS)
    nat i;

    task->cap = cap;

    // Loop indefinitely until all the workers have exited and there
    // are no Haskell threads left.  We used to bail out after 50
    // iterations of this loop, but that occasionally left a worker
    // running which caused problems later (the closeMutex() below
    // isn't safe, for one thing).

    for (i = 0; /* i < 50 */; i++) {
        ASSERT(sched_state == SCHED_SHUTTING_DOWN);

	debugTrace(DEBUG_sched, 
		   "shutting down capability %d, attempt %d", cap->no, i);
	ACQUIRE_LOCK(&cap->lock);
	if (cap->running_task) {
	    RELEASE_LOCK(&cap->lock);
	    debugTrace(DEBUG_sched, "not owner, yielding");
	    yieldThread();
	    continue;
	}
	cap->running_task = task;

        if (cap->spare_workers) {
            // Look for workers that have died without removing
            // themselves from the list; this could happen if the OS
            // summarily killed the thread, for example.  This
            // actually happens on Windows when the system is
            // terminating the program, and the RTS is running in a
            // DLL.
            Task *t, *prev;
            prev = NULL;
            for (t = cap->spare_workers; t != NULL; t = t->next) {
                if (!osThreadIsAlive(t->id)) {
                    debugTrace(DEBUG_sched, 
                               "worker thread %p has died unexpectedly", (void *)(size_t)t->id);
                    cap->n_spare_workers--;
                    if (!prev) {
                        cap->spare_workers = t->next;
                    } else {
                        prev->next = t->next;
                    }
                    prev = t;
                }
            }
        }

	if (!emptyRunQueue(cap) || cap->spare_workers) {
	    debugTrace(DEBUG_sched, 
		       "runnable threads or workers still alive, yielding");
	    releaseCapability_(cap,rtsFalse); // this will wake up a worker
	    RELEASE_LOCK(&cap->lock);
	    yieldThread();
	    continue;
	}

        // If "safe", then busy-wait for any threads currently doing
        // foreign calls.  If we're about to unload this DLL, for
        // example, we need to be sure that there are no OS threads
        // that will try to return to code that has been unloaded.
        // We can be a bit more relaxed when this is a standalone
        // program that is about to terminate, and let safe=false.
        if (cap->suspended_ccalls && safe) {
	    debugTrace(DEBUG_sched, 
		       "thread(s) are involved in foreign calls, yielding");
            cap->running_task = NULL;
	    RELEASE_LOCK(&cap->lock);
            // The IO manager thread might have been slow to start up,
            // so the first attempt to kill it might not have
            // succeeded.  Just in case, try again - the kill message
            // will only be sent once.
            //
            // To reproduce this deadlock: run ffi002(threaded1)
            // repeatedly on a loaded machine.
            ioManagerDie();
            yieldThread();
            continue;
        }

        traceSparkCounters(cap);
	RELEASE_LOCK(&cap->lock);
	break;
    }
    // we now have the Capability, its run queue and spare workers
    // list are both empty.

    // ToDo: we can't drop this mutex, because there might still be
    // threads performing foreign calls that will eventually try to 
    // return via resumeThread() and attempt to grab cap->lock.
    // closeMutex(&cap->lock);
#endif
}

void
shutdownCapabilities(Task *task, rtsBool safe)
{
    nat i;
    for (i=0; i < n_capabilities; i++) {
        ASSERT(task->incall->tso == NULL);
        shutdownCapability(&capabilities[i], task, safe);
    }
#if defined(THREADED_RTS)
    ASSERT(checkSparkCountInvariant());
#endif
}

static void
freeCapability (Capability *cap)
{
    stgFree(cap->mut_lists);
    stgFree(cap->saved_mut_lists);
#if defined(THREADED_RTS)
    freeSparkPool(cap->sparks);
#endif
    traceCapsetRemoveCap(CAPSET_OSPROCESS_DEFAULT, cap->no);
    traceCapsetRemoveCap(CAPSET_CLOCKDOMAIN_DEFAULT, cap->no);
    traceCapDelete(cap);
}

void
freeCapabilities (void)
{
#if defined(THREADED_RTS)
    nat i;
    for (i=0; i < n_capabilities; i++) {
        freeCapability(&capabilities[i]);
    }
#else
    freeCapability(&MainCapability);
#endif
    traceCapsetDelete(CAPSET_OSPROCESS_DEFAULT);
    traceCapsetDelete(CAPSET_CLOCKDOMAIN_DEFAULT);
}

/* ---------------------------------------------------------------------------
   Mark everything directly reachable from the Capabilities.  When
   using multiple GC threads, each GC thread marks all Capabilities
   for which (c `mod` n == 0), for Capability c and thread n.
   ------------------------------------------------------------------------ */

void
markCapability (evac_fn evac, void *user, Capability *cap,
                rtsBool no_mark_sparks USED_IF_THREADS)
{
    InCall *incall;

    // Each GC thread is responsible for following roots from the
    // Capability of the same number.  There will usually be the same
    // or fewer Capabilities as GC threads, but just in case there
    // are more, we mark every Capability whose number is the GC
    // thread's index plus a multiple of the number of GC threads.
    evac(user, (StgClosure **)(void *)&cap->run_queue_hd);
    evac(user, (StgClosure **)(void *)&cap->run_queue_tl);
#if defined(THREADED_RTS)
    evac(user, (StgClosure **)(void *)&cap->inbox);
#endif
    for (incall = cap->suspended_ccalls; incall != NULL;
         incall=incall->next) {
        evac(user, (StgClosure **)(void *)&incall->suspended_tso);
    }

#if defined(THREADED_RTS)
    if (!no_mark_sparks) {
        traverseSparkQueue (evac, user, cap);
    }
#endif

    // Free STM structures for this Capability
    stmPreGCHook(cap);
}

void
markCapabilities (evac_fn evac, void *user)
{
    nat n;
    for (n = 0; n < n_capabilities; n++) {
        markCapability(evac, user, &capabilities[n], rtsFalse);
    }
}

#if defined(THREADED_RTS)
rtsBool checkSparkCountInvariant (void)
{
    SparkCounters sparks = { 0, 0, 0, 0, 0, 0 };
    StgWord64 remaining = 0;
    nat i;

    for (i = 0; i < n_capabilities; i++) {
        sparks.created   += capabilities[i].spark_stats.created;
        sparks.dud       += capabilities[i].spark_stats.dud;
        sparks.overflowed+= capabilities[i].spark_stats.overflowed;
        sparks.converted += capabilities[i].spark_stats.converted;
        sparks.gcd       += capabilities[i].spark_stats.gcd;
        sparks.fizzled   += capabilities[i].spark_stats.fizzled;
        remaining        += sparkPoolSize(capabilities[i].sparks);
    }
    
    /* The invariant is
     *   created = converted + remaining + gcd + fizzled
     */
    debugTrace(DEBUG_sparks,"spark invariant: %ld == %ld + %ld + %ld + %ld "
                            "(created == converted + remaining + gcd + fizzled)",
                            sparks.created, sparks.converted, remaining,
                            sparks.gcd, sparks.fizzled);

    return (sparks.created ==
              sparks.converted + remaining + sparks.gcd + sparks.fizzled);

}
#endif