path: root/rts/RtsAPI.c

/* ----------------------------------------------------------------------------
 *
 * (c) The GHC Team, 1998-2001
 *
 * API for invoking Haskell functions via the RTS
 *
 * --------------------------------------------------------------------------*/

#include "rts/PosixSource.h"
#include "Rts.h"
#include "RtsAPI.h"
#include "HsFFI.h"

#include "RtsUtils.h"
#include "Prelude.h"
#include "Schedule.h"
#include "Capability.h"
#include "StableName.h"
#include "StablePtr.h"
#include "Threads.h"
#include "Weak.h"
#include "sm/NonMoving.h"

/* ----------------------------------------------------------------------------
   Building Haskell objects from C datatypes.
   ------------------------------------------------------------------------- */
HaskellObj
rts_mkChar (Capability *cap, HsChar c)
{
  StgClosure *p;
  // See Note [Precomputed static closures]
  if (c <= MAX_CHARLIKE) {
    p = (StgClosure *)CHARLIKE_CLOSURE(c);
  } else {
    p = (StgClosure *)allocate(cap, CONSTR_sizeW(0,1));
    SET_HDR(p, Czh_con_info, CCS_SYSTEM);
    p->payload[0] = (StgClosure *)(StgWord)(StgChar)c;
  }
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkInt (Capability *cap, HsInt i)
{
  StgClosure *p;
  // See Note [Precomputed static closures]
  if (i >= MIN_INTLIKE && i <= MAX_INTLIKE) {
    p = (StgClosure *)INTLIKE_CLOSURE(i);
  } else {
    p = (StgClosure *)allocate(cap,CONSTR_sizeW(0,1));
    SET_HDR(p, Izh_con_info, CCS_SYSTEM);
    *(StgInt *)p->payload = i;
  }
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkInt8 (Capability *cap, HsInt8 i)
{
  StgClosure *p = (StgClosure *)allocate(cap,CONSTR_sizeW(0,1));
  SET_HDR(p, I8zh_con_info, CCS_SYSTEM);
  *(StgInt8 *)p->payload = i;
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkInt16 (Capability *cap, HsInt16 i)
{
  StgClosure *p = (StgClosure *)allocate(cap,CONSTR_sizeW(0,1));
  SET_HDR(p, I16zh_con_info, CCS_SYSTEM);
  *(StgInt16 *)p->payload = i;
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkInt32 (Capability *cap, HsInt32 i)
{
  StgClosure *p = (StgClosure *)allocate(cap,CONSTR_sizeW(0,1));
  SET_HDR(p, I32zh_con_info, CCS_SYSTEM);
  *(StgInt32 *)p->payload = i;
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkInt64 (Capability *cap, HsInt64 i)
{
  StgClosure *p = (StgClosure *)allocate(cap,CONSTR_sizeW(0,sizeofW(StgInt64)));
  SET_HDR(p, I64zh_con_info, CCS_SYSTEM);
  ASSIGN_Int64((P_)&(p->payload[0]), i);
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkWord (Capability *cap, HsWord i)
{
  StgClosure *p = (StgClosure *)allocate(cap,CONSTR_sizeW(0,1));
  SET_HDR(p, Wzh_con_info, CCS_SYSTEM);
  *(StgWord *)p->payload = i;
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkWord8 (Capability *cap, HsWord8 w)
{
  StgClosure *p = (StgClosure *)allocate(cap,CONSTR_sizeW(0,1));
  SET_HDR(p, W8zh_con_info, CCS_SYSTEM);
  *(StgWord8 *)p->payload = w;
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkWord16 (Capability *cap, HsWord16 w)
{
  StgClosure *p = (StgClosure *)allocate(cap,CONSTR_sizeW(0,1));
  SET_HDR(p, W16zh_con_info, CCS_SYSTEM);
  *(StgWord16 *)p->payload = w;
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkWord32 (Capability *cap, HsWord32 w)
{
  StgClosure *p = (StgClosure *)allocate(cap,CONSTR_sizeW(0,1));
  SET_HDR(p, W32zh_con_info, CCS_SYSTEM);
  *(StgWord32 *)p->payload = w;
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkWord64 (Capability *cap, HsWord64 w)
{
  StgClosure *p = (StgClosure *)allocate(cap,CONSTR_sizeW(0,sizeofW(StgWord64)));
  SET_HDR(p, W64zh_con_info, CCS_SYSTEM);
  ASSIGN_Word64((P_)&(p->payload[0]), w);
  return TAG_CLOSURE(1, p);
}


HaskellObj
rts_mkFloat (Capability *cap, HsFloat f)
{
  StgClosure *p = (StgClosure *)allocate(cap,CONSTR_sizeW(0,1));
  SET_HDR(p, Fzh_con_info, CCS_SYSTEM);
  ASSIGN_FLT((P_)p->payload, (StgFloat)f);
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkDouble (Capability *cap, HsDouble d)
{
  StgClosure *p = (StgClosure *)allocate(cap,CONSTR_sizeW(0,sizeofW(StgDouble)));
  SET_HDR(p, Dzh_con_info, CCS_SYSTEM);
  ASSIGN_DBL((P_)p->payload, (StgDouble)d);
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkStablePtr (Capability *cap, HsStablePtr s)
{
  StgClosure *p = (StgClosure *)allocate(cap,sizeofW(StgHeader)+1);
  SET_HDR(p, StablePtr_con_info, CCS_SYSTEM);
  p->payload[0] = (StgClosure *)s;
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkPtr (Capability *cap, HsPtr a)
{
  StgClosure *p = (StgClosure *)allocate(cap,sizeofW(StgHeader)+1);
  SET_HDR(p, Ptr_con_info, CCS_SYSTEM);
  p->payload[0] = (StgClosure *)a;
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkFunPtr (Capability *cap, HsFunPtr a)
{
  StgClosure *p = (StgClosure *)allocate(cap,sizeofW(StgHeader)+1);
  SET_HDR(p, FunPtr_con_info, CCS_SYSTEM);
  p->payload[0] = (StgClosure *)a;
  return TAG_CLOSURE(1, p);
}

HaskellObj
rts_mkBool (Capability *cap STG_UNUSED, HsBool b)
{
  if (b) {
    return TAG_CLOSURE(2, (StgClosure *)True_closure);
  } else {
    return TAG_CLOSURE(1, (StgClosure *)False_closure);
  }
}

HaskellObj
rts_mkString (Capability *cap, char *s)
{
  return rts_apply(cap, (StgClosure *)unpackCString_closure, rts_mkPtr(cap,s));
}

HaskellObj
rts_apply (Capability *cap, HaskellObj f, HaskellObj arg)
{
    StgThunk *ap;

    ap = (StgThunk *)allocate(cap,sizeofW(StgThunk) + 2);
    // Here we don't want to use CCS_SYSTEM, because it's a hidden cost centre,
    // and evaluating Haskell code under a hidden cost centre leads to
    // confusing profiling output. (#7753)
    SET_HDR(ap, (StgInfoTable *)&stg_ap_2_upd_info, CCS_MAIN);
    ap->payload[0] = f;
    ap->payload[1] = arg;
    return (StgClosure *)ap;
}

/* ----------------------------------------------------------------------------
   Deconstructing Haskell objects

   We would like to assert that we have the right kind of object in
   each case, but this is problematic because in GHCi the info table
   for the D# constructor (say) might be dynamically loaded.  Hence we
   omit these assertions for now.
   ------------------------------------------------------------------------- */

HsChar
rts_getChar (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == Czh_con_info ||
    //        p->header.info == Czh_static_info);
    return (StgChar)(StgWord)(UNTAG_CLOSURE(p)->payload[0]);
}

HsInt
rts_getInt (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == Izh_con_info ||
    //        p->header.info == Izh_static_info);
    return *(HsInt *)(UNTAG_CLOSURE(p)->payload);
}

HsInt8
rts_getInt8 (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == I8zh_con_info ||
    //        p->header.info == I8zh_static_info);
    return *(HsInt8 *)(UNTAG_CLOSURE(p)->payload);
}

HsInt16
rts_getInt16 (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == I16zh_con_info ||
    //        p->header.info == I16zh_static_info);
    return *(HsInt16 *)(UNTAG_CLOSURE(p)->payload);
}

HsInt32
rts_getInt32 (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == I32zh_con_info ||
    //        p->header.info == I32zh_static_info);
    return *(HsInt32 *)(UNTAG_CLOSURE(p)->payload);
}

HsInt64
rts_getInt64 (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == I64zh_con_info ||
    //        p->header.info == I64zh_static_info);
    return PK_Int64((P_)&(UNTAG_CLOSURE(p)->payload[0]));
}

HsWord
rts_getWord (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == Wzh_con_info ||
    //        p->header.info == Wzh_static_info);
    return *(HsWord *)(UNTAG_CLOSURE(p)->payload);
}

HsWord8
rts_getWord8 (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == W8zh_con_info ||
    //        p->header.info == W8zh_static_info);
    return *(HsWord8 *)(UNTAG_CLOSURE(p)->payload);
}

HsWord16
rts_getWord16 (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == W16zh_con_info ||
    //        p->header.info == W16zh_static_info);
    return *(HsWord16 *)(UNTAG_CLOSURE(p)->payload);
}

HsWord32
rts_getWord32 (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == W32zh_con_info ||
    //        p->header.info == W32zh_static_info);
    return *(HsWord32 *)(UNTAG_CLOSURE(p)->payload);
}

HsWord64
rts_getWord64 (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == W64zh_con_info ||
    //        p->header.info == W64zh_static_info);
    return PK_Word64((P_)&(UNTAG_CLOSURE(p)->payload[0]));
}

HsFloat
rts_getFloat (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == Fzh_con_info ||
    //        p->header.info == Fzh_static_info);
    return (float)(PK_FLT((P_)UNTAG_CLOSURE(p)->payload));
}

HsDouble
rts_getDouble (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == Dzh_con_info ||
    //        p->header.info == Dzh_static_info);
    return (double)(PK_DBL((P_)UNTAG_CLOSURE(p)->payload));
}

HsStablePtr
rts_getStablePtr (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == StablePtr_con_info ||
    //        p->header.info == StablePtr_static_info);
    return (StgStablePtr)(UNTAG_CLOSURE(p)->payload[0]);
}

HsPtr
rts_getPtr (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == Ptr_con_info ||
    //        p->header.info == Ptr_static_info);
    return (Capability *)(UNTAG_CLOSURE(p)->payload[0]);
}

HsFunPtr
rts_getFunPtr (HaskellObj p)
{
    // See comment above:
    // ASSERT(p->header.info == FunPtr_con_info ||
    //        p->header.info == FunPtr_static_info);
    return (void *)(UNTAG_CLOSURE(p)->payload[0]);
}

HsBool
rts_getBool (HaskellObj p)
{
    const StgWord tag = GET_CLOSURE_TAG(p);
    if (tag > 0) {
        return tag - 1;
    }

    const StgInfoTable *info;

    info = get_itbl((const StgClosure *)UNTAG_CONST_CLOSURE(p));
    if (info->srt == 0) { // srt is the constructor tag
        return 0;
    } else {
        return 1;
    }
}

/* -----------------------------------------------------------------------------
   Creating threads
   -------------------------------------------------------------------------- */

INLINE_HEADER void pushClosure   (StgTSO *tso, StgWord c) {
  tso->stackobj->sp--;
  tso->stackobj->sp[0] = (W_) c;
}

StgTSO *
createGenThread (Capability *cap, W_ stack_size,  StgClosure *closure)
{
  StgTSO *t;
  t = createThread (cap, stack_size);
  pushClosure(t, (W_)closure);
  pushClosure(t, (W_)&stg_enter_info);
  return t;
}

StgTSO *
createIOThread (Capability *cap, W_ stack_size,  StgClosure *closure)
{
  StgTSO *t;
  t = createThread (cap, stack_size);
  pushClosure(t, (W_)&stg_ap_v_info);
  pushClosure(t, (W_)closure);
  pushClosure(t, (W_)&stg_enter_info);
  return t;
}

/*
 * Same as above, but also evaluate the result of the IO action
 * to whnf while we're at it.
 */

StgTSO *
createStrictIOThread(Capability *cap, W_ stack_size,  StgClosure *closure)
{
  StgTSO *t;
  t = createThread(cap, stack_size);
  pushClosure(t, (W_)&stg_forceIO_info);
  pushClosure(t, (W_)&stg_ap_v_info);
  pushClosure(t, (W_)closure);
  pushClosure(t, (W_)&stg_enter_info);
  return t;
}

/* ----------------------------------------------------------------------------
   Evaluating Haskell expressions

   The running task (capability->running_task) must be bounded i.e. you must
   call newBoundTask() before calling these functions. Note that rts_lock() and
   rts_pause() both call newBoundTask().
   ------------------------------------------------------------------------- */

void rts_eval (/* inout */ Capability **cap,
               /* in    */ HaskellObj p,
               /* out */   HaskellObj *ret)
{
    StgTSO *tso;

    tso = createGenThread(*cap, RtsFlags.GcFlags.initialStkSize, p);
    scheduleWaitThread(tso,ret,cap);
}

void rts_eval_ (/* inout */ Capability **cap,
                /* in    */ HaskellObj p,
                /* in    */ unsigned int stack_size,
                /* out   */ HaskellObj *ret)
{
    StgTSO *tso;

    tso = createGenThread(*cap, stack_size, p);
    scheduleWaitThread(tso,ret,cap);
}

/*
 * rts_evalIO() evaluates a value of the form (IO a), forcing the action's
 * result to WHNF before returning.
 */
void rts_evalIO (/* inout */ Capability **cap,
                 /* in    */ HaskellObj p,
                 /* out */   HaskellObj *ret)
{
    StgTSO* tso;

    tso = createStrictIOThread(*cap, RtsFlags.GcFlags.initialStkSize, p);
    scheduleWaitThread(tso,ret,cap);
}

/*
 * rts_inCall() is similar to rts_evalIO, but expects to be called as an incall,
 * and is not expected to be called by user code directly.
 */
void rts_inCall (/* inout */ Capability **cap,
                 /* in    */ HaskellObj p,
                 /* out */   HaskellObj *ret)
{
    StgTSO* tso;

    tso = createStrictIOThread(*cap, RtsFlags.GcFlags.initialStkSize, p);
    if ((*cap)->running_task->preferred_capability != -1) {
        // enabled_capabilities should not change between here and waitCapability()
        ASSERT((*cap)->no == ((*cap)->running_task->preferred_capability % enabled_capabilities));
        // we requested explicit affinity; don't move this thread from now on.
        tso->flags |= TSO_LOCKED;
    }
    scheduleWaitThread(tso,ret,cap);
}

/*
 * rts_evalStableIOMain() is suitable for calling main Haskell thread
 * stored in (StablePtr (IO a)) it calls rts_evalStableIO but wraps
 * function in GHC.TopHandler.runMainIO that installs top_handlers.
 * See #12903.
 */
void rts_evalStableIOMain(/* inout */ Capability **cap,
                          /* in    */ HsStablePtr s,
                          /* out   */ HsStablePtr *ret)
{
    StgTSO* tso;
    StgClosure *p, *r, *w;
    SchedulerStatus stat;

    p = (StgClosure *)deRefStablePtr(s);
    w = rts_apply(*cap, &base_GHCziTopHandler_runMainIO_closure, p);
    tso = createStrictIOThread(*cap, RtsFlags.GcFlags.initialStkSize, w);
    // async exceptions are always blocked by default in the created
    // thread.  See #1048.
    tso->flags |= TSO_BLOCKEX | TSO_INTERRUPTIBLE;
    scheduleWaitThread(tso,&r,cap);
    stat = rts_getSchedStatus(*cap);

    if (stat == Success && ret != NULL) {
        ASSERT(r != NULL);
        *ret = getStablePtr((StgPtr)r);
    }
}

/*
 * rts_evalStableIO() is suitable for calling from Haskell.  It
 * evaluates a value of the form (StablePtr (IO a)), forcing the
 * action's result to WHNF before returning.  The result is returned
 * in a StablePtr.
 */
void rts_evalStableIO (/* inout */ Capability **cap,
                       /* in    */ HsStablePtr s,
                       /* out */   HsStablePtr *ret)
{
    StgTSO* tso;
    StgClosure *p, *r;
    SchedulerStatus stat;

    p = (StgClosure *)deRefStablePtr(s);
    tso = createStrictIOThread(*cap, RtsFlags.GcFlags.initialStkSize, p);
    // async exceptions are always blocked by default in the created
    // thread.  See #1048.
    tso->flags |= TSO_BLOCKEX | TSO_INTERRUPTIBLE;
    scheduleWaitThread(tso,&r,cap);
    stat = rts_getSchedStatus(*cap);

    if (stat == Success && ret != NULL) {
        ASSERT(r != NULL);
        *ret = getStablePtr((StgPtr)r);
    }
}

/*
 * Like rts_evalIO(), but doesn't force the action's result.
 */
void rts_evalLazyIO (/* inout */ Capability **cap,
                     /* in    */ HaskellObj p,
                     /* out */   HaskellObj *ret)
{
    StgTSO *tso;

    tso = createIOThread(*cap, RtsFlags.GcFlags.initialStkSize, p);
    scheduleWaitThread(tso,ret,cap);
}

void rts_evalLazyIO_ (/* inout */ Capability **cap,
                      /* in    */ HaskellObj p,
                      /* in    */ unsigned int stack_size,
                      /* out   */ HaskellObj *ret)
{
    StgTSO *tso;

    tso = createIOThread(*cap, stack_size, p);
    scheduleWaitThread(tso,ret,cap);
}

/* Convenience function for decoding the returned status. */

void
rts_checkSchedStatus (char* site, Capability *cap)
{
    SchedulerStatus rc = cap->running_task->incall->rstat;
    switch (rc) {
    case Success:
        return;
    case Killed:
        errorBelch("%s: uncaught exception",site);
        stg_exit(EXIT_FAILURE);
    case Interrupted:
        errorBelch("%s: interrupted", site);
#if defined(THREADED_RTS)
        // The RTS is shutting down, and the process will probably
        // soon exit.  We don't want to preempt the shutdown
        // by exiting the whole process here, so we just terminate the
        // current thread.  Don't forget to release the cap first though.
        rts_unlock(cap);
        shutdownThread();
#else
        stg_exit(EXIT_FAILURE);
#endif
    default:
        errorBelch("%s: Return code (%d) not ok",(site),(rc));
        stg_exit(EXIT_FAILURE);
    }
}

SchedulerStatus
rts_getSchedStatus (Capability *cap)
{
    return cap->running_task->incall->rstat;
}

#if defined(THREADED_RTS)
// The task that paused the RTS. The rts_pausing_task variable is owned by the
// task that owns all capabilities (there is at most one such task).
//
// It's possible to remove this and instead define the pausing task as whichever
// task owns all capabilities, but using `rts_pausing_task` leads to marginally
// cleaner code/API and better error messages.
Task * rts_pausing_task = NULL;
#endif

Capability *
rts_lock (void)
{
    Capability *cap;
    Task *task;

    // Bound the current task. This is necessary to support rts_eval* functions.
    task = newBoundTask();

    if (task->running_finalizers) {
        errorBelch("error: a C finalizer called back into Haskell.\n"
                   "   This was previously allowed, but is disallowed in GHC 6.10.2 and later.\n"
                   "   To create finalizers that may call back into Haskell, use\n"
                   "   Foreign.Concurrent.newForeignPtr instead of Foreign.newForeignPtr.");
        stg_exit(EXIT_FAILURE);
    }

#if defined(THREADED_RTS)
    if (rts_pausing_task == task) {
        errorBelch("error: rts_lock: The RTS is already paused by this thread.\n"
                   "   There is no need to call rts_lock if you have already called rts_pause.");
        stg_exit(EXIT_FAILURE);
    }
#endif

    cap = NULL;
    waitForCapability(&cap, task);

    if (task->incall->prev_stack == NULL) {
      // This is a new outermost call from C into Haskell land.
      // Until the corresponding call to rts_unlock, this task
      // is doing work on behalf of the RTS.
      traceTaskCreate(task, cap);
    }

    return (Capability *)cap;
}

// Exiting the RTS: we hold a Capability that is not necessarily the
// same one that was originally returned by rts_lock(), because
// rts_evalIO() etc. may return a new one.  Now that we have
// investigated the return value, we can release the Capability,
// and free the Task (in that order).

void
rts_unlock (Capability *cap)
{
    Task *task;

    task = cap->running_task;
    ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);

    // Now release the Capability. With the capability released, GC
    // may happen. NB. does not try to put the current Task on the
    // worker queue.
    // NB. keep cap->lock held while we call exitMyTask(). This
    // is necessary during shutdown, where we want the invariant that
    // after shutdownCapability(), all the Tasks associated with the
    // Capability have completed their shutdown too. Otherwise we
    // could have exitMyTask()/workerTaskStop() running at some
    // random point in the future, which causes problems for
    // freeTaskManager().
    ACQUIRE_LOCK(&cap->lock);
    releaseCapability_(cap,false);

    // Finally, we can release the Task to the free list.
    exitMyTask();
    RELEASE_LOCK(&cap->lock);

    if (task->incall == NULL) {
      // This is the end of an outermost call from C into Haskell land.
      // From here on, the task goes back to C land and we should not count
      // it as doing work on behalf of the RTS.
      traceTaskDelete(task);
    }
}

struct PauseToken_ {
    Capability *capability;
};

Capability *pauseTokenCapability(PauseToken *pauseToken) {
    return pauseToken->capability;
}

#if defined(THREADED_RTS)

// See Note [Locking and Pausing the RTS]
PauseToken *rts_pause (void)
{

    // Wait for any nonmoving collection to finish before pausing the RTS.
    // The nonmoving collector needs to synchronise with the mutator,
    // so pausing the mutator while a collection is ongoing might lead to deadlock or
    // capabilities being prematurely re-awoken.
    if (RtsFlags.GcFlags.useNonmoving) {
      ACQUIRE_LOCK(&nonmoving_collection_mutex);
    }


    // It is an error if this thread already paused the RTS. If another
    // thread has paused the RTS, then rts_pause will block until rts_resume is
    // called (and compete with other threads calling rts_pause). The blocking
    // behavior is implied by the use of `stopAllCapabilities`.
    Task * task = getMyTask();
    if (rts_pausing_task == task)
    {
        // This task already passed the RTS.
        errorBelch("error: rts_pause: This thread has already paused the RTS.");
        stg_exit(EXIT_FAILURE);
    }

    // The current task must not own a capability. This is true for non-worker
    // threads e.g. when making a safe FFI call. We allow pausing when
    // `task->cap->running_task != task` because the capability can be taken by
    // other capabilities. Doing this check is justified because rts_pause is a
    // user facing function and we want good error reporting. We also don't
    // expect rts_pause to be performance critical.
    //
    // N.B. we use a relaxed load since there is no easy way to synchronize
    // here and this check is ultimately just a convenience for the user..
    if (task->cap && RELAXED_LOAD(&task->cap->running_task) == task)
    {
        // This task owns a capability (and it can't be taken by other capabilities).
        errorBelch(task->cap->in_haskell
            ? ("error: rts_pause: attempting to pause via an unsafe FFI call.\n"
               "   Perhaps a 'foreign import unsafe' should be 'safe'?")
            : ("error: rts_pause: attempting to pause from a Task that owns a capability.\n"
               "   Have you already acquired a capability e.g. with rts_lock?"));
        stg_exit(EXIT_FAILURE);
    }

    // Bound the current task. This is necessary to support rts_eval* functions.
    task = newBoundTask();
    stopAllCapabilities(NULL, task);

    // Now we own all capabilities so we own rts_pausing_task and may set it.
    rts_pausing_task = task;

    PauseToken *token = stgMallocBytes(sizeof(PauseToken), "rts_pause");
    token->capability = task->cap;
    return token;
}

static void assert_isPausedOnMyTask(const char *functionName);

// See Note [Locking and Pausing the RTS]. The pauseToken argument is here just
// for symmetry with rts_pause and to match the pattern of rts_lock/rts_unlock.
void rts_resume (PauseToken *pauseToken)
{
    assert_isPausedOnMyTask("rts_resume");
    Task * task = getMyTask();

    // Now we own all capabilities so we own rts_pausing_task and may write to
    // it.
    rts_pausing_task = NULL;

    // releaseAllCapabilities will not block because the current task owns all
    // capabilities.
    releaseAllCapabilities(getNumCapabilities(), NULL, task);
    exitMyTask();
    stgFree(pauseToken);

    if (RtsFlags.GcFlags.useNonmoving) {
      RELEASE_LOCK(&nonmoving_collection_mutex);
    }
}

// See RtsAPI.h
bool rts_isPaused(void)
{
    return rts_pausing_task != NULL;
}

// Check that the rts_pause was called on this thread/task and this thread owns
// all capabilities. If not, outputs an error and exits with EXIT_FAILURE.
static void assert_isPausedOnMyTask(const char *functionName)
{
    Task * task = getMyTask();
    if (rts_pausing_task == NULL)
    {
        errorBelch (
            "error: %s: the rts is not paused. Did you forget to call rts_pause?",
            functionName);
        stg_exit(EXIT_FAILURE);
    }

    if (task != rts_pausing_task)
    {
        // We don't have ownership of rts_pausing_task, so it may have changed
        // just after the above read. Still, we are guaranteed that
        // rts_pausing_task won't be set to the current task (because the
        // current task is here now!), so the error messages are still correct.
        errorBelch (
            "error: %s: called from a different OS thread than rts_pause.",
            functionName);

        stg_exit(EXIT_FAILURE);
    }

    // Check that we own all capabilities.
    for (unsigned int i = 0; i < getNumCapabilities(); i++)
    {
        Capability *cap = getCapability(i);
        if (cap->running_task != task)
        {
            errorBelch (
                "error: %s: the pausing thread does not own all capabilities.\n"
                "   Have you manually released a capability after calling rts_pause?",
                functionName);
            stg_exit(EXIT_FAILURE);
        }
    }
}

// See RtsAPI.h
void rts_listThreads(ListThreadsCb cb, void *user)
{
    assert_isPausedOnMyTask("rts_listThreads");

    // The rts is paused and can only be resumed by the current thread. Hence it
    // is safe to read global thread data.

    for (uint32_t g=0; g < RtsFlags.GcFlags.generations; g++) {
        StgTSO *tso = generations[g].threads;
        while (tso != END_TSO_QUEUE) {
            cb(user, tso);
            tso = tso->global_link;
        }
    }
}

struct list_roots_ctx {
    ListRootsCb cb;
    void *user;
};

// This is an evac_fn.
static void list_roots_helper(void *user, StgClosure **p) {
    struct list_roots_ctx *ctx = (struct list_roots_ctx *) user;
    ctx->cb(ctx->user, *p);
}

// See RtsAPI.h
void rts_listMiscRoots (ListRootsCb cb, void *user)
{
    assert_isPausedOnMyTask("rts_listMiscRoots");

    struct list_roots_ctx ctx;
    ctx.cb = cb;
    ctx.user = user;

    threadStableNameTable(&list_roots_helper, (void *)&ctx);
    threadStablePtrTable(&list_roots_helper, (void *)&ctx);
}

#else
PauseToken STG_NORETURN
*rts_pause (void)
{
    errorBelch("Warning: Pausing the RTS is only possible for "
               "multithreaded RTS.");
    stg_exit(EXIT_FAILURE);
}

void STG_NORETURN
rts_resume (PauseToken *pauseToken STG_UNUSED)
{
    errorBelch("Warning: Resuming the RTS is only possible for "
               "multithreaded RTS.");
    stg_exit(EXIT_FAILURE);
}

bool rts_isPaused()
{
    errorBelch("Warning: Pausing/Resuming the RTS is only possible for "
               "multithreaded RTS.");
    return false;
}

// See RtsAPI.h
void rts_listThreads(ListThreadsCb cb STG_UNUSED, void *user STG_UNUSED)
{
    errorBelch("Warning: rts_listThreads is only possible for multithreaded RTS.");
}

// See RtsAPI.h
void rts_listMiscRoots (ListRootsCb cb STG_UNUSED, void *user STG_UNUSED)
{
    errorBelch("Warning: rts_listMiscRoots is only possible for multithreaded RTS.");
}
#endif

void rts_done (void)
{
    freeMyTask();
}

/* -----------------------------------------------------------------------------
   tryPutMVar from outside Haskell

   The C call

      hs_try_putmvar(cap, mvar)

   is equivalent to the Haskell call

      tryPutMVar mvar ()

   but it is

     * non-blocking: takes a bounded, short, amount of time
     * asynchronous: the actual putMVar may be performed after the
       call returns.  That's why hs_try_putmvar() doesn't return a
       result to say whether the put succeeded.

   NOTE: this call transfers ownership of the StablePtr to the RTS, which will
   free it after the tryPutMVar has taken place.  The reason is that otherwise,
   it would be very difficult for the caller to arrange to free the StablePtr
   in all circumstances.

   For more details, see the section "Waking up Haskell threads from C" in the
   User's Guide.
   -------------------------------------------------------------------------- */

void hs_try_putmvar (/* in */ int capability,
                     /* in */ HsStablePtr mvar)
{
    Task *task = getMyTask();
    Capability *cap;
    Capability *task_old_cap USED_IF_THREADS;

    if (capability < 0) {
        capability = task->preferred_capability;
        if (capability < 0) {
            capability = 0;
        }
    }
    cap = getCapability(capability % enabled_capabilities);

#if !defined(THREADED_RTS)

    performTryPutMVar(cap, (StgMVar*)deRefStablePtr(mvar), Unit_closure);
    freeStablePtr(mvar);

#else

    ACQUIRE_LOCK(&cap->lock);
    // If the capability is free, we can perform the tryPutMVar immediately
    if (cap->running_task == NULL) {
        cap->running_task = task;
        task_old_cap = task->cap;
        task->cap = cap;
        RELEASE_LOCK(&cap->lock);

        performTryPutMVar(cap, (StgMVar*)deRefStablePtr(mvar), Unit_closure);

        freeStablePtr(mvar);

        // Wake up the capability, which will start running the thread that we
        // just awoke (if there was one).
        releaseCapability(cap);
        task->cap = task_old_cap;
    } else {
        PutMVar *p = stgMallocBytes(sizeof(PutMVar),"hs_try_putmvar");
        // We cannot deref the StablePtr if we don't have a capability,
        // so we have to store it and deref it later.
        p->mvar = mvar;
        p->link = cap->putMVars;
        cap->putMVars = p;
        RELEASE_LOCK(&cap->lock);
    }

#endif
}