summaryrefslogtreecommitdiff
path: root/rts/RaiseAsync.c
blob: b668b6a17809d269af355e19df69dc4c74263ca0 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
/* ---------------------------------------------------------------------------
 *
 * (c) The GHC Team, 1998-2006
 *
 * Asynchronous exceptions
 *
 * --------------------------------------------------------------------------*/

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

#include "sm/Storage.h"
#include "Threads.h"
#include "Trace.h"
#include "RaiseAsync.h"
#include "Schedule.h"
#include "Updates.h"
#include "STM.h"
#include "sm/Sanity.h"
#include "Profiling.h"
#include "Messages.h"
#if defined(mingw32_HOST_OS)
#include "win32/MIOManager.h"
#endif

static void blockedThrowTo (Capability *cap,
                            StgTSO *target, MessageThrowTo *msg);

static void removeFromQueues(Capability *cap, StgTSO *tso);

static void removeFromMVarBlockedQueue (StgTSO *tso);

static void throwToSendMsg (Capability *cap USED_IF_THREADS,
                            Capability *target_cap USED_IF_THREADS,
                            MessageThrowTo *msg USED_IF_THREADS);

/* -----------------------------------------------------------------------------
   throwToSingleThreaded

   This version of throwTo is safe to use if and only if one of the
   following holds:

     - !THREADED_RTS

     - all the other threads in the system are stopped (eg. during GC).

     - we surely own the target TSO (eg. we just took it from the
       run queue of the current capability, or we are running it).

   It doesn't cater for blocking the source thread until the exception
   has been raised.
   -------------------------------------------------------------------------- */

static void
throwToSingleThreaded__ (Capability *cap, StgTSO *tso, StgClosure *exception,
                         bool stop_at_atomically, StgUpdateFrame *stop_here)
{
    // Thread already dead?
    if (tso->what_next == ThreadComplete || tso->what_next == ThreadKilled) {
        return;
    }

    // Remove it from any blocking queues
    removeFromQueues(cap,tso);

    raiseAsync(cap, tso, exception, stop_at_atomically, stop_here);
}

void
throwToSingleThreaded (Capability *cap, StgTSO *tso, StgClosure *exception)
{
    throwToSingleThreaded__(cap, tso, exception, false, NULL);
}

void
throwToSingleThreaded_ (Capability *cap, StgTSO *tso, StgClosure *exception,
                        bool stop_at_atomically)
{
    throwToSingleThreaded__ (cap, tso, exception, stop_at_atomically, NULL);
}

void // cannot return a different TSO
suspendComputation (Capability *cap, StgTSO *tso, StgUpdateFrame *stop_here)
{
    throwToSingleThreaded__ (cap, tso, NULL, false, stop_here);
}

/* -----------------------------------------------------------------------------
   throwToSelf

   Useful for throwing an async exception in a thread from the
   runtime.  It handles unlocking the throwto message returned by
   throwTo().

   Note [Throw to self when masked]
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   When a StackOverflow occurs when the thread is masked, we want to
   defer the exception to when the thread becomes unmasked/hits an
   interruptible point.  We already have a mechanism for doing this,
   the blocked_exceptions list, but the use here is a bit unusual,
   because an exception is normally only added to this list upon
   an asynchronous 'throwTo' call (with all of the relevant
   multithreaded nonsense). Morally, a stack overflow should be an
   asynchronous exception sent by a thread to itself, and it should
   have the same semantics.  But there are a few key differences:

   - If you actually tried to send an asynchronous exception to
     yourself using throwTo, the exception would actually immediately
     be delivered.  This is because throwTo itself is considered an
     interruptible point, so the exception is always deliverable. Thus,
     ordinarily, we never end up with a message to oneself in the
     blocked_exceptions queue.

   - In the case of a StackOverflow, we don't actually care about the
     wakeup semantics; when an exception is delivered, the thread that
     originally threw the exception should be woken up, since throwTo
     blocks until the exception is successfully thrown.  Fortunately,
     it is harmless to wakeup a thread that doesn't actually need waking
     up, e.g. ourselves.

   - No synchronization is necessary, because we own the TSO and the
     capability.  You can observe this by tracing through the execution
     of throwTo.  We skip synchronizing the message and inter-capability
     communication.

   We think this doesn't break any invariants, but do be careful!
   -------------------------------------------------------------------------- */

void
throwToSelf (Capability *cap, StgTSO *tso, StgClosure *exception)
{
    MessageThrowTo *m;

    m = throwTo(cap, tso, tso, exception);

    if (m != NULL) {
        // throwTo leaves it locked
        unlockClosure((StgClosure*)m, &stg_MSG_THROWTO_info);
    }
}

/* -----------------------------------------------------------------------------
   throwTo

   This function may be used to throw an exception from one thread to
   another, during the course of normal execution.  This is a tricky
   task: the target thread might be running on another CPU, or it
   may be blocked and could be woken up at any point by another CPU.
   We have some delicate synchronisation to do.

   The underlying scheme when multiple Capabilities are in use is
   message passing: when the target of a throwTo is on another
   Capability, we send a message (a MessageThrowTo closure) to that
   Capability.

   If the throwTo needs to block because the target TSO is masking
   exceptions (the TSO_BLOCKEX flag), then the message is placed on
   the blocked_exceptions queue attached to the target TSO.  When the
   target TSO enters the unmasked state again, it must check the
   queue.  The blocked_exceptions queue is not locked; only the
   Capability owning the TSO may modify it.

   To make things simpler for throwTo, we always create the message
   first before deciding what to do.  The message may get sent, or it
   may get attached to a TSO's blocked_exceptions queue, or the
   exception may get thrown immediately and the message dropped,
   depending on the current state of the target.

   Currently we send a message if the target belongs to another
   Capability, and it is

     - NotBlocked, BlockedOnMsgThrowTo,
       BlockedOnCCall_Interruptible

     - or it is masking exceptions (TSO_BLOCKEX)

   Currently, if the target is BlockedOnMVar, BlockedOnSTM,
   or BlockedOnBlackHole then we acquire ownership of the
   TSO by locking its parent container (e.g. the MVar) and then raise the
   exception.  We might change these cases to be more message-passing-like in
   the future.

   Returns:

   NULL               exception was raised, ok to continue

   MessageThrowTo *   exception was not raised; the source TSO
                      should now put itself in the state
                      BlockedOnMsgThrowTo, and when it is ready
                      it should unlock the message using
                      unlockClosure(msg, &stg_MSG_THROWTO_info);
                      If it decides not to raise the exception after
                      all, it can revoke it safely with
                      unlockClosure(msg, &stg_MSG_NULL_info);

   -------------------------------------------------------------------------- */

MessageThrowTo *
throwTo (Capability *cap,       // the Capability we hold
         StgTSO *source,        // the TSO sending the exception (or NULL)
         StgTSO *target,        // the TSO receiving the exception
         StgClosure *exception) // the exception closure
{
    MessageThrowTo *msg;

    msg = (MessageThrowTo *) allocate(cap, sizeofW(MessageThrowTo));
    // the message starts locked; see below
    SET_HDR(msg, &stg_WHITEHOLE_info, CCS_SYSTEM);
    msg->source      = source;
    msg->target      = target;
    msg->exception   = exception;

    switch (throwToMsg(cap, msg))
    {
    case THROWTO_SUCCESS:
        // unlock the message now, otherwise we leave a WHITEHOLE in
        // the heap (#6103)
        SET_HDR(msg, &stg_MSG_THROWTO_info, CCS_SYSTEM);
        return NULL;

    case THROWTO_BLOCKED:
    default:
        // the caller will unlock the message when it is ready.  We
        // cannot unlock it yet, because the calling thread will need
        // to tidy up its state first.
        return msg;
    }
}


uint32_t
throwToMsg (Capability *cap, MessageThrowTo *msg)
{
    StgWord status;
    StgTSO *target = ACQUIRE_LOAD(&msg->target);
    Capability *target_cap;

    goto check_target;

retry:
    write_barrier();
    debugTrace(DEBUG_sched, "throwTo: retrying...");

check_target:
    ASSERT(target != END_TSO_QUEUE);

    // Thread already dead?
    StgWord16 what_next = SEQ_CST_LOAD(&target->what_next);
    if (what_next == ThreadComplete
        || what_next == ThreadKilled) {
        return THROWTO_SUCCESS;
    }

    debugTraceCap(DEBUG_sched, cap,
                  "throwTo: from thread %lu to thread %lu",
                  (unsigned long)msg->source->id,
                  (unsigned long)msg->target->id);

#if defined(DEBUG)
    traceThreadStatus(DEBUG_sched, target);
#endif

    target_cap = target->cap;
    if (target->cap != cap) {
        throwToSendMsg(cap, target_cap, msg);
        return THROWTO_BLOCKED;
    }

    status = target->why_blocked;

    switch (status) {
    case NotBlocked:
    {
        if ((target->flags & TSO_BLOCKEX) == 0) {
            // It's on our run queue and not blocking exceptions
            raiseAsync(cap, target, msg->exception, false, NULL);
            return THROWTO_SUCCESS;
        } else {
            blockedThrowTo(cap,target,msg);
            return THROWTO_BLOCKED;
        }
    }

    case BlockedOnMsgThrowTo:
    {
        const StgInfoTable *i;
        MessageThrowTo *m;

        m = target->block_info.throwto;

        // target is local to this cap, but has sent a throwto
        // message to another cap.
        //
        // The source message is locked.  We need to revoke the
        // target's message so that we can raise the exception, so
        // we attempt to lock it.

        // There's a possibility of a deadlock if two threads are both
        // trying to throwTo each other (or more generally, a cycle of
        // threads).  To break the symmetry we compare the addresses
        // of the MessageThrowTo objects, and the one for which m <
        // msg gets to spin, while the other can only try to lock
        // once, but must then back off and unlock both before trying
        // again.
        if (m < msg) {
            i = lockClosure((StgClosure *)m);
        } else {
            i = tryLockClosure((StgClosure *)m);
            if (i == NULL) {
//            debugBelch("collision\n");
                throwToSendMsg(cap, target->cap, msg);
                return THROWTO_BLOCKED;
            }
        }

        if (i == &stg_MSG_NULL_info) {
            // we know there's a MSG_TRY_WAKEUP on the way, so we
            // might as well just do it now.  The message will
            // be a no-op when it arrives.
            unlockClosure((StgClosure*)m, i);
            tryWakeupThread(cap, target);
            goto retry;
        }

        if (i != &stg_MSG_THROWTO_info) {
            // if it's a MSG_NULL, this TSO has been woken up by another Cap
            unlockClosure((StgClosure*)m, i);
            goto retry;
        }

        if ((target->flags & TSO_BLOCKEX) &&
            ((target->flags & TSO_INTERRUPTIBLE) == 0)) {
            unlockClosure((StgClosure*)m, i);
            blockedThrowTo(cap,target,msg);
            return THROWTO_BLOCKED;
        }

        // nobody else can wake up this TSO after we claim the message
        doneWithMsgThrowTo(cap, m);

        raiseAsync(cap, target, msg->exception, false, NULL);
        return THROWTO_SUCCESS;
    }

    case BlockedOnMVar:
    case BlockedOnMVarRead:
    {
        /*
          To establish ownership of this TSO, we need to acquire a
          lock on the MVar that it is blocked on.
        */
        StgMVar *mvar;
        StgInfoTable *info USED_IF_THREADS;

        mvar = (StgMVar *)target->block_info.closure;

        // ASSUMPTION: tso->block_info must always point to a
        // closure.  In the threaded RTS it does.
        switch (get_itbl((StgClosure *)mvar)->type) {
        case MVAR_CLEAN:
        case MVAR_DIRTY:
            break;
        default:
            goto retry;
        }

        info = lockClosure((StgClosure *)mvar);

        // we have the MVar, let's check whether the thread
        // is still blocked on the same MVar.
        if ((target->why_blocked != BlockedOnMVar
             && target->why_blocked != BlockedOnMVarRead)
            || (StgMVar *)target->block_info.closure != mvar) {
            unlockClosure((StgClosure *)mvar, info);
            goto retry;
        }

        if (target->_link == END_TSO_QUEUE) {
            // the MVar operation has already completed.  There is a
            // MSG_TRY_WAKEUP on the way, but we can just wake up the
            // thread now anyway and ignore the message when it
            // arrives.
            unlockClosure((StgClosure *)mvar, info);
            tryWakeupThread(cap, target);
            goto retry;
        }

        if ((target->flags & TSO_BLOCKEX) &&
            ((target->flags & TSO_INTERRUPTIBLE) == 0)) {
            blockedThrowTo(cap,target,msg);
            unlockClosure((StgClosure *)mvar, info);
            return THROWTO_BLOCKED;
        } else {
            // revoke the MVar operation
            removeFromMVarBlockedQueue(target);
            raiseAsync(cap, target, msg->exception, false, NULL);
            unlockClosure((StgClosure *)mvar, info);
            return THROWTO_SUCCESS;
        }
    }

    case BlockedOnBlackHole:
    {
        if (target->flags & TSO_BLOCKEX) {
            // BlockedOnBlackHole is not interruptible.
            blockedThrowTo(cap,target,msg);
            return THROWTO_BLOCKED;
        } else {
            // Revoke the message by replacing it with IND. We're not
            // locking anything here, so we might still get a TRY_WAKEUP
            // message from the owner of the blackhole some time in the
            // future, but that doesn't matter.
            ASSERT(target->block_info.bh->header.info == &stg_MSG_BLACKHOLE_info);
            OVERWRITE_INFO(target->block_info.bh, &stg_IND_info);
            raiseAsync(cap, target, msg->exception, false, NULL);
            return THROWTO_SUCCESS;
        }
    }

    case BlockedOnSTM:
        if ((target->flags & TSO_BLOCKEX) &&
            ((target->flags & TSO_INTERRUPTIBLE) == 0)) {
            blockedThrowTo(cap,target,msg);
            return THROWTO_BLOCKED;
        } else {
            raiseAsync(cap, target, msg->exception, false, NULL);
            return THROWTO_SUCCESS;
        }

    case BlockedOnCCall_Interruptible:
#if defined(THREADED_RTS)
    {
        Task *task = NULL;
        // walk suspended_ccalls to find the correct worker thread
        InCall *incall;
        for (incall = cap->suspended_ccalls; incall != NULL; incall = incall->next) {
            if (incall->suspended_tso == target) {
                task = incall->task;
                break;
            }
        }
        if (task != NULL) {
            blockedThrowTo(cap, target, msg);
            if (!((target->flags & TSO_BLOCKEX) &&
                  ((target->flags & TSO_INTERRUPTIBLE) == 0))) {
                interruptWorkerTask(task);
            }
            return THROWTO_BLOCKED;
        } else {
            debugTraceCap(DEBUG_sched, cap, "throwTo: could not find worker thread to kill");
        }
        // fall to next
    }
    FALLTHROUGH;
#endif
    case BlockedOnCCall:
        blockedThrowTo(cap,target,msg);
        return THROWTO_BLOCKED;

#if !defined(THREADED_RTS)
    case BlockedOnRead:
    case BlockedOnWrite:
    case BlockedOnDelay:
#if defined(mingw32_HOST_OS)
    case BlockedOnDoProc:
#endif
        if ((target->flags & TSO_BLOCKEX) &&
            ((target->flags & TSO_INTERRUPTIBLE) == 0)) {
            blockedThrowTo(cap,target,msg);
            return THROWTO_BLOCKED;
        } else {
            removeFromQueues(cap,target);
            raiseAsync(cap, target, msg->exception, false, NULL);
            return THROWTO_SUCCESS;
        }
#endif

    case ThreadMigrating:
        // if it is ThreadMigrating and tso->cap is ours, then it
        // *must* be migrating *to* this capability.  If it were
        // migrating away from the capability, then tso->cap would
        // point to the destination.
        //
        // There is a MSG_WAKEUP in the message queue for this thread,
        // but we can just do it preemptively:
        tryWakeupThread(cap, target);
        // and now retry, the thread should be runnable.
        goto retry;

    default:
        barf("throwTo: unrecognised why_blocked (%d)", target->why_blocked);
    }
    barf("throwTo");
}

static void
throwToSendMsg (Capability *cap STG_UNUSED,
                Capability *target_cap USED_IF_THREADS,
                MessageThrowTo *msg USED_IF_THREADS)

{
#if defined(THREADED_RTS)
    debugTraceCap(DEBUG_sched, cap, "throwTo: sending a throwto message to cap %lu", (unsigned long)target_cap->no);

    sendMessage(cap, target_cap, (Message*)msg);
#endif
}

// Block a throwTo message on the target TSO's blocked_exceptions
// queue.  The current Capability must own the target TSO in order to
// modify the blocked_exceptions queue.
void
blockedThrowTo (Capability *cap, StgTSO *target, MessageThrowTo *msg)
{
    debugTraceCap(DEBUG_sched, cap, "throwTo: blocking on thread %lu",
                  (unsigned long)target->id);

    ASSERT(target->cap == cap);

    dirty_TSO(cap,target); // we will modify the blocked_exceptions queue
    msg->link = target->blocked_exceptions;
    target->blocked_exceptions = msg;
}

/* -----------------------------------------------------------------------------
   Waking up threads blocked in throwTo

   There are two ways to do this: maybePerformBlockedException() will
   perform the throwTo() for the thread at the head of the queue
   immediately, and leave the other threads on the queue.
   maybePerformBlockedException() also checks the TSO_BLOCKEX flag
   before raising an exception.

   awakenBlockedExceptionQueue() will wake up all the threads in the
   queue, but not perform any throwTo() immediately.  This might be
   more appropriate when the target thread is the one actually running
   (see Exception.cmm).

   Returns: non-zero if an exception was raised, zero otherwise.
   -------------------------------------------------------------------------- */

int
maybePerformBlockedException (Capability *cap, StgTSO *tso)
{
    MessageThrowTo *msg;
    const StgInfoTable *i;
    StgTSO *source;

    if (tso->what_next == ThreadComplete || tso->what_next == ThreadFinished) {
        if (tso->blocked_exceptions != END_BLOCKED_EXCEPTIONS_QUEUE) {
            awakenBlockedExceptionQueue(cap,tso);
            return 1;
        } else {
            return 0;
        }
    }

    if (tso->blocked_exceptions != END_BLOCKED_EXCEPTIONS_QUEUE &&
        (tso->flags & TSO_BLOCKEX) != 0) {
        debugTraceCap(DEBUG_sched, cap, "throwTo: thread %" FMT_StgThreadID
                      " has blocked exceptions but is inside block", tso->id);
    }

    if (tso->blocked_exceptions != END_BLOCKED_EXCEPTIONS_QUEUE
        && ((tso->flags & TSO_BLOCKEX) == 0
            || ((tso->flags & TSO_INTERRUPTIBLE) && interruptible(tso)))) {

        // We unblock just the first thread on the queue, and perform
        // its throw immediately.
    loop:
        msg = tso->blocked_exceptions;
        if (msg == END_BLOCKED_EXCEPTIONS_QUEUE) return 0;
        i = lockClosure((StgClosure*)msg);
        tso->blocked_exceptions = (MessageThrowTo*)msg->link;
        if (i == &stg_MSG_NULL_info) {
            unlockClosure((StgClosure*)msg,i);
            goto loop;
        }

        throwToSingleThreaded(cap, msg->target, msg->exception);
        source = msg->source;
        doneWithMsgThrowTo(cap, msg);
        tryWakeupThread(cap, source);
        return 1;
    }
    return 0;
}

// awakenBlockedExceptionQueue(): Just wake up the whole queue of
// blocked exceptions.

void
awakenBlockedExceptionQueue (Capability *cap, StgTSO *tso)
{
    MessageThrowTo *msg;
    const StgInfoTable *i;
    StgTSO *source;

    for (msg = tso->blocked_exceptions; msg != END_BLOCKED_EXCEPTIONS_QUEUE;
         msg = (MessageThrowTo*)msg->link) {
        i = lockClosure((StgClosure *)msg);
        if (i != &stg_MSG_NULL_info) {
            source = msg->source;
            doneWithMsgThrowTo(cap, msg);
            tryWakeupThread(cap, source);
        } else {
            unlockClosure((StgClosure *)msg,i);
        }
    }
    tso->blocked_exceptions = END_BLOCKED_EXCEPTIONS_QUEUE;
}

/* -----------------------------------------------------------------------------
   Remove a thread from blocking queues.

   This is for use when we raise an exception in another thread, which
   may be blocked.

   Precondition: we have exclusive access to the TSO, via the same set
   of conditions as throwToSingleThreaded() (c.f.).
   -------------------------------------------------------------------------- */

static void
removeFromMVarBlockedQueue (StgTSO *tso)
{
    StgMVar *mvar = (StgMVar*)tso->block_info.closure;
    StgMVarTSOQueue *q = (StgMVarTSOQueue*)tso->_link;

    if (q == (StgMVarTSOQueue*)END_TSO_QUEUE) {
        // already removed from this MVar
        return;
    }

    // Assume the MVar is locked. (not assertable; sometimes it isn't
    // actually WHITEHOLE'd).

    // We want to remove the MVAR_TSO_QUEUE object from the queue.  It
    // isn't doubly-linked so we can't actually remove it; instead we
    // just overwrite it with an IND if possible and let the GC short
    // it out.  However, we have to be careful to maintain the deque
    // structure:

    if (mvar->head == q) {
        mvar->head = q->link;
        OVERWRITE_INFO(q, &stg_IND_info);
        if (mvar->tail == q) {
            mvar->tail = (StgMVarTSOQueue*)END_TSO_QUEUE;
        }
    }
    else if (mvar->tail == q) {
        // we can't replace it with an IND in this case, because then
        // we lose the tail pointer when the GC shorts out the IND.
        // So we use MSG_NULL as a kind of non-dupable indirection;
        // these are ignored by takeMVar/putMVar.
        OVERWRITE_INFO(q, &stg_MSG_NULL_info);
    }
    else {
        OVERWRITE_INFO(q, &stg_IND_info);
    }

    // revoke the MVar operation
    tso->_link = END_TSO_QUEUE;
}

static void
removeFromQueues(Capability *cap, StgTSO *tso)
{
  switch (tso->why_blocked) {

  case NotBlocked:
  case ThreadMigrating:
      return;

  case BlockedOnSTM:
    // Be careful: nothing to do here!  We tell the scheduler that the
    // thread is runnable and we leave it to the stack-walking code to
    // abort the transaction while unwinding the stack.  We should
    // perhaps have a debugging test to make sure that this really
    // happens and that the 'zombie' transaction does not get
    // committed.
    goto done;

  case BlockedOnMVar:
  case BlockedOnMVarRead:
      removeFromMVarBlockedQueue(tso);
      goto done;

  case BlockedOnBlackHole:
      // nothing to do
      goto done;

  case BlockedOnMsgThrowTo:
  {
      MessageThrowTo *m = tso->block_info.throwto;
      // The message is locked by us, unless we got here via
      // deleteAllThreads(), in which case we own all the
      // capabilities.
      // ASSERT(m->header.info == &stg_WHITEHOLE_info);

      // unlock and revoke it at the same time
      doneWithMsgThrowTo(cap, m);
      break;
  }

#if !defined(THREADED_RTS)
  case BlockedOnRead:
  case BlockedOnWrite:
#if defined(mingw32_HOST_OS)
  case BlockedOnDoProc:
#endif
      removeThreadFromDeQueue(cap, &blocked_queue_hd, &blocked_queue_tl, tso);
#if defined(mingw32_HOST_OS)
      /* (Cooperatively) signal that the worker thread should abort
       * the request.
       */
      abandonWorkRequest(tso->block_info.async_result->reqID);
#endif
      goto done;

  case BlockedOnDelay:
        removeThreadFromQueue(cap, &sleeping_queue, tso);
        goto done;
#endif

  default:
      barf("removeFromQueues: %d", tso->why_blocked);
  }

 done:
  tso->why_blocked = NotBlocked;
  appendToRunQueue(cap, tso);
}

/* -----------------------------------------------------------------------------
 * raiseAsync()
 *
 * The following function implements the magic for raising an
 * asynchronous exception in an existing thread.
 *
 * We first remove the thread from any queue on which it might be
 * blocked.  The possible blockages are MVARs, BLOCKING_QUEUESs, and
 * TSO blocked_exception queues.
 *
 * We strip the stack down to the innermost CATCH_FRAME, building
 * thunks in the heap for all the active computations, so they can
 * be restarted if necessary.  When we reach a CATCH_FRAME, we build
 * an application of the handler to the exception, and push it on
 * the top of the stack.
 *
 * How exactly do we save all the active computations?  We create an
 * AP_STACK for every UpdateFrame on the stack.  Entering one of these
 * AP_STACKs pushes everything from the corresponding update frame
 * upwards onto the stack.  (Actually, it pushes everything up to the
 * next update frame plus a pointer to the next AP_STACK object.
 * Entering the next AP_STACK object pushes more onto the stack until we
 * reach the last AP_STACK object - at which point the stack should look
 * exactly as it did when we killed the TSO and we can continue
 * execution by entering the closure on top of the stack.
 *
 * We can also kill a thread entirely - this happens if either (a) the
 * exception passed to raiseAsync is NULL, or (b) there's no
 * CATCH_FRAME on the stack.  In either case, we strip the entire
 * stack and replace the thread with a zombie.
 *
 * ToDo: in THREADED_RTS mode, this function is only safe if either
 * (a) we hold all the Capabilities (eg. in GC, or if there is only
 * one Capability), or (b) we own the Capability that the TSO is
 * currently blocked on or on the run queue of.
 *
 * -------------------------------------------------------------------------- */

StgTSO *
raiseAsync(Capability *cap, StgTSO *tso, StgClosure *exception,
           bool stop_at_atomically, StgUpdateFrame *stop_here)
{
    const StgRetInfoTable *info;
    StgPtr sp, frame;
    StgClosure *updatee;
    uint32_t i;
    StgStack *stack;

    debugTraceCap(DEBUG_sched, cap,
                  "raising exception in thread %" FMT_StgThreadID ".", tso->id);

#if defined(PROFILING)
    /*
     * Debugging tool: on raising an  exception, show where we are.
     * See also Exception.cmm:stg_raisezh.
     * This wasn't done for asynchronous exceptions originally; see #1450
     */
    if (RtsFlags.ProfFlags.showCCSOnException && exception != NULL)
    {
        fprintCCS_stderr(tso->prof.cccs,exception,tso);
    }
#endif
    // ASSUMES: the thread is not already complete or dead
    // Upper layers should deal with that.
    ASSERT(tso->what_next != ThreadComplete &&
           tso->what_next != ThreadKilled);

    // only if we own this TSO (except that deleteThread() calls this
    ASSERT(tso->cap == cap);

    stack = tso->stackobj;

    // mark it dirty; we're about to change its stack.
    dirty_TSO(cap, tso);
    dirty_STACK(cap, stack);

    sp = stack->sp;

    if (stop_here != NULL) {
        updatee = stop_here->updatee;
    } else {
        updatee = NULL;
    }

    // The stack freezing code assumes there's a closure pointer on
    // the top of the stack, so we have to arrange that this is the case...
    //
    if (sp[0] == (W_)&stg_enter_info) {
        sp++;
    } else {
        sp--;
        sp[0] = (W_)&stg_dummy_ret_closure;
    }

    frame = sp + 1;
    while (stop_here == NULL || frame < (StgPtr)stop_here) {

        // 1. Let the top of the stack be the "current closure"
        //
        // 2. Walk up the stack until we find either an UPDATE_FRAME or a
        // CATCH_FRAME.
        //
        // 3. If it's an UPDATE_FRAME, then make an AP_STACK containing the
        // current closure applied to the chunk of stack up to (but not
        // including) the update frame.  This closure becomes the "current
        // closure".  Go back to step 2.
        //
        // 4. If it's a CATCH_FRAME, then leave the exception handler on
        // top of the stack applied to the exception.
        //
        // 5. If it's a STOP_FRAME, then kill the thread.
        //
        // 6. If it's an UNDERFLOW_FRAME, then continue with the next
        //    stack chunk.
        //
        // NB: if we pass an ATOMICALLY_FRAME then abort the associated
        // transaction

        info = get_ret_itbl((StgClosure *)frame);

        switch (info->i.type) {

        case UPDATE_FRAME:
        {
            StgAP_STACK * ap;
            uint32_t words;

            // First build an AP_STACK consisting of the stack chunk above the
            // current update frame, with the top word on the stack as the
            // fun field.
            //
            words = frame - sp - 1;
            ap = (StgAP_STACK *)allocate(cap,AP_STACK_sizeW(words));

            ap->size = words;
            ap->fun  = (StgClosure *)sp[0];

            sp++;
            for(i=0; i < words; ++i) {
                ap->payload[i] = (StgClosure *)*sp++;
            }

            write_barrier(); // XXX: Necessary?
            SET_HDR(ap,&stg_AP_STACK_info,
                    ((StgClosure *)frame)->header.prof.ccs /* ToDo */);
            TICK_ALLOC_UP_THK(WDS(words+1),0);

            //IF_DEBUG(scheduler,
            //       debugBelch("sched: Updating ");
            //       printPtr((P_)((StgUpdateFrame *)frame)->updatee);
            //       debugBelch(" with ");
            //       printObj((StgClosure *)ap);
            //  );

            if (((StgUpdateFrame *)frame)->updatee == updatee) {
                // If this update frame points to the same closure as
                // the update frame further down the stack
                // (stop_here), then don't perform the update.  We
                // want to keep the blackhole in this case, so we can
                // detect and report the loop (#2783).
                ap = (StgAP_STACK*)updatee;
            } else {
                // Perform the update
                // TODO: this may waste some work, if the thunk has
                // already been updated by another thread.
                updateThunk(cap, tso,
                            ((StgUpdateFrame *)frame)->updatee, (StgClosure *)ap);
            }

            sp += sizeofW(StgUpdateFrame) - 1;
            sp[0] = (W_)ap; // push onto stack
            frame = sp + 1;
            continue; //no need to bump frame
        }

        case UNDERFLOW_FRAME:
        {
            StgAP_STACK * ap;
            uint32_t words;

            // First build an AP_STACK consisting of the stack chunk above the
            // current update frame, with the top word on the stack as the
            // fun field.
            //
            words = frame - sp - 1;
            ap = (StgAP_STACK *)allocate(cap,AP_STACK_sizeW(words));

            ap->size = words;
            ap->fun  = (StgClosure *)sp[0];
            sp++;
            for(i=0; i < words; ++i) {
                ap->payload[i] = (StgClosure *)*sp++;
            }

            SET_HDR(ap,&stg_AP_STACK_NOUPD_info,stack->header.prof.ccs);
            TICK_ALLOC_SE_THK(WDS(words+1),0);

            stack->sp = sp;
            threadStackUnderflow(cap,tso);
            stack = tso->stackobj;
            sp = stack->sp;

            sp--;
            sp[0] = (W_)ap;
            frame = sp + 1;
            continue;
        }

        case STOP_FRAME:
        {
            // We've stripped the entire stack, the thread is now dead.
            tso->what_next = ThreadKilled;
            stack->sp = frame + sizeofW(StgStopFrame);
            goto done;
        }

        case CATCH_FRAME:
            // If we find a CATCH_FRAME, and we've got an exception to raise,
            // then build the THUNK raise(exception), and leave it on
            // top of the CATCH_FRAME ready to enter.
            //
        {
            StgCatchFrame *cf = (StgCatchFrame *)frame;
            StgThunk *raise;

            if (exception == NULL) break;

            // we've got an exception to raise, so let's pass it to the
            // handler in this frame.
            //
            raise = (StgThunk *)allocate(cap,sizeofW(StgThunk)+1);
            TICK_ALLOC_SE_THK(WDS(1),0);
            SET_HDR(raise,&stg_raise_info,cf->header.prof.ccs);
            raise->payload[0] = exception;

            // throw away the stack from Sp up to the CATCH_FRAME.
            //
            sp = frame - 1;

            /* Ensure that async exceptions are blocked now, so we don't get
             * a surprise exception before we get around to executing the
             * handler.
             */
            tso->flags |= TSO_BLOCKEX;
            if ((cf->exceptions_blocked & TSO_INTERRUPTIBLE) == 0) {
                tso->flags &= ~TSO_INTERRUPTIBLE;
            } else {
                tso->flags |= TSO_INTERRUPTIBLE;
            }

            /* Put the newly-built THUNK on top of the stack, ready to execute
             * when the thread restarts.
             */
            sp[0] = (W_)raise;
            sp[-1] = (W_)&stg_enter_info;
            stack->sp = sp-1;
            RELAXED_STORE(&tso->what_next, ThreadRunGHC);
            goto done;
        }

        case ATOMICALLY_FRAME:
            if (stop_at_atomically) {
                ASSERT(tso->trec->enclosing_trec == NO_TREC);
                stmCondemnTransaction(cap, tso -> trec);
                stack->sp = frame - 2;
                // The ATOMICALLY_FRAME expects to be returned a
                // result from the transaction, which it stores in the
                // stack frame.  Hence we arrange to return a dummy
                // result, so that the GC doesn't get upset (#3578).
                // Perhaps a better way would be to have a different
                // ATOMICALLY_FRAME instance for condemned
                // transactions, but I don't fully understand the
                // interaction with STM invariants.
                stack->sp[1] = (W_)&stg_NO_TREC_closure;
                stack->sp[0] = (W_)&stg_ret_p_info;
                tso->what_next = ThreadRunGHC;
                goto done;
            }
            else
            {
                // Freezing an STM transaction.  Just aborting the
                // transaction would be wrong; this is what we used to
                // do, and it goes wrong if the ATOMICALLY_FRAME ever
                // gets back onto the stack again, which it will do if
                // the transaction is inside unsafePerformIO or
                // unsafeInterleaveIO and hence inside an UPDATE_FRAME.
                //
                // So we want to make it so that if the enclosing
                // computation is resumed, we will re-execute the
                // transaction.  We therefore:
                //
                //   1. abort the current transaction
                //   3. replace the stack up to and including the
                //      atomically frame with a closure representing
                //      a call to "atomically x", where x is the code
                //      of the transaction.
                //   4. continue stripping the stack
                //
                StgTRecHeader *trec = tso->trec;
                StgTRecHeader *outer = trec->enclosing_trec;

                StgThunk *atomically;
                StgAtomicallyFrame *af = (StgAtomicallyFrame*)frame;

                debugTraceCap(DEBUG_stm, cap,
                              "raiseAsync: freezing atomically frame")
                stmAbortTransaction(cap, trec);
                stmFreeAbortedTRec(cap, trec);
                tso->trec = outer;

                atomically = (StgThunk*)allocate(cap,sizeofW(StgThunk)+1);
                TICK_ALLOC_SE_THK(1,0);
                SET_HDR(atomically,&stg_atomically_info,af->header.prof.ccs);
                atomically->payload[0] = af->code;

                // discard stack up to and including the ATOMICALLY_FRAME
                frame += sizeofW(StgAtomicallyFrame);
                sp = frame - 1;

                // replace the ATOMICALLY_FRAME with call to atomically#
                sp[0] = (W_)atomically;
                continue;
            }

        case CATCH_STM_FRAME:
        case CATCH_RETRY_FRAME:
            // CATCH frames within an atomically block: abort the
            // inner transaction and continue.  Eventually we will
            // hit the outer transaction that will get frozen (see
            // above).
            //
            // In this case (unlike ordinary exceptions) we do not care
            // whether the transaction is valid or not because its
            // possible validity cannot have caused the exception
            // and will not be visible after the abort.
        {
            StgTRecHeader *trec = tso -> trec;
            StgTRecHeader *outer = trec -> enclosing_trec;
            debugTraceCap(DEBUG_stm, cap,
                          "found atomically block delivering async exception");
            stmAbortTransaction(cap, trec);
            stmFreeAbortedTRec(cap, trec);
            tso -> trec = outer;
            break;
        };

        default:
            break;
        }

        // move on to the next stack frame
        frame += stack_frame_sizeW((StgClosure *)frame);
    }

done:
    IF_DEBUG(sanity, checkTSO(tso));

    // wake it up
    if (tso->why_blocked != NotBlocked) {
        tso->why_blocked = NotBlocked;
        appendToRunQueue(cap,tso);
    }

    return tso;
}