summaryrefslogtreecommitdiffstats
path: root/libutils/Threads.cpp
blob: b09d51068359885fb2f865f9eff99e7d2fa32a7d (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
/*
 * Copyright (C) 2007 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// #define LOG_NDEBUG 0
#define LOG_TAG "libutils.threads"

#include <assert.h>
#include <errno.h>
#include <memory.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#if defined(HAVE_PTHREADS)
# include <pthread.h>
# include <sched.h>
# include <sys/resource.h>
#ifdef HAVE_ANDROID_OS
# include <private/bionic_pthread.h>
#endif
#elif defined(HAVE_WIN32_THREADS)
# include <windows.h>
# include <stdint.h>
# include <process.h>
# define HAVE_CREATETHREAD  // Cygwin, vs. HAVE__BEGINTHREADEX for MinGW
#endif

#if defined(HAVE_PRCTL)
#include <sys/prctl.h>
#endif

#include <utils/threads.h>
#include <utils/Log.h>

#include <cutils/sched_policy.h>

#ifdef HAVE_ANDROID_OS
# define __android_unused
#else
# define __android_unused __attribute__((__unused__))
#endif

/*
 * ===========================================================================
 *      Thread wrappers
 * ===========================================================================
 */

using namespace android;

// ----------------------------------------------------------------------------
#if defined(HAVE_PTHREADS)
// ----------------------------------------------------------------------------

/*
 * Create and run a new thread.
 *
 * We create it "detached", so it cleans up after itself.
 */

typedef void* (*android_pthread_entry)(void*);

struct thread_data_t {
    thread_func_t   entryFunction;
    void*           userData;
    int             priority;
    char *          threadName;

    // we use this trampoline when we need to set the priority with
    // nice/setpriority, and name with prctl.
    static int trampoline(const thread_data_t* t) {
        thread_func_t f = t->entryFunction;
        void* u = t->userData;
        int prio = t->priority;
        char * name = t->threadName;
        delete t;
        setpriority(PRIO_PROCESS, 0, prio);
        if (prio >= ANDROID_PRIORITY_BACKGROUND) {
            set_sched_policy(0, SP_BACKGROUND);
        } else {
            set_sched_policy(0, SP_FOREGROUND);
        }
        
        if (name) {
            androidSetThreadName(name);
            free(name);
        }
        return f(u);
    }
};

void androidSetThreadName(const char* name) {
#if defined(HAVE_PRCTL)
    // Mac OS doesn't have this, and we build libutil for the host too
    int hasAt = 0;
    int hasDot = 0;
    const char *s = name;
    while (*s) {
        if (*s == '.') hasDot = 1;
        else if (*s == '@') hasAt = 1;
        s++;
    }
    int len = s - name;
    if (len < 15 || hasAt || !hasDot) {
        s = name;
    } else {
        s = name + len - 15;
    }
    prctl(PR_SET_NAME, (unsigned long) s, 0, 0, 0);
#endif
}

int androidCreateRawThreadEtc(android_thread_func_t entryFunction,
                               void *userData,
                               const char* threadName __android_unused,
                               int32_t threadPriority,
                               size_t threadStackSize,
                               android_thread_id_t *threadId)
{
    pthread_attr_t attr; 
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);

#ifdef HAVE_ANDROID_OS  /* valgrind is rejecting RT-priority create reqs */
    if (threadPriority != PRIORITY_DEFAULT || threadName != NULL) {
        // Now that the pthread_t has a method to find the associated
        // android_thread_id_t (pid) from pthread_t, it would be possible to avoid
        // this trampoline in some cases as the parent could set the properties
        // for the child.  However, there would be a race condition because the
        // child becomes ready immediately, and it doesn't work for the name.
        // prctl(PR_SET_NAME) only works for self; prctl(PR_SET_THREAD_NAME) was
        // proposed but not yet accepted.
        thread_data_t* t = new thread_data_t;
        t->priority = threadPriority;
        t->threadName = threadName ? strdup(threadName) : NULL;
        t->entryFunction = entryFunction;
        t->userData = userData;
        entryFunction = (android_thread_func_t)&thread_data_t::trampoline;
        userData = t;            
    }
#endif

    if (threadStackSize) {
        pthread_attr_setstacksize(&attr, threadStackSize);
    }
    
    errno = 0;
    pthread_t thread;
    int result = pthread_create(&thread, &attr,
                    (android_pthread_entry)entryFunction, userData);
    pthread_attr_destroy(&attr);
    if (result != 0) {
        ALOGE("androidCreateRawThreadEtc failed (entry=%p, res=%d, errno=%d)\n"
             "(android threadPriority=%d)",
            entryFunction, result, errno, threadPriority);
        return 0;
    }

    // Note that *threadID is directly available to the parent only, as it is
    // assigned after the child starts.  Use memory barrier / lock if the child
    // or other threads also need access.
    if (threadId != NULL) {
        *threadId = (android_thread_id_t)thread; // XXX: this is not portable
    }
    return 1;
}

#ifdef HAVE_ANDROID_OS
static pthread_t android_thread_id_t_to_pthread(android_thread_id_t thread)
{
    return (pthread_t) thread;
}
#endif

android_thread_id_t androidGetThreadId()
{
    return (android_thread_id_t)pthread_self();
}

// ----------------------------------------------------------------------------
#elif defined(HAVE_WIN32_THREADS)
// ----------------------------------------------------------------------------

/*
 * Trampoline to make us __stdcall-compliant.
 *
 * We're expected to delete "vDetails" when we're done.
 */
struct threadDetails {
    int (*func)(void*);
    void* arg;
};
static __stdcall unsigned int threadIntermediary(void* vDetails)
{
    struct threadDetails* pDetails = (struct threadDetails*) vDetails;
    int result;

    result = (*(pDetails->func))(pDetails->arg);

    delete pDetails;

    ALOG(LOG_VERBOSE, "thread", "thread exiting\n");
    return (unsigned int) result;
}

/*
 * Create and run a new thread.
 */
static bool doCreateThread(android_thread_func_t fn, void* arg, android_thread_id_t *id)
{
    HANDLE hThread;
    struct threadDetails* pDetails = new threadDetails; // must be on heap
    unsigned int thrdaddr;

    pDetails->func = fn;
    pDetails->arg = arg;

#if defined(HAVE__BEGINTHREADEX)
    hThread = (HANDLE) _beginthreadex(NULL, 0, threadIntermediary, pDetails, 0,
                    &thrdaddr);
    if (hThread == 0)
#elif defined(HAVE_CREATETHREAD)
    hThread = CreateThread(NULL, 0,
                    (LPTHREAD_START_ROUTINE) threadIntermediary,
                    (void*) pDetails, 0, (DWORD*) &thrdaddr);
    if (hThread == NULL)
#endif
    {
        ALOG(LOG_WARN, "thread", "WARNING: thread create failed\n");
        return false;
    }

#if defined(HAVE_CREATETHREAD)
    /* close the management handle */
    CloseHandle(hThread);
#endif

    if (id != NULL) {
      	*id = (android_thread_id_t)thrdaddr;
    }

    return true;
}

int androidCreateRawThreadEtc(android_thread_func_t fn,
                               void *userData,
                               const char* /*threadName*/,
                               int32_t /*threadPriority*/,
                               size_t /*threadStackSize*/,
                               android_thread_id_t *threadId)
{
    return doCreateThread(  fn, userData, threadId);
}

android_thread_id_t androidGetThreadId()
{
    return (android_thread_id_t)GetCurrentThreadId();
}

// ----------------------------------------------------------------------------
#else
#error "Threads not supported"
#endif

// ----------------------------------------------------------------------------

int androidCreateThread(android_thread_func_t fn, void* arg)
{
    return createThreadEtc(fn, arg);
}

int androidCreateThreadGetID(android_thread_func_t fn, void *arg, android_thread_id_t *id)
{
    return createThreadEtc(fn, arg, "android:unnamed_thread",
                           PRIORITY_DEFAULT, 0, id);
}

static android_create_thread_fn gCreateThreadFn = androidCreateRawThreadEtc;

int androidCreateThreadEtc(android_thread_func_t entryFunction,
                            void *userData,
                            const char* threadName,
                            int32_t threadPriority,
                            size_t threadStackSize,
                            android_thread_id_t *threadId)
{
    return gCreateThreadFn(entryFunction, userData, threadName,
        threadPriority, threadStackSize, threadId);
}

void androidSetCreateThreadFunc(android_create_thread_fn func)
{
    gCreateThreadFn = func;
}

pid_t androidGetTid()
{
#ifdef HAVE_GETTID
    return gettid();
#else
    return getpid();
#endif
}

#ifdef HAVE_ANDROID_OS
int androidSetThreadPriority(pid_t tid, int pri)
{
    int rc = 0;
    
#if defined(HAVE_PTHREADS)
    int lasterr = 0;

    if (pri >= ANDROID_PRIORITY_BACKGROUND) {
        rc = set_sched_policy(tid, SP_BACKGROUND);
    } else if (getpriority(PRIO_PROCESS, tid) >= ANDROID_PRIORITY_BACKGROUND) {
        rc = set_sched_policy(tid, SP_FOREGROUND);
    }

    if (rc) {
        lasterr = errno;
    }

    if (setpriority(PRIO_PROCESS, tid, pri) < 0) {
        rc = INVALID_OPERATION;
    } else {
        errno = lasterr;
    }
#endif
    
    return rc;
}

int androidGetThreadPriority(pid_t tid) {
#if defined(HAVE_PTHREADS)
    return getpriority(PRIO_PROCESS, tid);
#else
    return ANDROID_PRIORITY_NORMAL;
#endif
}

#endif

namespace android {

/*
 * ===========================================================================
 *      Mutex class
 * ===========================================================================
 */

#if defined(HAVE_PTHREADS)
// implemented as inlines in threads.h
#elif defined(HAVE_WIN32_THREADS)

Mutex::Mutex()
{
    HANDLE hMutex;

    assert(sizeof(hMutex) == sizeof(mState));

    hMutex = CreateMutex(NULL, FALSE, NULL);
    mState = (void*) hMutex;
}

Mutex::Mutex(const char* name)
{
    // XXX: name not used for now
    HANDLE hMutex;

    assert(sizeof(hMutex) == sizeof(mState));

    hMutex = CreateMutex(NULL, FALSE, NULL);
    mState = (void*) hMutex;
}

Mutex::Mutex(int type, const char* name)
{
    // XXX: type and name not used for now
    HANDLE hMutex;

    assert(sizeof(hMutex) == sizeof(mState));

    hMutex = CreateMutex(NULL, FALSE, NULL);
    mState = (void*) hMutex;
}

Mutex::~Mutex()
{
    CloseHandle((HANDLE) mState);
}

status_t Mutex::lock()
{
    DWORD dwWaitResult;
    dwWaitResult = WaitForSingleObject((HANDLE) mState, INFINITE);
    return dwWaitResult != WAIT_OBJECT_0 ? -1 : NO_ERROR;
}

void Mutex::unlock()
{
    if (!ReleaseMutex((HANDLE) mState))
        ALOG(LOG_WARN, "thread", "WARNING: bad result from unlocking mutex\n");
}

status_t Mutex::tryLock()
{
    DWORD dwWaitResult;

    dwWaitResult = WaitForSingleObject((HANDLE) mState, 0);
    if (dwWaitResult != WAIT_OBJECT_0 && dwWaitResult != WAIT_TIMEOUT)
        ALOG(LOG_WARN, "thread", "WARNING: bad result from try-locking mutex\n");
    return (dwWaitResult == WAIT_OBJECT_0) ? 0 : -1;
}

#else
#error "Somebody forgot to implement threads for this platform."
#endif


/*
 * ===========================================================================
 *      Condition class
 * ===========================================================================
 */

#if defined(HAVE_PTHREADS)
// implemented as inlines in threads.h
#elif defined(HAVE_WIN32_THREADS)

/*
 * Windows doesn't have a condition variable solution.  It's possible
 * to create one, but it's easy to get it wrong.  For a discussion, and
 * the origin of this implementation, see:
 *
 *  http://www.cs.wustl.edu/~schmidt/win32-cv-1.html
 *
 * The implementation shown on the page does NOT follow POSIX semantics.
 * As an optimization they require acquiring the external mutex before
 * calling signal() and broadcast(), whereas POSIX only requires grabbing
 * it before calling wait().  The implementation here has been un-optimized
 * to have the correct behavior.
 */
typedef struct WinCondition {
    // Number of waiting threads.
    int                 waitersCount;

    // Serialize access to waitersCount.
    CRITICAL_SECTION    waitersCountLock;

    // Semaphore used to queue up threads waiting for the condition to
    // become signaled.
    HANDLE              sema;

    // An auto-reset event used by the broadcast/signal thread to wait
    // for all the waiting thread(s) to wake up and be released from
    // the semaphore.
    HANDLE              waitersDone;

    // This mutex wouldn't be necessary if we required that the caller
    // lock the external mutex before calling signal() and broadcast().
    // I'm trying to mimic pthread semantics though.
    HANDLE              internalMutex;

    // Keeps track of whether we were broadcasting or signaling.  This
    // allows us to optimize the code if we're just signaling.
    bool                wasBroadcast;

    status_t wait(WinCondition* condState, HANDLE hMutex, nsecs_t* abstime)
    {
        // Increment the wait count, avoiding race conditions.
        EnterCriticalSection(&condState->waitersCountLock);
        condState->waitersCount++;
        //printf("+++ wait: incr waitersCount to %d (tid=%ld)\n",
        //    condState->waitersCount, getThreadId());
        LeaveCriticalSection(&condState->waitersCountLock);
    
        DWORD timeout = INFINITE;
        if (abstime) {
            nsecs_t reltime = *abstime - systemTime();
            if (reltime < 0)
                reltime = 0;
            timeout = reltime/1000000;
        }
        
        // Atomically release the external mutex and wait on the semaphore.
        DWORD res =
            SignalObjectAndWait(hMutex, condState->sema, timeout, FALSE);
    
        //printf("+++ wait: awake (tid=%ld)\n", getThreadId());
    
        // Reacquire lock to avoid race conditions.
        EnterCriticalSection(&condState->waitersCountLock);
    
        // No longer waiting.
        condState->waitersCount--;
    
        // Check to see if we're the last waiter after a broadcast.
        bool lastWaiter = (condState->wasBroadcast && condState->waitersCount == 0);
    
        //printf("+++ wait: lastWaiter=%d (wasBc=%d wc=%d)\n",
        //    lastWaiter, condState->wasBroadcast, condState->waitersCount);
    
        LeaveCriticalSection(&condState->waitersCountLock);
    
        // If we're the last waiter thread during this particular broadcast
        // then signal broadcast() that we're all awake.  It'll drop the
        // internal mutex.
        if (lastWaiter) {
            // Atomically signal the "waitersDone" event and wait until we
            // can acquire the internal mutex.  We want to do this in one step
            // because it ensures that everybody is in the mutex FIFO before
            // any thread has a chance to run.  Without it, another thread
            // could wake up, do work, and hop back in ahead of us.
            SignalObjectAndWait(condState->waitersDone, condState->internalMutex,
                INFINITE, FALSE);
        } else {
            // Grab the internal mutex.
            WaitForSingleObject(condState->internalMutex, INFINITE);
        }
    
        // Release the internal and grab the external.
        ReleaseMutex(condState->internalMutex);
        WaitForSingleObject(hMutex, INFINITE);
    
        return res == WAIT_OBJECT_0 ? NO_ERROR : -1;
    }
} WinCondition;

/*
 * Constructor.  Set up the WinCondition stuff.
 */
Condition::Condition()
{
    WinCondition* condState = new WinCondition;

    condState->waitersCount = 0;
    condState->wasBroadcast = false;
    // semaphore: no security, initial value of 0
    condState->sema = CreateSemaphore(NULL, 0, 0x7fffffff, NULL);
    InitializeCriticalSection(&condState->waitersCountLock);
    // auto-reset event, not signaled initially
    condState->waitersDone = CreateEvent(NULL, FALSE, FALSE, NULL);
    // used so we don't have to lock external mutex on signal/broadcast
    condState->internalMutex = CreateMutex(NULL, FALSE, NULL);

    mState = condState;
}

/*
 * Destructor.  Free Windows resources as well as our allocated storage.
 */
Condition::~Condition()
{
    WinCondition* condState = (WinCondition*) mState;
    if (condState != NULL) {
        CloseHandle(condState->sema);
        CloseHandle(condState->waitersDone);
        delete condState;
    }
}


status_t Condition::wait(Mutex& mutex)
{
    WinCondition* condState = (WinCondition*) mState;
    HANDLE hMutex = (HANDLE) mutex.mState;
    
    return ((WinCondition*)mState)->wait(condState, hMutex, NULL);
}

status_t Condition::waitRelative(Mutex& mutex, nsecs_t reltime)
{
    WinCondition* condState = (WinCondition*) mState;
    HANDLE hMutex = (HANDLE) mutex.mState;
    nsecs_t absTime = systemTime()+reltime;

    return ((WinCondition*)mState)->wait(condState, hMutex, &absTime);
}

/*
 * Signal the condition variable, allowing one thread to continue.
 */
void Condition::signal()
{
    WinCondition* condState = (WinCondition*) mState;

    // Lock the internal mutex.  This ensures that we don't clash with
    // broadcast().
    WaitForSingleObject(condState->internalMutex, INFINITE);

    EnterCriticalSection(&condState->waitersCountLock);
    bool haveWaiters = (condState->waitersCount > 0);
    LeaveCriticalSection(&condState->waitersCountLock);

    // If no waiters, then this is a no-op.  Otherwise, knock the semaphore
    // down a notch.
    if (haveWaiters)
        ReleaseSemaphore(condState->sema, 1, 0);

    // Release internal mutex.
    ReleaseMutex(condState->internalMutex);
}

/*
 * Signal the condition variable, allowing all threads to continue.
 *
 * First we have to wake up all threads waiting on the semaphore, then
 * we wait until all of the threads have actually been woken before
 * releasing the internal mutex.  This ensures that all threads are woken.
 */
void Condition::broadcast()
{
    WinCondition* condState = (WinCondition*) mState;

    // Lock the internal mutex.  This keeps the guys we're waking up
    // from getting too far.
    WaitForSingleObject(condState->internalMutex, INFINITE);

    EnterCriticalSection(&condState->waitersCountLock);
    bool haveWaiters = false;

    if (condState->waitersCount > 0) {
        haveWaiters = true;
        condState->wasBroadcast = true;
    }

    if (haveWaiters) {
        // Wake up all the waiters.
        ReleaseSemaphore(condState->sema, condState->waitersCount, 0);

        LeaveCriticalSection(&condState->waitersCountLock);

        // Wait for all awakened threads to acquire the counting semaphore.
        // The last guy who was waiting sets this.
        WaitForSingleObject(condState->waitersDone, INFINITE);

        // Reset wasBroadcast.  (No crit section needed because nobody
        // else can wake up to poke at it.)
        condState->wasBroadcast = 0;
    } else {
        // nothing to do
        LeaveCriticalSection(&condState->waitersCountLock);
    }

    // Release internal mutex.
    ReleaseMutex(condState->internalMutex);
}

#else
#error "condition variables not supported on this platform"
#endif

// ----------------------------------------------------------------------------

/*
 * This is our thread object!
 */

Thread::Thread(bool canCallJava)
    :   mCanCallJava(canCallJava),
        mThread(thread_id_t(-1)),
        mLock("Thread::mLock"),
        mStatus(NO_ERROR),
        mExitPending(false), mRunning(false)
#ifdef HAVE_ANDROID_OS
        , mTid(-1)
#endif
{
}

Thread::~Thread()
{
}

status_t Thread::readyToRun()
{
    return NO_ERROR;
}

status_t Thread::run(const char* name, int32_t priority, size_t stack)
{
    Mutex::Autolock _l(mLock);

    if (mRunning) {
        // thread already started
        return INVALID_OPERATION;
    }

    // reset status and exitPending to their default value, so we can
    // try again after an error happened (either below, or in readyToRun())
    mStatus = NO_ERROR;
    mExitPending = false;
    mThread = thread_id_t(-1);
    
    // hold a strong reference on ourself
    mHoldSelf = this;

    mRunning = true;

    bool res;
    if (mCanCallJava) {
        res = createThreadEtc(_threadLoop,
                this, name, priority, stack, &mThread);
    } else {
        res = androidCreateRawThreadEtc(_threadLoop,
                this, name, priority, stack, &mThread);
    }
    
    if (res == false) {
        mStatus = UNKNOWN_ERROR;   // something happened!
        mRunning = false;
        mThread = thread_id_t(-1);
        mHoldSelf.clear();  // "this" may have gone away after this.

        return UNKNOWN_ERROR;
    }
    
    // Do not refer to mStatus here: The thread is already running (may, in fact
    // already have exited with a valid mStatus result). The NO_ERROR indication
    // here merely indicates successfully starting the thread and does not
    // imply successful termination/execution.
    return NO_ERROR;

    // Exiting scope of mLock is a memory barrier and allows new thread to run
}

int Thread::_threadLoop(void* user)
{
    Thread* const self = static_cast<Thread*>(user);

    sp<Thread> strong(self->mHoldSelf);
    wp<Thread> weak(strong);
    self->mHoldSelf.clear();

#ifdef HAVE_ANDROID_OS
    // this is very useful for debugging with gdb
    self->mTid = gettid();
#endif

    bool first = true;

    do {
        bool result;
        if (first) {
            first = false;
            self->mStatus = self->readyToRun();
            result = (self->mStatus == NO_ERROR);

            if (result && !self->exitPending()) {
                // Binder threads (and maybe others) rely on threadLoop
                // running at least once after a successful ::readyToRun()
                // (unless, of course, the thread has already been asked to exit
                // at that point).
                // This is because threads are essentially used like this:
                //   (new ThreadSubclass())->run();
                // The caller therefore does not retain a strong reference to
                // the thread and the thread would simply disappear after the
                // successful ::readyToRun() call instead of entering the
                // threadLoop at least once.
                result = self->threadLoop();
            }
        } else {
            result = self->threadLoop();
        }

        // establish a scope for mLock
        {
        Mutex::Autolock _l(self->mLock);
        if (result == false || self->mExitPending) {
            self->mExitPending = true;
            self->mRunning = false;
            // clear thread ID so that requestExitAndWait() does not exit if
            // called by a new thread using the same thread ID as this one.
            self->mThread = thread_id_t(-1);
            // note that interested observers blocked in requestExitAndWait are
            // awoken by broadcast, but blocked on mLock until break exits scope
            self->mThreadExitedCondition.broadcast();
            break;
        }
        }
        
        // Release our strong reference, to let a chance to the thread
        // to die a peaceful death.
        strong.clear();
        // And immediately, re-acquire a strong reference for the next loop
        strong = weak.promote();
    } while(strong != 0);
    
    return 0;
}

void Thread::requestExit()
{
    Mutex::Autolock _l(mLock);
    mExitPending = true;
}

status_t Thread::requestExitAndWait()
{
    Mutex::Autolock _l(mLock);
    if (mThread == getThreadId()) {
        ALOGW(
        "Thread (this=%p): don't call waitForExit() from this "
        "Thread object's thread. It's a guaranteed deadlock!",
        this);

        return WOULD_BLOCK;
    }
    
    mExitPending = true;

    while (mRunning == true) {
        mThreadExitedCondition.wait(mLock);
    }
    // This next line is probably not needed any more, but is being left for
    // historical reference. Note that each interested party will clear flag.
    mExitPending = false;

    return mStatus;
}

status_t Thread::join()
{
    Mutex::Autolock _l(mLock);
    if (mThread == getThreadId()) {
        ALOGW(
        "Thread (this=%p): don't call join() from this "
        "Thread object's thread. It's a guaranteed deadlock!",
        this);

        return WOULD_BLOCK;
    }

    while (mRunning == true) {
        mThreadExitedCondition.wait(mLock);
    }

    return mStatus;
}

bool Thread::isRunning() const {
    Mutex::Autolock _l(mLock);
    return mRunning;
}

#ifdef HAVE_ANDROID_OS
pid_t Thread::getTid() const
{
    // mTid is not defined until the child initializes it, and the caller may need it earlier
    Mutex::Autolock _l(mLock);
    pid_t tid;
    if (mRunning) {
        pthread_t pthread = android_thread_id_t_to_pthread(mThread);
        tid = __pthread_gettid(pthread);
    } else {
        ALOGW("Thread (this=%p): getTid() is undefined before run()", this);
        tid = -1;
    }
    return tid;
}
#endif

bool Thread::exitPending() const
{
    Mutex::Autolock _l(mLock);
    return mExitPending;
}



};  // namespace android