compat/win32/pthread.c

   1 /*
   2  * Copyright (C) 2009 Andrzej K. Haczewski <ahaczewski@gmail.com>
   3  *
   4  * DISCLAIMER: The implementation is Git-specific, it is subset of original
   5  * Pthreads API, without lots of other features that Git doesn't use.
   6  * Git also makes sure that the passed arguments are valid, so there's
   7  * no need for double-checking.
   8  */
   9
  10 #include "../../git-compat-util.h"
  11 #include "pthread.h"
  12
  13 #include <errno.h>
  14 #include <limits.h>
  15
  16 static unsigned __stdcall win32_start_routine(void *arg)
  17 {
  18         pthread_t *thread = arg;
  19         thread->tid = GetCurrentThreadId();
  20         thread->arg = thread->start_routine(thread->arg);
  21         return 0;
  22 }
  23
  24 int pthread_create(pthread_t *thread, const void *unused,
  25                    void *(*start_routine)(void*), void *arg)
  26 {
  27         thread->arg = arg;
  28         thread->start_routine = start_routine;
  29         thread->handle = (HANDLE)
  30                 _beginthreadex(NULL, 0, win32_start_routine, thread, 0, NULL);
  31
  32         if (!thread->handle)
  33                 return errno;
  34         else
  35                 return 0;
  36 }
  37
  38 int win32_pthread_join(pthread_t *thread, void **value_ptr)
  39 {
  40         DWORD result = WaitForSingleObject(thread->handle, INFINITE);
  41         switch (result) {
  42                 case WAIT_OBJECT_0:
  43                         if (value_ptr)
  44                                 *value_ptr = thread->arg;
  45                         return 0;
  46                 case WAIT_ABANDONED:
  47                         return EINVAL;
  48                 default:
  49                         return err_win_to_posix(GetLastError());
  50         }
  51 }
  52
  53 pthread_t pthread_self(void)
  54 {
  55         pthread_t t = { 0 };
  56         t.tid = GetCurrentThreadId();
  57         return t;
  58 }
  59
  60 int pthread_cond_init(pthread_cond_t *cond, const void *unused)
  61 {
  62         cond->waiters = 0;
  63         cond->was_broadcast = 0;
  64         InitializeCriticalSection(&cond->waiters_lock);
  65
  66         cond->sema = CreateSemaphore(NULL, 0, LONG_MAX, NULL);
  67         if (!cond->sema)
  68                 die("CreateSemaphore() failed");
  69
  70         cond->continue_broadcast = CreateEvent(NULL,    /* security */
  71                                 FALSE,                  /* auto-reset */
  72                                 FALSE,                  /* not signaled */
  73                                 NULL);                  /* name */
  74         if (!cond->continue_broadcast)
  75                 die("CreateEvent() failed");
  76
  77         return 0;
  78 }
  79
  80 int pthread_cond_destroy(pthread_cond_t *cond)
  81 {
  82         CloseHandle(cond->sema);
  83         CloseHandle(cond->continue_broadcast);
  84         DeleteCriticalSection(&cond->waiters_lock);
  85         return 0;
  86 }
  87
  88 int pthread_cond_wait(pthread_cond_t *cond, CRITICAL_SECTION *mutex)
  89 {
  90         int last_waiter;
  91
  92         EnterCriticalSection(&cond->waiters_lock);
  93         cond->waiters++;
  94         LeaveCriticalSection(&cond->waiters_lock);
  95
  96         /*
  97          * Unlock external mutex and wait for signal.
  98          * NOTE: we've held mutex locked long enough to increment
  99          * waiters count above, so there's no problem with
 100          * leaving mutex unlocked before we wait on semaphore.
 101          */
 102         LeaveCriticalSection(mutex);
 103
 104         /* let's wait - ignore return value */
 105         WaitForSingleObject(cond->sema, INFINITE);
 106
 107         /*
 108          * Decrease waiters count. If we are the last waiter, then we must
 109          * notify the broadcasting thread that it can continue.
 110          * But if we continued due to cond_signal, we do not have to do that
 111          * because the signaling thread knows that only one waiter continued.
 112          */
 113         EnterCriticalSection(&cond->waiters_lock);
 114         cond->waiters--;
 115         last_waiter = cond->was_broadcast && cond->waiters == 0;
 116         LeaveCriticalSection(&cond->waiters_lock);
 117
 118         if (last_waiter) {
 119                 /*
 120                  * cond_broadcast was issued while mutex was held. This means
 121                  * that all other waiters have continued, but are contending
 122                  * for the mutex at the end of this function because the
 123                  * broadcasting thread did not leave cond_broadcast, yet.
 124                  * (This is so that it can be sure that each waiter has
 125                  * consumed exactly one slice of the semaphor.)
 126                  * The last waiter must tell the broadcasting thread that it
 127                  * can go on.
 128                  */
 129                 SetEvent(cond->continue_broadcast);
 130                 /*
 131                  * Now we go on to contend with all other waiters for
 132                  * the mutex. Auf in den Kampf!
 133                  */
 134         }
 135         /* lock external mutex again */
 136         EnterCriticalSection(mutex);
 137
 138         return 0;
 139 }
 140
 141 /*
 142  * IMPORTANT: This implementation requires that pthread_cond_signal
 143  * is called while the mutex is held that is used in the corresponding
 144  * pthread_cond_wait calls!
 145  */
 146 int pthread_cond_signal(pthread_cond_t *cond)
 147 {
 148         int have_waiters;
 149
 150         EnterCriticalSection(&cond->waiters_lock);
 151         have_waiters = cond->waiters > 0;
 152         LeaveCriticalSection(&cond->waiters_lock);
 153
 154         /*
 155          * Signal only when there are waiters
 156          */
 157         if (have_waiters)
 158                 return ReleaseSemaphore(cond->sema, 1, NULL) ?
 159                         0 : err_win_to_posix(GetLastError());
 160         else
 161                 return 0;
 162 }
 163
 164 /*
 165  * DOUBLY IMPORTANT: This implementation requires that pthread_cond_broadcast
 166  * is called while the mutex is held that is used in the corresponding
 167  * pthread_cond_wait calls!
 168  */
 169 int pthread_cond_broadcast(pthread_cond_t *cond)
 170 {
 171         EnterCriticalSection(&cond->waiters_lock);
 172
 173         if ((cond->was_broadcast = cond->waiters > 0)) {
 174                 /* wake up all waiters */
 175                 ReleaseSemaphore(cond->sema, cond->waiters, NULL);
 176                 LeaveCriticalSection(&cond->waiters_lock);
 177                 /*
 178                  * At this point all waiters continue. Each one takes its
 179                  * slice of the semaphor. Now it's our turn to wait: Since
 180                  * the external mutex is held, no thread can leave cond_wait,
 181                  * yet. For this reason, we can be sure that no thread gets
 182                  * a chance to eat *more* than one slice. OTOH, it means
 183                  * that the last waiter must send us a wake-up.
 184                  */
 185                 WaitForSingleObject(cond->continue_broadcast, INFINITE);
 186                 /*
 187                  * Since the external mutex is held, no thread can enter
 188                  * cond_wait, and, hence, it is safe to reset this flag
 189                  * without cond->waiters_lock held.
 190                  */
 191                 cond->was_broadcast = 0;
 192         } else {
 193                 LeaveCriticalSection(&cond->waiters_lock);
 194         }
 195         return 0;
 196 }