util/qemu-thread-win32.c - qemu - Git at Google

 /*
  * Win32 implementation for mutex/cond/thread functions
  *
  * Copyright Red Hat, Inc. 2010
  *
  * Author:
  *  Paolo Bonzini <pbonzini@redhat.com>
  *
  * This work is licensed under the terms of the GNU GPL, version 2 or later.
  * See the COPYING file in the top-level directory.
  *
  */
 #include "qemu/osdep.h"
 #include "qemu-common.h"
 #include "qemu/thread.h"
 #include "qemu/notify.h"
 #include <process.h>

 static bool name_threads;

 void qemu_thread_naming(bool enable)
 {
     /* But note we don't actually name them on Windows yet */
     name_threads = enable;

     fprintf(stderr, "qemu: thread naming not supported on this host\n");
 }

 static void error_exit(int err, const char *msg)
 {
     char *pstr;

     FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_ALLOCATE_BUFFER,
                   NULL, err, 0, (LPTSTR)&pstr, 2, NULL);
     fprintf(stderr, "qemu: %s: %s\n", msg, pstr);
     LocalFree(pstr);
     abort();
 }

 void qemu_mutex_init(QemuMutex *mutex)
 {
     mutex->owner = 0;
     InitializeCriticalSection(&mutex->lock);
 }

 void qemu_mutex_destroy(QemuMutex *mutex)
 {
     assert(mutex->owner == 0);
     DeleteCriticalSection(&mutex->lock);
 }

 void qemu_mutex_lock(QemuMutex *mutex)
 {
     EnterCriticalSection(&mutex->lock);

     /* Win32 CRITICAL_SECTIONs are recursive.  Assert that we're not
      * using them as such.
      */
     assert(mutex->owner == 0);
     mutex->owner = GetCurrentThreadId();
 }

 int qemu_mutex_trylock(QemuMutex *mutex)
 {
     int owned;

     owned = TryEnterCriticalSection(&mutex->lock);
     if (owned) {
         assert(mutex->owner == 0);
         mutex->owner = GetCurrentThreadId();
     }
     return !owned;
 }

 void qemu_mutex_unlock(QemuMutex *mutex)
 {
     assert(mutex->owner == GetCurrentThreadId());
     mutex->owner = 0;
     LeaveCriticalSection(&mutex->lock);
 }

 void qemu_rec_mutex_init(QemuRecMutex *mutex)
 {
     InitializeCriticalSection(&mutex->lock);
 }

 void qemu_rec_mutex_destroy(QemuRecMutex *mutex)
 {
     DeleteCriticalSection(&mutex->lock);
 }

 void qemu_rec_mutex_lock(QemuRecMutex *mutex)
 {
     EnterCriticalSection(&mutex->lock);
 }

 int qemu_rec_mutex_trylock(QemuRecMutex *mutex)
 {
     return !TryEnterCriticalSection(&mutex->lock);
 }

 void qemu_rec_mutex_unlock(QemuRecMutex *mutex)
 {
     LeaveCriticalSection(&mutex->lock);
 }

 void qemu_cond_init(QemuCond *cond)
 {
     memset(cond, 0, sizeof(*cond));

     cond->sema = CreateSemaphore(NULL, 0, LONG_MAX, NULL);
     if (!cond->sema) {
         error_exit(GetLastError(), __func__);
     }
     cond->continue_event = CreateEvent(NULL,    /* security */
                                        FALSE,   /* auto-reset */
                                        FALSE,   /* not signaled */
                                        NULL);   /* name */
     if (!cond->continue_event) {
         error_exit(GetLastError(), __func__);
     }
 }

 void qemu_cond_destroy(QemuCond *cond)
 {
     BOOL result;
     result = CloseHandle(cond->continue_event);
     if (!result) {
         error_exit(GetLastError(), __func__);
     }
     cond->continue_event = 0;
     result = CloseHandle(cond->sema);
     if (!result) {
         error_exit(GetLastError(), __func__);
     }
     cond->sema = 0;
 }

 void qemu_cond_signal(QemuCond *cond)
 {
     DWORD result;

     /*
      * Signal only when there are waiters.  cond->waiters is
      * incremented by pthread_cond_wait under the external lock,
      * so we are safe about that.
      */
     if (cond->waiters == 0) {
         return;
     }

     /*
      * Waiting threads decrement it outside the external lock, but
      * only if another thread is executing pthread_cond_broadcast and
      * has the mutex.  So, it also cannot be decremented concurrently
      * with this particular access.
      */
     cond->target = cond->waiters - 1;
     result = SignalObjectAndWait(cond->sema, cond->continue_event,
                                  INFINITE, FALSE);
     if (result == WAIT_ABANDONED || result == WAIT_FAILED) {
         error_exit(GetLastError(), __func__);
     }
 }

 void qemu_cond_broadcast(QemuCond *cond)
 {
     BOOLEAN result;
     /*
      * As in pthread_cond_signal, access to cond->waiters and
      * cond->target is locked via the external mutex.
      */
     if (cond->waiters == 0) {
         return;
     }

     cond->target = 0;
     result = ReleaseSemaphore(cond->sema, cond->waiters, NULL);
     if (!result) {
         error_exit(GetLastError(), __func__);
     }

     /*
      * At this point all waiters continue. Each one takes its
      * slice of the semaphore. Now it's our turn to wait: Since
      * the external mutex is held, no thread can leave cond_wait,
      * yet. For this reason, we can be sure that no thread gets
      * a chance to eat *more* than one slice. OTOH, it means
      * that the last waiter must send us a wake-up.
      */
     WaitForSingleObject(cond->continue_event, INFINITE);
 }

 void qemu_cond_wait(QemuCond *cond, QemuMutex *mutex)
 {
     /*
      * This access is protected under the mutex.
      */
     cond->waiters++;

     /*
      * Unlock external mutex and wait for signal.
      * NOTE: we've held mutex locked long enough to increment
      * waiters count above, so there's no problem with
      * leaving mutex unlocked before we wait on semaphore.
      */
     qemu_mutex_unlock(mutex);
     WaitForSingleObject(cond->sema, INFINITE);

     /* Now waiters must rendez-vous with the signaling thread and
      * let it continue.  For cond_broadcast this has heavy contention
      * and triggers thundering herd.  So goes life.
      *
      * Decrease waiters count.  The mutex is not taken, so we have
      * to do this atomically.
      *
      * All waiters contend for the mutex at the end of this function
      * until the signaling thread relinquishes it.  To ensure
      * each waiter consumes exactly one slice of the semaphore,
      * the signaling thread stops until it is told by the last
      * waiter that it can go on.
      */
     if (InterlockedDecrement(&cond->waiters) == cond->target) {
         SetEvent(cond->continue_event);
     }

     qemu_mutex_lock(mutex);
 }

 void qemu_sem_init(QemuSemaphore *sem, int init)
 {
     /* Manual reset.  */
     sem->sema = CreateSemaphore(NULL, init, LONG_MAX, NULL);
 }

 void qemu_sem_destroy(QemuSemaphore *sem)
 {
     CloseHandle(sem->sema);
 }

 void qemu_sem_post(QemuSemaphore *sem)
 {
     ReleaseSemaphore(sem->sema, 1, NULL);
 }

 int qemu_sem_timedwait(QemuSemaphore *sem, int ms)
 {
     int rc = WaitForSingleObject(sem->sema, ms);
     if (rc == WAIT_OBJECT_0) {
         return 0;
     }
     if (rc != WAIT_TIMEOUT) {
         error_exit(GetLastError(), __func__);
     }
     return -1;
 }

 void qemu_sem_wait(QemuSemaphore *sem)
 {
     if (WaitForSingleObject(sem->sema, INFINITE) != WAIT_OBJECT_0) {
         error_exit(GetLastError(), __func__);
     }
 }

 /* Wrap a Win32 manual-reset event with a fast userspace path.  The idea
  * is to reset the Win32 event lazily, as part of a test-reset-test-wait
  * sequence.  Such a sequence is, indeed, how QemuEvents are used by
  * RCU and other subsystems!
  *
  * Valid transitions:
  * - free->set, when setting the event
  * - busy->set, when setting the event, followed by futex_wake
  * - set->free, when resetting the event
  * - free->busy, when waiting
  *
  * set->busy does not happen (it can be observed from the outside but
  * it really is set->free->busy).
  *
  * busy->free provably cannot happen; to enforce it, the set->free transition
  * is done with an OR, which becomes a no-op if the event has concurrently
  * transitioned to free or busy (and is faster than cmpxchg).
  */

 #define EV_SET         0
 #define EV_FREE        1
 #define EV_BUSY       -1

 void qemu_event_init(QemuEvent *ev, bool init)
 {
     /* Manual reset.  */
     ev->event = CreateEvent(NULL, TRUE, TRUE, NULL);
     ev->value = (init ? EV_SET : EV_FREE);
 }

 void qemu_event_destroy(QemuEvent *ev)
 {
     CloseHandle(ev->event);
 }

 void qemu_event_set(QemuEvent *ev)
 {
     /* qemu_event_set has release semantics, but because it *loads*
      * ev->value we need a full memory barrier here.
      */
     smp_mb();
     if (atomic_read(&ev->value) != EV_SET) {
         if (atomic_xchg(&ev->value, EV_SET) == EV_BUSY) {
             /* There were waiters, wake them up.  */
             SetEvent(ev->event);
         }
     }
 }

 void qemu_event_reset(QemuEvent *ev)
 {
     unsigned value;

     value = atomic_read(&ev->value);
     smp_mb_acquire();
     if (value == EV_SET) {
         /* If there was a concurrent reset (or even reset+wait),
          * do nothing.  Otherwise change EV_SET->EV_FREE.
          */
         atomic_or(&ev->value, EV_FREE);
     }
 }

 void qemu_event_wait(QemuEvent *ev)
 {
     unsigned value;

     value = atomic_read(&ev->value);
     smp_mb_acquire();
     if (value != EV_SET) {
         if (value == EV_FREE) {
             /* qemu_event_set is not yet going to call SetEvent, but we are
              * going to do another check for EV_SET below when setting EV_BUSY.
              * At that point it is safe to call WaitForSingleObject.
              */
             ResetEvent(ev->event);

             /* Tell qemu_event_set that there are waiters.  No need to retry
              * because there cannot be a concurent busy->free transition.
              * After the CAS, the event will be either set or busy.
              */
             if (atomic_cmpxchg(&ev->value, EV_FREE, EV_BUSY) == EV_SET) {
                 value = EV_SET;
             } else {
                 value = EV_BUSY;
             }
         }
         if (value == EV_BUSY) {
             WaitForSingleObject(ev->event, INFINITE);
         }
     }
 }

 struct QemuThreadData {
     /* Passed to win32_start_routine.  */
     void             *(*start_routine)(void *);
     void             *arg;
     short             mode;
     NotifierList      exit;

     /* Only used for joinable threads. */
     bool              exited;
     void             *ret;
     CRITICAL_SECTION  cs;
 };

 static bool atexit_registered;
 static NotifierList main_thread_exit;

 static __thread QemuThreadData *qemu_thread_data;

 static void run_main_thread_exit(void)
 {
     notifier_list_notify(&main_thread_exit, NULL);
 }

 void qemu_thread_atexit_add(Notifier *notifier)
 {
     if (!qemu_thread_data) {
         if (!atexit_registered) {
             atexit_registered = true;
             atexit(run_main_thread_exit);
         }
         notifier_list_add(&main_thread_exit, notifier);
     } else {
         notifier_list_add(&qemu_thread_data->exit, notifier);
     }
 }

 void qemu_thread_atexit_remove(Notifier *notifier)
 {
     notifier_remove(notifier);
 }

 static unsigned __stdcall win32_start_routine(void *arg)
 {
     QemuThreadData *data = (QemuThreadData *) arg;
     void *(*start_routine)(void *) = data->start_routine;
     void *thread_arg = data->arg;

     qemu_thread_data = data;
     qemu_thread_exit(start_routine(thread_arg));
     abort();
 }

 void qemu_thread_exit(void *arg)
 {
     QemuThreadData *data = qemu_thread_data;

     notifier_list_notify(&data->exit, NULL);
     if (data->mode == QEMU_THREAD_JOINABLE) {
         data->ret = arg;
         EnterCriticalSection(&data->cs);
         data->exited = true;
         LeaveCriticalSection(&data->cs);
     } else {
         g_free(data);
     }
     _endthreadex(0);
 }

 void *qemu_thread_join(QemuThread *thread)
 {
     QemuThreadData *data;
     void *ret;
     HANDLE handle;

     data = thread->data;
     if (data->mode == QEMU_THREAD_DETACHED) {
         return NULL;
     }

     /*
      * Because multiple copies of the QemuThread can exist via
      * qemu_thread_get_self, we need to store a value that cannot
      * leak there.  The simplest, non racy way is to store the TID,
      * discard the handle that _beginthreadex gives back, and
      * get another copy of the handle here.
      */
     handle = qemu_thread_get_handle(thread);
     if (handle) {
         WaitForSingleObject(handle, INFINITE);
         CloseHandle(handle);
     }
     ret = data->ret;
     DeleteCriticalSection(&data->cs);
     g_free(data);
     return ret;
 }

 void qemu_thread_create(QemuThread *thread, const char *name,
                        void *(*start_routine)(void *),
                        void *arg, int mode)
 {
     HANDLE hThread;
     struct QemuThreadData *data;

     data = g_malloc(sizeof *data);
     data->start_routine = start_routine;
     data->arg = arg;
     data->mode = mode;
     data->exited = false;
     notifier_list_init(&data->exit);

     if (data->mode != QEMU_THREAD_DETACHED) {
         InitializeCriticalSection(&data->cs);
     }

     hThread = (HANDLE) _beginthreadex(NULL, 0, win32_start_routine,
                                       data, 0, &thread->tid);
     if (!hThread) {
         error_exit(GetLastError(), __func__);
     }
     CloseHandle(hThread);
     thread->data = data;
 }

 void qemu_thread_get_self(QemuThread *thread)
 {
     thread->data = qemu_thread_data;
     thread->tid = GetCurrentThreadId();
 }

 HANDLE qemu_thread_get_handle(QemuThread *thread)
 {
     QemuThreadData *data;
     HANDLE handle;

     data = thread->data;
     if (data->mode == QEMU_THREAD_DETACHED) {
         return NULL;
     }

     EnterCriticalSection(&data->cs);
     if (!data->exited) {
         handle = OpenThread(SYNCHRONIZE | THREAD_SUSPEND_RESUME, FALSE,
                             thread->tid);
     } else {
         handle = NULL;
     }
     LeaveCriticalSection(&data->cs);
     return handle;
 }

 bool qemu_thread_is_self(QemuThread *thread)
 {
     return GetCurrentThreadId() == thread->tid;
 }
	/*
	* Win32 implementation for mutex/cond/thread functions
	*
	* Copyright Red Hat, Inc. 2010
	*
	* Author:
	* Paolo Bonzini <pbonzini@redhat.com>
	*
	* This work is licensed under the terms of the GNU GPL, version 2 or later.
	* See the COPYING file in the top-level directory.
	*
	*/
	#include "qemu/osdep.h"
	#include "qemu-common.h"
	#include "qemu/thread.h"
	#include "qemu/notify.h"
	#include <process.h>

	static bool name_threads;

	void qemu_thread_naming(bool enable)
	{
	/* But note we don't actually name them on Windows yet */
	name_threads = enable;

	fprintf(stderr, "qemu: thread naming not supported on this host\n");
	}

	static void error_exit(int err, const char *msg)
	{
	char *pstr;

	FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM \| FORMAT_MESSAGE_ALLOCATE_BUFFER,
	NULL, err, 0, (LPTSTR)&pstr, 2, NULL);
	fprintf(stderr, "qemu: %s: %s\n", msg, pstr);
	LocalFree(pstr);
	abort();
	}

	void qemu_mutex_init(QemuMutex *mutex)
	{
	mutex->owner = 0;
	InitializeCriticalSection(&mutex->lock);
	}

	void qemu_mutex_destroy(QemuMutex *mutex)
	{
	assert(mutex->owner == 0);
	DeleteCriticalSection(&mutex->lock);
	}

	void qemu_mutex_lock(QemuMutex *mutex)
	{
	EnterCriticalSection(&mutex->lock);

	/* Win32 CRITICAL_SECTIONs are recursive. Assert that we're not
	* using them as such.
	*/
	assert(mutex->owner == 0);
	mutex->owner = GetCurrentThreadId();
	}

	int qemu_mutex_trylock(QemuMutex *mutex)
	{
	int owned;

	owned = TryEnterCriticalSection(&mutex->lock);
	if (owned) {
	assert(mutex->owner == 0);
	mutex->owner = GetCurrentThreadId();
	}
	return !owned;
	}

	void qemu_mutex_unlock(QemuMutex *mutex)
	{
	assert(mutex->owner == GetCurrentThreadId());
	mutex->owner = 0;
	LeaveCriticalSection(&mutex->lock);
	}

	void qemu_rec_mutex_init(QemuRecMutex *mutex)
	{
	InitializeCriticalSection(&mutex->lock);
	}

	void qemu_rec_mutex_destroy(QemuRecMutex *mutex)
	{
	DeleteCriticalSection(&mutex->lock);
	}

	void qemu_rec_mutex_lock(QemuRecMutex *mutex)
	{
	EnterCriticalSection(&mutex->lock);
	}

	int qemu_rec_mutex_trylock(QemuRecMutex *mutex)
	{
	return !TryEnterCriticalSection(&mutex->lock);
	}

	void qemu_rec_mutex_unlock(QemuRecMutex *mutex)
	{
	LeaveCriticalSection(&mutex->lock);
	}

	void qemu_cond_init(QemuCond *cond)
	{
	memset(cond, 0, sizeof(*cond));

	cond->sema = CreateSemaphore(NULL, 0, LONG_MAX, NULL);
	if (!cond->sema) {
	error_exit(GetLastError(), __func__);
	}
	cond->continue_event = CreateEvent(NULL, /* security */
	FALSE, /* auto-reset */
	FALSE, /* not signaled */
	NULL); /* name */
	if (!cond->continue_event) {
	error_exit(GetLastError(), __func__);
	}
	}

	void qemu_cond_destroy(QemuCond *cond)
	{
	BOOL result;
	result = CloseHandle(cond->continue_event);
	if (!result) {
	error_exit(GetLastError(), __func__);
	}
	cond->continue_event = 0;
	result = CloseHandle(cond->sema);
	if (!result) {
	error_exit(GetLastError(), __func__);
	}
	cond->sema = 0;
	}

	void qemu_cond_signal(QemuCond *cond)
	{
	DWORD result;

	/*
	* Signal only when there are waiters. cond->waiters is
	* incremented by pthread_cond_wait under the external lock,
	* so we are safe about that.
	*/
	if (cond->waiters == 0) {
	return;
	}

	/*
	* Waiting threads decrement it outside the external lock, but
	* only if another thread is executing pthread_cond_broadcast and
	* has the mutex. So, it also cannot be decremented concurrently
	* with this particular access.
	*/
	cond->target = cond->waiters - 1;
	result = SignalObjectAndWait(cond->sema, cond->continue_event,
	INFINITE, FALSE);
	if (result == WAIT_ABANDONED \|\| result == WAIT_FAILED) {
	error_exit(GetLastError(), __func__);
	}
	}

	void qemu_cond_broadcast(QemuCond *cond)
	{
	BOOLEAN result;
	/*
	* As in pthread_cond_signal, access to cond->waiters and
	* cond->target is locked via the external mutex.
	*/
	if (cond->waiters == 0) {
	return;
	}

	cond->target = 0;
	result = ReleaseSemaphore(cond->sema, cond->waiters, NULL);
	if (!result) {
	error_exit(GetLastError(), __func__);
	}

	/*
	* At this point all waiters continue. Each one takes its
	* slice of the semaphore. Now it's our turn to wait: Since
	* the external mutex is held, no thread can leave cond_wait,
	* yet. For this reason, we can be sure that no thread gets
	* a chance to eat more than one slice. OTOH, it means
	* that the last waiter must send us a wake-up.
	*/
	WaitForSingleObject(cond->continue_event, INFINITE);
	}

	void qemu_cond_wait(QemuCond cond, QemuMutex mutex)
	{
	/*
	* This access is protected under the mutex.
	*/
	cond->waiters++;

	/*
	* Unlock external mutex and wait for signal.
	* NOTE: we've held mutex locked long enough to increment
	* waiters count above, so there's no problem with
	* leaving mutex unlocked before we wait on semaphore.
	*/
	qemu_mutex_unlock(mutex);
	WaitForSingleObject(cond->sema, INFINITE);

	/* Now waiters must rendez-vous with the signaling thread and
	* let it continue. For cond_broadcast this has heavy contention
	* and triggers thundering herd. So goes life.
	*
	* Decrease waiters count. The mutex is not taken, so we have
	* to do this atomically.
	*
	* All waiters contend for the mutex at the end of this function
	* until the signaling thread relinquishes it. To ensure
	* each waiter consumes exactly one slice of the semaphore,
	* the signaling thread stops until it is told by the last
	* waiter that it can go on.
	*/
	if (InterlockedDecrement(&cond->waiters) == cond->target) {
	SetEvent(cond->continue_event);
	}

	qemu_mutex_lock(mutex);
	}

	void qemu_sem_init(QemuSemaphore *sem, int init)
	{
	/* Manual reset. */
	sem->sema = CreateSemaphore(NULL, init, LONG_MAX, NULL);
	}

	void qemu_sem_destroy(QemuSemaphore *sem)
	{
	CloseHandle(sem->sema);
	}

	void qemu_sem_post(QemuSemaphore *sem)
	{
	ReleaseSemaphore(sem->sema, 1, NULL);
	}

	int qemu_sem_timedwait(QemuSemaphore *sem, int ms)
	{
	int rc = WaitForSingleObject(sem->sema, ms);
	if (rc == WAIT_OBJECT_0) {
	return 0;
	}
	if (rc != WAIT_TIMEOUT) {
	error_exit(GetLastError(), __func__);
	}
	return -1;
	}

	void qemu_sem_wait(QemuSemaphore *sem)
	{
	if (WaitForSingleObject(sem->sema, INFINITE) != WAIT_OBJECT_0) {
	error_exit(GetLastError(), __func__);
	}
	}

	/* Wrap a Win32 manual-reset event with a fast userspace path. The idea
	* is to reset the Win32 event lazily, as part of a test-reset-test-wait
	* sequence. Such a sequence is, indeed, how QemuEvents are used by
	* RCU and other subsystems!
	*
	* Valid transitions:
	* - free->set, when setting the event
	* - busy->set, when setting the event, followed by futex_wake
	* - set->free, when resetting the event
	* - free->busy, when waiting
	*
	* set->busy does not happen (it can be observed from the outside but
	* it really is set->free->busy).
	*
	* busy->free provably cannot happen; to enforce it, the set->free transition
	* is done with an OR, which becomes a no-op if the event has concurrently
	* transitioned to free or busy (and is faster than cmpxchg).
	*/

	#define EV_SET 0
	#define EV_FREE 1
	#define EV_BUSY -1

	void qemu_event_init(QemuEvent *ev, bool init)
	{
	/* Manual reset. */
	ev->event = CreateEvent(NULL, TRUE, TRUE, NULL);
	ev->value = (init ? EV_SET : EV_FREE);
	}

	void qemu_event_destroy(QemuEvent *ev)
	{
	CloseHandle(ev->event);
	}

	void qemu_event_set(QemuEvent *ev)
	{
	/* qemu_event_set has release semantics, but because it loads
	* ev->value we need a full memory barrier here.
	*/
	smp_mb();
	if (atomic_read(&ev->value) != EV_SET) {
	if (atomic_xchg(&ev->value, EV_SET) == EV_BUSY) {
	/* There were waiters, wake them up. */
	SetEvent(ev->event);
	}
	}
	}

	void qemu_event_reset(QemuEvent *ev)
	{
	unsigned value;

	value = atomic_read(&ev->value);
	smp_mb_acquire();
	if (value == EV_SET) {
	/* If there was a concurrent reset (or even reset+wait),
	* do nothing. Otherwise change EV_SET->EV_FREE.
	*/
	atomic_or(&ev->value, EV_FREE);
	}
	}

	void qemu_event_wait(QemuEvent *ev)
	{
	unsigned value;

	value = atomic_read(&ev->value);
	smp_mb_acquire();
	if (value != EV_SET) {
	if (value == EV_FREE) {
	/* qemu_event_set is not yet going to call SetEvent, but we are
	* going to do another check for EV_SET below when setting EV_BUSY.
	* At that point it is safe to call WaitForSingleObject.
	*/
	ResetEvent(ev->event);

	/* Tell qemu_event_set that there are waiters. No need to retry
	* because there cannot be a concurent busy->free transition.
	* After the CAS, the event will be either set or busy.
	*/
	if (atomic_cmpxchg(&ev->value, EV_FREE, EV_BUSY) == EV_SET) {
	value = EV_SET;
	} else {
	value = EV_BUSY;
	}
	}
	if (value == EV_BUSY) {
	WaitForSingleObject(ev->event, INFINITE);
	}
	}
	}

	struct QemuThreadData {
	/* Passed to win32_start_routine. */
	void (start_routine)(void *);
	void *arg;
	short mode;
	NotifierList exit;

	/* Only used for joinable threads. */
	bool exited;
	void *ret;
	CRITICAL_SECTION cs;
	};

	static bool atexit_registered;
	static NotifierList main_thread_exit;

	static __thread QemuThreadData *qemu_thread_data;

	static void run_main_thread_exit(void)
	{
	notifier_list_notify(&main_thread_exit, NULL);
	}

	void qemu_thread_atexit_add(Notifier *notifier)
	{
	if (!qemu_thread_data) {
	if (!atexit_registered) {
	atexit_registered = true;
	atexit(run_main_thread_exit);
	}
	notifier_list_add(&main_thread_exit, notifier);
	} else {
	notifier_list_add(&qemu_thread_data->exit, notifier);
	}
	}

	void qemu_thread_atexit_remove(Notifier *notifier)
	{
	notifier_remove(notifier);
	}

	static unsigned __stdcall win32_start_routine(void *arg)
	{
	QemuThreadData data = (QemuThreadData ) arg;
	void (start_routine)(void *) = data->start_routine;
	void *thread_arg = data->arg;

	qemu_thread_data = data;
	qemu_thread_exit(start_routine(thread_arg));
	abort();
	}

	void qemu_thread_exit(void *arg)
	{
	QemuThreadData *data = qemu_thread_data;

	notifier_list_notify(&data->exit, NULL);
	if (data->mode == QEMU_THREAD_JOINABLE) {
	data->ret = arg;
	EnterCriticalSection(&data->cs);
	data->exited = true;
	LeaveCriticalSection(&data->cs);
	} else {
	g_free(data);
	}
	_endthreadex(0);
	}

	void qemu_thread_join(QemuThread thread)
	{
	QemuThreadData *data;
	void *ret;
	HANDLE handle;

	data = thread->data;
	if (data->mode == QEMU_THREAD_DETACHED) {
	return NULL;
	}

	/*
	* Because multiple copies of the QemuThread can exist via
	* qemu_thread_get_self, we need to store a value that cannot
	* leak there. The simplest, non racy way is to store the TID,
	* discard the handle that _beginthreadex gives back, and
	* get another copy of the handle here.
	*/
	handle = qemu_thread_get_handle(thread);
	if (handle) {
	WaitForSingleObject(handle, INFINITE);
	CloseHandle(handle);
	}
	ret = data->ret;
	DeleteCriticalSection(&data->cs);
	g_free(data);
	return ret;
	}

	void qemu_thread_create(QemuThread thread, const char name,
	void (start_routine)(void *),
	void *arg, int mode)
	{
	HANDLE hThread;
	struct QemuThreadData *data;

	data = g_malloc(sizeof *data);
	data->start_routine = start_routine;
	data->arg = arg;
	data->mode = mode;
	data->exited = false;
	notifier_list_init(&data->exit);

	if (data->mode != QEMU_THREAD_DETACHED) {
	InitializeCriticalSection(&data->cs);
	}

	hThread = (HANDLE) _beginthreadex(NULL, 0, win32_start_routine,
	data, 0, &thread->tid);
	if (!hThread) {
	error_exit(GetLastError(), __func__);
	}
	CloseHandle(hThread);
	thread->data = data;
	}

	void qemu_thread_get_self(QemuThread *thread)
	{
	thread->data = qemu_thread_data;
	thread->tid = GetCurrentThreadId();
	}

	HANDLE qemu_thread_get_handle(QemuThread *thread)
	{
	QemuThreadData *data;
	HANDLE handle;

	data = thread->data;
	if (data->mode == QEMU_THREAD_DETACHED) {
	return NULL;
	}

	EnterCriticalSection(&data->cs);
	if (!data->exited) {
	handle = OpenThread(SYNCHRONIZE \| THREAD_SUSPEND_RESUME, FALSE,
	thread->tid);
	} else {
	handle = NULL;
	}
	LeaveCriticalSection(&data->cs);
	return handle;
	}

	bool qemu_thread_is_self(QemuThread *thread)
	{
	return GetCurrentThreadId() == thread->tid;
	}