Issue #8411: new condition variable emulation under Windows for the new GIL,
by Kristján. Unfortunately the 3.x Windows buildbots are in a wreck, so we'll have to watch them when they become fit again.
This commit is contained in:
Antoine Pitrou
parent
817c9df7e5
commit
e1dd1747e8
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@ -106,7 +106,6 @@ do { \
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#define COND_INIT(cond) \
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if (pthread_cond_init(&cond, NULL)) { \
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Py_FatalError("pthread_cond_init(" #cond ") failed"); };
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#define COND_RESET(cond)
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#define COND_SIGNAL(cond) \
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if (pthread_cond_signal(&cond)) { \
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Py_FatalError("pthread_cond_signal(" #cond ") failed"); };
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@ -141,64 +140,120 @@ do { \
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#include <windows.h>
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#define MUTEX_T HANDLE
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#define MUTEX_INIT(mut) \
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if (!(mut = CreateMutex(NULL, FALSE, NULL))) { \
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Py_FatalError("CreateMutex(" #mut ") failed"); };
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#define MUTEX_T CRITICAL_SECTION
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#define MUTEX_INIT(mut) do { \
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if (!(InitializeCriticalSectionAndSpinCount(&(mut), 4000))) \
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Py_FatalError("CreateMutex(" #mut ") failed"); \
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} while (0)
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#define MUTEX_FINI(mut) \
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DeleteCriticalSection(&(mut))
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#define MUTEX_LOCK(mut) \
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if (WaitForSingleObject(mut, INFINITE) != WAIT_OBJECT_0) { \
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Py_FatalError("WaitForSingleObject(" #mut ") failed"); };
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EnterCriticalSection(&(mut))
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#define MUTEX_UNLOCK(mut) \
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if (!ReleaseMutex(mut)) { \
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Py_FatalError("ReleaseMutex(" #mut ") failed"); };
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LeaveCriticalSection(&(mut))
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/* We emulate condition variables with events. It is sufficient here.
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WaitForMultipleObjects() allows the event to be caught and the mutex
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to be taken atomically.
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As for SignalObjectAndWait(), its semantics are unfortunately a bit
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more foggy. Many sources on the Web define it as atomically releasing
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the first object while starting to wait on the second, but MSDN states
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it is *not* atomic...
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/* We emulate condition variables with a semaphore.
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We use a Semaphore rather than an auto-reset event, because although
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an auto-resent event might appear to solve the lost-wakeup bug (race
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condition between releasing the outer lock and waiting) because it
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maintains state even though a wait hasn't happened, there is still
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a lost wakeup problem if more than one thread are interrupted in the
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critical place. A semaphore solves that.
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Because it is ok to signal a condition variable with no one
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waiting, we need to keep track of the number of
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waiting threads. Otherwise, the semaphore's state could rise
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without bound.
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In any case, the emulation here is tailored for our particular use case.
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For example, we don't care how many threads are woken up when a condition
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gets signalled. Generic emulations of the pthread_cond_* API using
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Generic emulations of the pthread_cond_* API using
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Win32 functions can be found on the Web.
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The following read can be edificating (or not):
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http://www.cse.wustl.edu/~schmidt/win32-cv-1.html
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*/
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#define COND_T HANDLE
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typedef struct COND_T
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{
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HANDLE sem; /* the semaphore */
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int n_waiting; /* how many are unreleased */
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} COND_T;
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__inline static void _cond_init(COND_T *cond)
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{
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/* A semaphore with a large max value, The positive value
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* is only needed to catch those "lost wakeup" events and
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* race conditions when a timed wait elapses.
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*/
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if (!(cond->sem = CreateSemaphore(NULL, 0, 1000, NULL)))
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Py_FatalError("CreateSemaphore() failed");
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cond->n_waiting = 0;
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}
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__inline static void _cond_fini(COND_T *cond)
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{
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BOOL ok = CloseHandle(cond->sem);
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if (!ok)
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Py_FatalError("CloseHandle() failed");
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}
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__inline static void _cond_wait(COND_T *cond, MUTEX_T *mut)
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{
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++cond->n_waiting;
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MUTEX_UNLOCK(*mut);
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/* "lost wakeup bug" would occur if the caller were interrupted here,
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* but we are safe because we are using a semaphore wich has an internal
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* count.
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*/
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if (WaitForSingleObject(cond->sem, INFINITE) == WAIT_FAILED)
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Py_FatalError("WaitForSingleObject() failed");
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MUTEX_LOCK(*mut);
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}
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__inline static int _cond_timed_wait(COND_T *cond, MUTEX_T *mut,
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int us)
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{
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DWORD r;
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++cond->n_waiting;
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MUTEX_UNLOCK(*mut);
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r = WaitForSingleObject(cond->sem, us / 1000);
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if (r == WAIT_FAILED)
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Py_FatalError("WaitForSingleObject() failed");
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MUTEX_LOCK(*mut);
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if (r == WAIT_TIMEOUT)
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--cond->n_waiting;
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/* Here we have a benign race condition with _cond_signal. If the
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* wait operation has timed out, but before we can acquire the
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* mutex again to decrement n_waiting, a thread holding the mutex
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* still sees a positive n_waiting value and may call
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* ReleaseSemaphore and decrement n_waiting.
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* This will cause n_waiting to be decremented twice.
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* This is benign, though, because ReleaseSemaphore will also have
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* been called, leaving the semaphore state positive. We may
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* thus end up with semaphore in state 1, and n_waiting == -1, and
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* the next time someone calls _cond_wait(), that thread will
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* pass right through, decrementing the semaphore state and
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* incrementing n_waiting, thus correcting the extra _cond_signal.
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*/
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return r == WAIT_TIMEOUT;
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}
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__inline static void _cond_signal(COND_T *cond) {
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/* NOTE: This must be called with the mutex held */
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if (cond->n_waiting > 0) {
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if (!ReleaseSemaphore(cond->sem, 1, NULL))
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Py_FatalError("ReleaseSemaphore() failed");
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--cond->n_waiting;
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}
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}
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#define COND_INIT(cond) \
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/* auto-reset, non-signalled */ \
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if (!(cond = CreateEvent(NULL, FALSE, FALSE, NULL))) { \
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Py_FatalError("CreateMutex(" #cond ") failed"); };
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#define COND_RESET(cond) \
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if (!ResetEvent(cond)) { \
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Py_FatalError("ResetEvent(" #cond ") failed"); };
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_cond_init(&(cond))
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#define COND_FINI(cond) \
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_cond_fini(&(cond))
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#define COND_SIGNAL(cond) \
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if (!SetEvent(cond)) { \
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Py_FatalError("SetEvent(" #cond ") failed"); };
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_cond_signal(&(cond))
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#define COND_WAIT(cond, mut) \
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{ \
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if (SignalObjectAndWait(mut, cond, INFINITE, FALSE) != WAIT_OBJECT_0) \
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Py_FatalError("SignalObjectAndWait(" #mut ", " #cond") failed"); \
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MUTEX_LOCK(mut); \
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}
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#define COND_TIMED_WAIT(cond, mut, microseconds, timeout_result) \
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{ \
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DWORD r; \
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HANDLE objects[2] = { cond, mut }; \
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MUTEX_UNLOCK(mut); \
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r = WaitForMultipleObjects(2, objects, TRUE, microseconds / 1000); \
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if (r == WAIT_TIMEOUT) { \
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MUTEX_LOCK(mut); \
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timeout_result = 1; \
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} \
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else if (r != WAIT_OBJECT_0) \
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Py_FatalError("WaitForSingleObject(" #cond ") failed"); \
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else \
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timeout_result = 0; \
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}
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_cond_wait(&(cond), &(mut))
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#define COND_TIMED_WAIT(cond, mut, us, timeout_result) do { \
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(timeout_result) = _cond_timed_wait(&(cond), &(mut), us); \
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} while (0)
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#else
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@ -282,7 +337,6 @@ static void drop_gil(PyThreadState *tstate)
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the GIL and drop it again, and reset the condition
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before we even had a chance to wait for it. */
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COND_WAIT(switch_cond, switch_mutex);
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COND_RESET(switch_cond);
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}
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MUTEX_UNLOCK(switch_mutex);
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}
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@ -301,7 +355,6 @@ static void take_gil(PyThreadState *tstate)
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if (!_Py_atomic_load_relaxed(&gil_locked))
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goto _ready;
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COND_RESET(gil_cond);
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while (_Py_atomic_load_relaxed(&gil_locked)) {
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int timed_out = 0;
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unsigned long saved_switchnum;
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