xref: /third_party/python/Python/condvar.h (revision 7db96d56) 
1/*
2 * Portable condition variable support for windows and pthreads.
3 * Everything is inline, this header can be included where needed.
4 *
5 * APIs generally return 0 on success and non-zero on error,
6 * and the caller needs to use its platform's error mechanism to
7 * discover the error (errno, or GetLastError())
8 *
9 * Note that some implementations cannot distinguish between a
10 * condition variable wait time-out and successful wait. Most often
11 * the difference is moot anyway since the wait condition must be
12 * re-checked.
13 * PyCOND_TIMEDWAIT, in addition to returning negative on error,
14 * thus returns 0 on regular success, 1 on timeout
15 * or 2 if it can't tell.
16 *
17 * There are at least two caveats with using these condition variables,
18 * due to the fact that they may be emulated with Semaphores on
19 * Windows:
20 * 1) While PyCOND_SIGNAL() will wake up at least one thread, we
21 *    cannot currently guarantee that it will be one of the threads
22 *    already waiting in a PyCOND_WAIT() call.  It _could_ cause
23 *    the wakeup of a subsequent thread to try a PyCOND_WAIT(),
24 *    including the thread doing the PyCOND_SIGNAL() itself.
25 *    The same applies to PyCOND_BROADCAST(), if N threads are waiting
26 *    then at least N threads will be woken up, but not necessarily
27 *    those already waiting.
28 *    For this reason, don't make the scheduling assumption that a
29 *    specific other thread will get the wakeup signal
30 * 2) The _mutex_ must be held when calling PyCOND_SIGNAL() and
31 *    PyCOND_BROADCAST().
32 *    While e.g. the posix standard strongly recommends that the mutex
33 *    associated with the condition variable is held when a
34 *    pthread_cond_signal() call is made, this is not a hard requirement,
35 *    although scheduling will not be "reliable" if it isn't.  Here
36 *    the mutex is used for internal synchronization of the emulated
37 *    Condition Variable.
38 */
39
40#ifndef _CONDVAR_IMPL_H_
41#define _CONDVAR_IMPL_H_
42
43#include "Python.h"
44#include "pycore_condvar.h"
45
46#ifdef _POSIX_THREADS
47/*
48 * POSIX support
49 */
50
51/* These private functions are implemented in Python/thread_pthread.h */
52int _PyThread_cond_init(PyCOND_T *cond);
53void _PyThread_cond_after(long long us, struct timespec *abs);
54
55/* The following functions return 0 on success, nonzero on error */
56#define PyMUTEX_INIT(mut)       pthread_mutex_init((mut), NULL)
57#define PyMUTEX_FINI(mut)       pthread_mutex_destroy(mut)
58#define PyMUTEX_LOCK(mut)       pthread_mutex_lock(mut)
59#define PyMUTEX_UNLOCK(mut)     pthread_mutex_unlock(mut)
60
61#define PyCOND_INIT(cond)       _PyThread_cond_init(cond)
62#define PyCOND_FINI(cond)       pthread_cond_destroy(cond)
63#define PyCOND_SIGNAL(cond)     pthread_cond_signal(cond)
64#define PyCOND_BROADCAST(cond)  pthread_cond_broadcast(cond)
65#define PyCOND_WAIT(cond, mut)  pthread_cond_wait((cond), (mut))
66
67/* return 0 for success, 1 on timeout, -1 on error */
68Py_LOCAL_INLINE(int)
69PyCOND_TIMEDWAIT(PyCOND_T *cond, PyMUTEX_T *mut, long long us)
70{
71    struct timespec abs;
72    _PyThread_cond_after(us, &abs);
73    int ret = pthread_cond_timedwait(cond, mut, &abs);
74    if (ret == ETIMEDOUT) {
75        return 1;
76    }
77    if (ret) {
78        return -1;
79    }
80    return 0;
81}
82
83#elif defined(NT_THREADS)
84/*
85 * Windows (XP, 2003 server and later, as well as (hopefully) CE) support
86 *
87 * Emulated condition variables ones that work with XP and later, plus
88 * example native support on VISTA and onwards.
89 */
90
91#if _PY_EMULATED_WIN_CV
92
93/* The mutex is a CriticalSection object and
94   The condition variables is emulated with the help of a semaphore.
95
96   This implementation still has the problem that the threads woken
97   with a "signal" aren't necessarily those that are already
98   waiting.  It corresponds to listing 2 in:
99   http://birrell.org/andrew/papers/ImplementingCVs.pdf
100
101   Generic emulations of the pthread_cond_* API using
102   earlier Win32 functions can be found on the web.
103   The following read can be give background information to these issues,
104   but the implementations are all broken in some way.
105   http://www.cse.wustl.edu/~schmidt/win32-cv-1.html
106*/
107
108Py_LOCAL_INLINE(int)
109PyMUTEX_INIT(PyMUTEX_T *cs)
110{
111    InitializeCriticalSection(cs);
112    return 0;
113}
114
115Py_LOCAL_INLINE(int)
116PyMUTEX_FINI(PyMUTEX_T *cs)
117{
118    DeleteCriticalSection(cs);
119    return 0;
120}
121
122Py_LOCAL_INLINE(int)
123PyMUTEX_LOCK(PyMUTEX_T *cs)
124{
125    EnterCriticalSection(cs);
126    return 0;
127}
128
129Py_LOCAL_INLINE(int)
130PyMUTEX_UNLOCK(PyMUTEX_T *cs)
131{
132    LeaveCriticalSection(cs);
133    return 0;
134}
135
136
137Py_LOCAL_INLINE(int)
138PyCOND_INIT(PyCOND_T *cv)
139{
140    /* A semaphore with a "large" max value,  The positive value
141     * is only needed to catch those "lost wakeup" events and
142     * race conditions when a timed wait elapses.
143     */
144    cv->sem = CreateSemaphore(NULL, 0, 100000, NULL);
145    if (cv->sem==NULL)
146        return -1;
147    cv->waiting = 0;
148    return 0;
149}
150
151Py_LOCAL_INLINE(int)
152PyCOND_FINI(PyCOND_T *cv)
153{
154    return CloseHandle(cv->sem) ? 0 : -1;
155}
156
157/* this implementation can detect a timeout.  Returns 1 on timeout,
158 * 0 otherwise (and -1 on error)
159 */
160Py_LOCAL_INLINE(int)
161_PyCOND_WAIT_MS(PyCOND_T *cv, PyMUTEX_T *cs, DWORD ms)
162{
163    DWORD wait;
164    cv->waiting++;
165    PyMUTEX_UNLOCK(cs);
166    /* "lost wakeup bug" would occur if the caller were interrupted here,
167     * but we are safe because we are using a semaphore which has an internal
168     * count.
169     */
170    wait = WaitForSingleObjectEx(cv->sem, ms, FALSE);
171    PyMUTEX_LOCK(cs);
172    if (wait != WAIT_OBJECT_0)
173        --cv->waiting;
174        /* Here we have a benign race condition with PyCOND_SIGNAL.
175         * When failure occurs or timeout, it is possible that
176         * PyCOND_SIGNAL also decrements this value
177         * and signals releases the mutex.  This is benign because it
178         * just means an extra spurious wakeup for a waiting thread.
179         * ('waiting' corresponds to the semaphore's "negative" count and
180         * we may end up with e.g. (waiting == -1 && sem.count == 1).  When
181         * a new thread comes along, it will pass right through, having
182         * adjusted it to (waiting == 0 && sem.count == 0).
183         */
184
185    if (wait == WAIT_FAILED)
186        return -1;
187    /* return 0 on success, 1 on timeout */
188    return wait != WAIT_OBJECT_0;
189}
190
191Py_LOCAL_INLINE(int)
192PyCOND_WAIT(PyCOND_T *cv, PyMUTEX_T *cs)
193{
194    int result = _PyCOND_WAIT_MS(cv, cs, INFINITE);
195    return result >= 0 ? 0 : result;
196}
197
198Py_LOCAL_INLINE(int)
199PyCOND_TIMEDWAIT(PyCOND_T *cv, PyMUTEX_T *cs, long long us)
200{
201    return _PyCOND_WAIT_MS(cv, cs, (DWORD)(us/1000));
202}
203
204Py_LOCAL_INLINE(int)
205PyCOND_SIGNAL(PyCOND_T *cv)
206{
207    /* this test allows PyCOND_SIGNAL to be a no-op unless required
208     * to wake someone up, thus preventing an unbounded increase of
209     * the semaphore's internal counter.
210     */
211    if (cv->waiting > 0) {
212        /* notifying thread decreases the cv->waiting count so that
213         * a delay between notify and actual wakeup of the target thread
214         * doesn't cause a number of extra ReleaseSemaphore calls.
215         */
216        cv->waiting--;
217        return ReleaseSemaphore(cv->sem, 1, NULL) ? 0 : -1;
218    }
219    return 0;
220}
221
222Py_LOCAL_INLINE(int)
223PyCOND_BROADCAST(PyCOND_T *cv)
224{
225    int waiting = cv->waiting;
226    if (waiting > 0) {
227        cv->waiting = 0;
228        return ReleaseSemaphore(cv->sem, waiting, NULL) ? 0 : -1;
229    }
230    return 0;
231}
232
233#else /* !_PY_EMULATED_WIN_CV */
234
235Py_LOCAL_INLINE(int)
236PyMUTEX_INIT(PyMUTEX_T *cs)
237{
238    InitializeSRWLock(cs);
239    return 0;
240}
241
242Py_LOCAL_INLINE(int)
243PyMUTEX_FINI(PyMUTEX_T *cs)
244{
245    return 0;
246}
247
248Py_LOCAL_INLINE(int)
249PyMUTEX_LOCK(PyMUTEX_T *cs)
250{
251    AcquireSRWLockExclusive(cs);
252    return 0;
253}
254
255Py_LOCAL_INLINE(int)
256PyMUTEX_UNLOCK(PyMUTEX_T *cs)
257{
258    ReleaseSRWLockExclusive(cs);
259    return 0;
260}
261
262
263Py_LOCAL_INLINE(int)
264PyCOND_INIT(PyCOND_T *cv)
265{
266    InitializeConditionVariable(cv);
267    return 0;
268}
269Py_LOCAL_INLINE(int)
270PyCOND_FINI(PyCOND_T *cv)
271{
272    return 0;
273}
274
275Py_LOCAL_INLINE(int)
276PyCOND_WAIT(PyCOND_T *cv, PyMUTEX_T *cs)
277{
278    return SleepConditionVariableSRW(cv, cs, INFINITE, 0) ? 0 : -1;
279}
280
281/* This implementation makes no distinction about timeouts.  Signal
282 * 2 to indicate that we don't know.
283 */
284Py_LOCAL_INLINE(int)
285PyCOND_TIMEDWAIT(PyCOND_T *cv, PyMUTEX_T *cs, long long us)
286{
287    return SleepConditionVariableSRW(cv, cs, (DWORD)(us/1000), 0) ? 2 : -1;
288}
289
290Py_LOCAL_INLINE(int)
291PyCOND_SIGNAL(PyCOND_T *cv)
292{
293     WakeConditionVariable(cv);
294     return 0;
295}
296
297Py_LOCAL_INLINE(int)
298PyCOND_BROADCAST(PyCOND_T *cv)
299{
300     WakeAllConditionVariable(cv);
301     return 0;
302}
303
304
305#endif /* _PY_EMULATED_WIN_CV */
306
307#endif /* _POSIX_THREADS, NT_THREADS */
308
309#endif /* _CONDVAR_IMPL_H_ */
310