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| -rw-r--r-- | Python/condvar.h | 29 |
1 files changed, 29 insertions, 0 deletions
diff --git a/Python/condvar.h b/Python/condvar.h index 29641654c5..fe6bd74da8 100644 --- a/Python/condvar.h +++ b/Python/condvar.h @@ -13,6 +13,28 @@ * PyCOND_TIMEDWAIT, in addition to returning negative on error, * thus returns 0 on regular success, 1 on timeout * or 2 if it can't tell. + * + * There are at least two caveats with using these condition variables, + * due to the fact that they may be emulated with Semaphores on + * Windows: + * 1) While PyCOND_SIGNAL() will wake up at least one thread, we + * cannot currently guarantee that it will be one of the threads + * already waiting in a PyCOND_WAIT() call. It _could_ cause + * the wakeup of a subsequent thread to try a PyCOND_WAIT(), + * including the thread doing the PyCOND_SIGNAL() itself. + * The same applies to PyCOND_BROADCAST(), if N threads are waiting + * then at least N threads will be woken up, but not necessarily + * those already waiting. + * For this reason, don't make the scheduling assumption that a + * specific other thread will get the wakeup signal + * 2) The _mutex_ must be held when calling PyCOND_SIGNAL() and + * PyCOND_BROADCAST(). + * While e.g. the posix standard strongly recommends that the mutex + * associated with the condition variable is held when a + * pthread_cond_signal() call is made, this is not a hard requirement, + * although scheduling will not be "reliable" if it isn't. Here + * the mutex is used for internal synchronization of the emulated + * Condition Variable. */ #ifndef _CONDVAR_H_ @@ -134,10 +156,17 @@ PyCOND_TIMEDWAIT(PyCOND_T *cond, PyMUTEX_T *mut, long us) without bound. This also helps reduce the number of "spurious wakeups" that would otherwise happen. + This implementation still has the problem that the threads woken + with a "signal" aren't necessarily those that are already + waiting. It corresponds to listing 2 in: + http://birrell.org/andrew/papers/ImplementingCVs.pdf + Generic emulations of the pthread_cond_* API using earlier Win32 functions can be found on the Web. The following read can be edificating (or not): http://www.cse.wustl.edu/~schmidt/win32-cv-1.html + + See also */ typedef CRITICAL_SECTION PyMUTEX_T; |