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Issue #15038: Document caveats with the emulated condition variables.

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Kristjan Valur Jonsson 2012-06-19 16:30:28 +00:00
parent 066dacf662
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29
Python/condvar.h
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@ -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;