Subj : Re: Win32 condition variables redux
To   : comp.programming.threads
From : Charles Bryant
Date : Wed Aug 03 2005 11:38 pm

In article <59GdnTUDgpKpuWzfRVn-2A@comcast.com>,
Chris Thomasson <_no_damn_spam_cristom@_no_damn_comcast.net_spam> wrote:
>
>An event object per-thread and linked-lists for waitsets can create a 
>"fairly efficient" and correct condvar for windows.
...
>Something like this:
>
><pseudo-code with timeout logic>

skip down to:

>void condvar_broadcast( condvar_t *_this )
...
>    LeaveCriticalSection( &_this->mutex );
>
>    while ( thread )
>    {
>      next = thread->next;

Note that at this point we don't hold any mutex

while in:

>void prv_condvar_pop( condvar_t *_this, per_thread_t *thread )
>{
...
>  thread->next = 0;
>  thread->prev = 0;
>}

called from

>int prv_condvar_timeout( condvar_t *_this, per_thread_t *thread )
>{
>  int err = 0, wait = 0;
>  LONG count = 0;
>  EnterCriticalSection( &_this->mutex );
>  if ( thread->state ) {
>    prv_condvar_pop( _this, thread );


So if a thread gets a timeout just as a condvar_broadcast() is running
through the list of threads, then the thread may corrupt the list of
threads to wake, leading to threads never being woken.

To test, use this scenario:


Modify the code to insert 'Sleep(10 seconds)' just after the
	LeaveCriticalSection in condvar_broadcast()

Run the following:
	Start N threads which do condvar_timedwait() with no timeout.
	Sleep one second to ensure that they have gone asleep
	Start a thread which does condvar_timedwait() with a timeout
		of 4 seconds
	wait 1 second
	Start N threads which do condvar_timedwait() with no timeout.
	wait 1 second
	perform condvar_broadcast()
	wait 15 seconds

	check how many threads were woken. It should be 2N+1, but I
	believe it will be N+1

.