Re: [OT] Win32 condition variable implementation

mark benedetto king <mbk@lowlatency.com> writes:

> On Wed, Sep 10, 2003 at 08:29:39AM -0400, mark benedetto king wrote:
>
> wait(cond,mutex):
> acquire(cond->mutex)
> cond->num_waiters++
> release(cond->mutex)
> release(mutex)
> down(cond->sem)
> up(cond->ack)
> acquire(mutex)
>
> deliver(cond,count):
> up(cond->sem, count)
> down(cond->ack, count)
> count->num_waiters -= count
>
> signal(cond):
> acquire(cond->mutex)
> deliver(cond, min(1, cond->num_waiters))
> release(cond->mutex)
>
> broadcast(cond):
> acquire(cond->mutex)
> deliver(cond, cond->num_waiters);
> release(cond->mutex)

I am not familiar with the sematics of the Win32 "counting
semaphores", your up/down in the algorithm above, but that algorithm
doesn't look right. What behaviour you are trying to achieve, are you
after the POSIX behaviour where broadcast wakes every waiting thread?
What about this scenario

  thread 1 waits
  thread 2 waits
  thread 3 broadcasts
  thread 1 wakes up and runs
  thread 1 waits again
  thread 1 wakes up and runs
  thread 1 waits again

The broadcast didn't wake thread 2. That could be enough to cause
deadlock if the application relied on POSIX behaviour.

-- 
Philip Martin
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