[boost] [Threads] uncoditional scoped lock?

From: Sohail Somani (s.somani_at_[hidden])
Date: 2007-01-12 13:39:28

Hi,

Somewhat longer-than-normal-noise follows :)

Consider the following pseudo-code which has a race condition:

condition g_some_cond;
mutex g_condition_mtx;

void thread_fn()
{
  {
    ... // get lock
    g_some_cond.wait(condition_lock);
  }
  // do work
}

void main_thread()
{
  thread_group grp;
  // Start all threads and have them wait on the condition
  for(i in [0,N)) grp.create_thread(&thread_fn);

  {
    ... // get lock
    g_some_cond.notify_all(); // let threads continue
  }
}

The race condition is of course that it is possible that not all threads
actually start waiting on the condition before notify_all() is called.
In this case, it is possible to solve the race condition in two ways: 

 1) Have the last thread that reaches the condition wait, signal to the
main thread
 2) Have the main thread checking the condition before notify_all()

I don't really like the first case because it requires bi-directional
signalling between the threads. Though in my ignorance, I really do
consider it the correct solution. The second solution would look
something like this:

while(true)
{
  scoped_lock cond_lock(g_condition_mtx);
  // .num_waiting_threads() is made up
  if(N_EXPECTED_THREADS==g_some_cond.num_waiting_threads()){break;}
  // release lock
}

Should the second pattern be encapsulated in the condition object
somehow? It is almost like timed_wait(mtx,xt,pred) but there is no
timeout. So maybe untimed_wait()? Is there another way to do this?

Thanks for your eyes!

Sohail