Re: [boost] [Threads] uncoditional scoped lock?

From: Sohail Somani (s.somani_at_[hidden])
Date: 2007-01-12 13:41:53

I should have mentioned that one can't use wait(mtx,pred) because this
needs to be used in the signalling thread. It might be better to have
this as a scoped_lock constructor?

Anyway...

> -----Original Message-----
> From: Sohail Somani 
> Sent: Friday, January 12, 2007 10:39 AM
> To: 'boost_at_[hidden]'
> Subject: [Threads] uncoditional scoped lock?
> 
> 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