From: Jerry Lawson (Jerry.Lawson_at_[hidden])
Date: 2007-01-12 14:26:46
I would think the best way to synchronize this type of operation, where you
want all the threads to "wait until it we are all ready" is via
boost::barrier.
> -----Original Message-----
> From: boost-bounces_at_[hidden]
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Sohail Somani
> Sent: Friday, January 12, 2007 12:39 PM
> To: boost_at_[hidden]
> Subject: [boost] [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
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