Re: [Boost-users] Condition Variable and optimization

Subject: Re: [Boost-users] Condition Variable and optimization
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2011-12-15 13:56:11

AMDG

On 12/15/2011 10:29 AM, John M. Dlugosz wrote:
> The example in the documentation for Condition Variables illustrates the
> implication that these are Mesa-style (non-blocking signals) with this
> example:
> 

cond.wait() does block, but spurious wakeups
are allowed, hence the loop.

> boost::condition_variable  cond;
> boost::mutex  mut;
> bool  data_ready;
> 
> void  process_data();
> 
> void  wait_for_data_to_process()
> {
>     boost::unique_lock<boost::mutex>  lock(mut);
>     while(!data_ready)
>     {
>         cond.wait(lock);
>     }
>     process_data();
> }
> 
> 
> 
> This needs to declare data_ready as mutable, so the compiler doesn't
> reduce the while loop to a no-op and assume the value doesn't change, or
> just hold it in a register so it doesn't see the change made by another
> thread.
> 
> More generally, how do you write it in this case?
> 
>     while ( !collection.empty() )  cond.wait(lock);
> 
> where the condition is a function of an existing class?  The compiler
> might inline that function and give the same issue.
> 

If the compiler doesn't know anything about
condition variables, it will see an opaque
function call that can potentially clobber
any non-local variables.  If the compiler
does know about condition variables, then
it knows that it can't optimize like this.

> I think a form of wait that takes a predicate ought to be introduced,
> that prevents this particular issue and ensures correct use.
> 

It already exists.

In Christ,
Steven Watanabe