Re: [boost] boost::signal thread safety (implied question: is boost::function thread safe ?)

From: Timmo Stange (ts_at_[hidden])
Date: 2007-03-03 23:17:49

Steven Watanabe wrote:

>> Given the relative "distance" between this initialization code and a
>> possible access of either manager or invoker member in the vtable, I
>> think the problem is of a rather academical nature. But the code is
>> still not strictly correct from my understanding.
> 
> I'm afraid that it is a very real problem.  In pseudo-code
> 
> if(!vtable_initialized) {
>     stored_vtable_initialized = true;
>     stored_vtable.vtable_type(f);
> }
> if(stored_vtable.assign_to(f, functor)) vtable = &stored_vtable;
> else vtable = 0;
> 
> Even with a single CPU if the thread is
> interrupted immidiately after the assignment
> to stored_vtable_initialized, another thread
> can reach stored_vtable.assign_to(f, functor)
> before stored_vtable is initialized.

Right, that would be a legal implementation I just did not
consider.

This is a little sad since it could probably be easily fixed by
a compiler vendor if the standard would cover it (with a proper
memory model even for the SMP case). But as it is, this nice
little optimization probably has to go.

Regards

Timmo Stange