[dmd-concurrency] shared arrays

[Steve Schveighoffer]

----- Original Message ----

> From: Sean Kelly <sean at invisibleduck.org>
> On Jan 14, 2010, at 4:10 AM, Steve Schveighoffer wrote:
> 
> > Having implemented the array append patch to fix stomping, and reading the 
> dmd-concurrency debate, I realized that what I implemented is no good for shared 
> arrays.
> > 
> > In fact, I wonder how shared arrays can support any array operations
> 
> I sent an email about this a while back.  In short, if we're going to allow 
> array ops on shared arrays at all I think they'll have to use atomic ops to 
> "lock" the array for the length of the update.  Basically, set the 1 bit of the 
> ptr field to indicate the array is locked.  This gets tricky when more than one 
> shared array is involved because the lock has to be acquired on each, and 
> because different ops may try to lock arrays in different orders, if a spinlock 
> acquire times out the code will have to release all the locks it's acquired, 
> wait some random interval, and try again.  Pretty complicated stuff if the goal 
> is just to support shared array ops.

This does not work:

shared(char)[] x;

void foo()
{
   shared(char)[] y = x;
   y[] = "hi"; // how does this now lock the 1 bit in x?
}

You'd need to lock the array data itself.  But in any case, I think it may be too ideal to think we can support atomic array operations without manual synchronization.  I.e., a SynchronizedArray type similar to UniqueArray type may be required for this.

I was thinking more in the case, how to write the runtime so it handles atomic updates of the actual array data, meaning at least word-sized data isn't messed up.  I have no idea if things like this should be possible on shared arrays:

a[] = b[] + 5 + c[];

The simple case of data copying may be acceptable, since you simply want to do a shared read of each data item, and a shared write to the target.

But in any case, I don't know how to handle cases where data is a hybrid -- some data shared and some not shared.

-Steve