[dmd-concurrency] synchronized, shared, and regular methods inside the same class

[Kevin Bealer]

I read through a few papers on hazard pointers, and I still only know enough
gun safety to know that the gun is always loaded.  But I can't help but
ask...

What if this code:

void methodName(ref shared int a)
{
    do_stuff();

    int a1 = borrowShared(a);

     auto x = computeX(a1);
    auto y = computeY(a1);
    auto z = computeZ(a1);
    int xyz = combine(x, y, z);

    returnShared(a, xyz);

    more_stuff();
}

Was converted to this (borrowing Sean's example):

void methodName( ref shared int a )
{
   do_stuff();

   int temp1;
   do {
       int a1 = temp1 = a;

        auto x = computeX(a1);
       auto y = computeY(a1);
       auto z = computeZ(a1);
       a1 = combine(x, y, z);

   } while( !atomicStoreIf( a, a1, temp1 ) );  // atomicStoreIf is a CAS
   more_stuff();
}
It would only work for single variable modifications of course, and if
computeX,Y,Z is slow, it is probably pointless or a live-lock.  The user
would need to understand that this is really a loop, so maybe a different
syntax is needed:

 void methodName(ref shared int a)
{
    do_stuff();

    bool quit = false;

    forshare(a; a1; ! quit) {
         auto x = computeX(a1);
        auto y = computeY(a1);
        auto z = computeZ(a1);
         quit = /* iteration count test or something */;
        a1 = combine(x, y, z);
    }

    more_stuff();
}

I included the quit concept as an optional feature to remind users that this
might live-lock; they could of course say "if (...) break" at any time as
well.

(I'm sure there are some critics who will say that putting easy to reach
lock-free algorithm primitives next to simple to use language features is
like selling rubber snakes and real snakes from the same bin.)

Kevin
On Tue, Jan 5, 2010 at 6:58 PM, Andrei Alexandrescu <andrei at erdani.com>wrote:

> Kevin Bealer wrote:
>
>>  And I guess getting lock free systems right is normally
>> considered an "experts only" kind of thing, except maybe the one-pointer
>> versions like linked list.
>>
>
> Even lock-free slists are heinously difficult to define correctly...
>
> Andrei
>
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