Re: [boost] Re: Threads: Atomic Counter

From: Valentin Bonnard (Bonnard.V_at_[hidden])
Date: 2000-08-31 22:15:37

On Thu, Aug 31, 2000 at 12:30:22PM -0400, David Abrahams wrote:
> 
> ----- Original Message -----
> From: "William Kempf" <sirwillard_at_[hidden]>
> > > The following Win32 functions don't work right on Win95:
> > >
> > > InterlockedCompareExchange
> > > InterlockedCompareExchangePointer
> > > InterlockedExchangeAdd
> >
> > But they do on Win98.  Tricky issue here.  I included the
> > compare_exchange() only to bring up debate about its usefulness.
> > I've never found a need for them, but didn't want to eliminate them
> > just because I've never needed them, nor because Win95 didn't support
> > them.  Obviously they can be added to Win95 in the form of some
> > assembler instructions.
> 
> Just an additional data point: if I remember correctly, the Motorola 680x0,
> where x > 2, includes atomic instructions for linking list elements in/out
> of singly- and doubly-linked lists! These are called something like
> "Atomic-compare-and-exchange" IIRC.

Insert in singly linked list:

inserted->next = pos->next;
pos->next = inserted;

This can't be done atomically, even which get_and_set (but each assignment 
can be done atomically)

Remove from singly linked list:

// at this point removed_prev.next == removed
removed_prev->next = removed->next;

MOVE (next,removed), (next,removed_prev)

(Atomic on every 68k)

Remove from doubly linked list:

removed->prev->next = removed->next;
removed->next->prev = removed->prev;

IMO this can't be done atomically (but each assignment can be done
atomically).

The 68000 also has CAS:

if (*p == v)
  *p = u;
else
  v = *p;

that I have used to implement a lock.

-- 
Valentin Bonnard