Subj : Re: Memory visibility and MS Interlocked instructions
To   : comp.programming.threads
From : David Schwartz
Date : Thu Aug 25 2005 11:43 am


"John Hickin" <hickin@nortelnetworks.com> wrote in message 
news:dekva7$p36$1@zcars129.ca.nortel.com...

>   LONG volatile fValueHasBeenComputedState = 0;
>
>    void CacheComputedValue()
>    {
>      switch (InterlockedExchangeAdd(&fValueHasBeenComputedState,(LONG)1))
>      {
>        case 0;
>           iValue = ComputeValue();
>           InterlockedExchange(&fValueHasBeenComputedState, 2);
>           break;
>        case 1:
> 
> while(InterlockedExchangeAdd(&fValueHasBeenComputedState,(LONG)0)
> == (LONG)1) { }
>           // fall through
>        default:
>           // nothing to do
>           ;
>      }
>    }

    This is a disaster on a machine with more than 2 CPUs or 
hyper-threading. In the more than 2 CPUs case, two CPUs in the 'while' loop 
will totally saturate the FSB, causing the CPU that needs to release the 
lock to be unable to do so. In the hyper-threaded case, the virtual CPU in 
the while loop will steal resources from the thread that's doing useful 
work.

    DS

.