Subj : Re: [.NET] Volatile Fields [correction]
To   : comp.programming.threads
From : Chris Thomasson
Date : Mon Sep 19 2005 11:33 am


"Cool Guy" <coolguy@abc.xyz> wrote in message 
news:1cgq3c5ccqz6r.dlg@cool.guy.abc.xyz...
> David Hopwood <david.nospam.hopwood@blueyonder.co.uk> wrote:
>
>> The write to Y is not performed as a single atomic event; it has to be 
>> modelled
>> as two events: "Y = 1 is performed with respect to processor 2" and "Y = 
>> 1 is
>> performed with respect to processor 3". So it can happen that processor 2 
>> sees
>> Y == 1 before writing Z = 1, but processor 3 does not see Y == 1 until 
>> after
>> it sees Z == 1.
>
> <snip>
>
> So it's impossible to make such code thread-safe, then?

This should do it for x86:

Processor 1:  Y = 1; L = 1;
Processor 2:  if (L == 1) { Z = 1; L = 2; }
Processor 3:  if (L == 2) { assert(Y == 1); }

.