Subj : Re: [.NET] Volatile Fields [correction]
To   : comp.programming.threads
From : Sean Kelly
Date : Mon Sep 19 2005 05:49 pm

Cool Guy wrote:
> 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?

Simply by following the (sparsely) documented x86 memory model?  Yes.
Though Alexander suggested a method that should work on existing CPUs:
replace load instructions with CMPXCHG.  This relies on a comment in
the docs for CMPXCHG that says a write is always performed when this
instruction is executed, even if the value is not changed as a result
of the operation.  Hardware folks may argue that this is an
implementation detail however (as Andy Glew did), so future x86 CPUs
may not guarantee the same behavior.


Sean

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