[m-dev.] thread safe mutable updates

[Ralph Becket]

Julien Fischer, Monday, 14 August 2006:
> 
> Peter Wang recently pointed at that the current "thread_safe" mutables
> are really not; further more there is currently no way of atomically
> updating the value of mutable based on its present value.
> 
> e.g. the following cannot be carried out as an atomic operation.
> 
> 	...
> 	get_varname(X0, !IO),
> 	X = Transform(X0),
> 	set_varname(X, !IO),
> 	...
> 
> The problem with the above is that it is possible for another thread to
> change the value of varname while Transform is being executed.
> 
> I have just posted a diff that removes the existing (broken) thread_safe
> mutable support.  Peter and I have discussed two proposals for extending
> mutables in the presence of threads:
> 
> (1) add a `thread_local' attribute that allows for (optional) thread-local
>     mutables.
> 
> (2) for mutables that are truly global (and hence potentially shared
>     between threads) add a new mutable operation `update' that allows
>     the value of the mutable to be atomically updated.

What about (3) just require users to use a mutex to gain access to a mutable
with transaction semantics?

I think that is cleaner than (1) or (2).

-- Ralph
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