[cpp-threads] Yet another visibility question

[Boehm, Hans]

> From:  Ben Hutchings
> > ==>
> > "All modifications to a particular memory location L occur in some 
> > particular total order, referred to as the modification order for L.
> > An evaluation A that performs a synchronization operation that 
> > modifies location L synchronizes-with an evaluation B that 
> reads the 
> > value written by A, or reads a value that was subsequently (in 
> > modification
> > order) written to L.
> <snip>
> 
> This wrongly matches each write with *every* read of the same 
> value, when what I think we want is to match each read with a 
> single write of the same value.  Example:
> 
> Atomic variables x, y initialised to 0
> Thread 1: x = 1;
>           x = 0;
>           y = 1;
>           x = 1;
> Thread 2: while (!x);
>           assert(y);
> 
> By my reading of the new wording, the second assignment of 1 
> to x in thread 1 must synchronise with the evaluation of x in 
> thread 2 that reads 1, so that the assertion succeeds.  
> Really we expect that thread 2 might synchronise only with 
> the first assignment of 1 to x in thread 1, so that the 
> assertion can fail.
> 
> I would like to suggest the following wording:
> 
> "All modifications to a particular memory location L occur in 
> some particular total order, referred to as the modification 
> order for L. For any evaluation B that reads the value of L, 
> there is some modification A such that A writes the value 
> read by B and A and each modification of L that occurs 
> earlier in the modification order synchronizes-with B."
We certainly agree on the intent, and on the fact that I didn't state it
well.  Your restatement is better, but maybe still a little hard to
parse.  Maybe Clark can do better still?

> 
> (Is static initialisation counted as a modification?  If not, 
> there needs to be a special case for that.)
That's also a good point.  I think that's currently less clear than it
should be.

Hans

> 
> Ben.
> 
> --
> Ben Hutchings
> It is easier to write an incorrect program than to understand 
> a correct one.
>