Re: [rvm-core] JMM: Questions about PPC-32, Magic and sun.misc.Unsafe

[David P G. <gr...@us...>]

"Thomas Preud'homme" <ro...@ce...> wrote on 04/12/2013 08:43:55 AM:
>
> I indeed understood you wrong. So instead of a remark I have a question.
Is
> the code proposed supposed to work even in presence of a word store in
the
> second word? Because then if the reservation granule is exactly
> word-size (the
> documentation only says it's a power of two at least the size of a word)
> nothing would detect the (stw) write to the second word. The following
> sequence would then be possible:
>
> lwarx // first word
> lwz // second word
> stw // second word
> stwcx // first word, succeed.
>
> If not (the only writes to second word are 64 bits write to the same 2
words
> in the same order), then indeed, it depends on the ordering and I didn't
find
> anything about this in the documentation.
>

This is exactly the scenario I was thinking of (but did not articulate as
clearly).  In practice, the reservation granule is often a cache line. So
if the two words straddle a cache-line boundary (which they could if there
are not 8 byte aligned), I think this sequence could happen, resulting in a
non-atomic read.

--dave