Re: what the benefit is by using annotation, like "@Immutable" ?
Andreas Leitgeb wrote:
Lew <noone@lewscanon.com> wrote:
Tom Anderson wrote:
To make this method safe, you either have to synchronize the whole thing,
or do the update of calculated and code atomically at the end.
Andreas Leitgeb wrote:
I'd have expected that if "calculated" was assigned *after* "code", that
would suffice without further synchronisation or volatile-ity of the fields.
Am I still too naive?
Yes. Don't omit what tom said about /happens-before/:
In fact, it's worse than that. Thread A could finish the method and update
both calculated and code, but because there is no happens-before relationship
between thread A and thread B, it's possible that B could come along later,
and see the updated calculated but *not* the updated code. So even without
an unlucky timeslice end, there is no guarantee of safety here.
There is still a misunderstanding - I'm just not sure if it's on my or your
side.
Thread 1 assigns two plain word-sized fields: a and then b.
can Thread 2 happen to see b's new value, and (after that) a's old value?
Tom's explanation was (as far as I understood it) based on the code-sample
where the flag was set before the code, and he rightly pointed out that this
gap may be even much longer than expected. Can it reverse, as well?
Depends on the memory order rules in the hardware. I'm most familiar
with the rules for SPARC v9, but other processors have their own rules.
In Total Store Order they cannot reverse. In Relaxed Memory Order and
Partial Store Order, they can reverse, from the point of view of other
processors, unless some action such as a membar #StoreStore forces ordering.
See
http://en.wikipedia.org/wiki/Memory_ordering#In_SMP_microprocessor_systems
Patricia
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