[concurrency-interest] How bad can volatile long++ be?

David Gallardo dgallardo at gmail.com
Mon Dec 10 13:49:42 EST 2007


++ is not atomic; while it may effectively be so on a single processor
machine, this is not the case on multiprocessor machines.

@D

On Dec 10, 2007 10:04 AM, Chris Purcell <chris.purcell.39 at gmail.com> wrote:
> I can't answer with authority whether ++ is atomic; I don't believe
> so. However, I can answer the rest of the question.
>
> The reads and writes will be atomic. Even so, the non-atomicity of the
> increment may still result in a wildly wrong number being produced.
>
> As an example, consider two threads incrementing a shared counter like
> this a thousand times each. If the counter starts at zero, in one
> unlikely but possible execution history, the final value of the
> counter may be only 2. That is, one thousand nine hundred and ninety
> eight increments can be lost. (The same final value is possible for
> any number of threads and any number of increments.)
>
> If you're only maintaining statistics, however, the inaccuracy of the
> counter shouldn't be an issue.
>
> Cheers,
> Chris
>
>
> On 12/10/07, Sam Berlin <sberlin at gmail.com> wrote:
> > Hi Folks,
> >
> > I'm fairly certain that ++ is not an atomic operation, even on
> > volatile variables, on longs (and quite possibly ints).  Given that is
> > true (which it very well might not be), is it suspectible to problems
> > where a wildly wrong number can be produced (due to different bytes
> > being updated at different times from different threads), or will it
> > just cause some increments to effectively not happen?
> >
> > Thanks!
> >  Sam
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