[Openmcl-devel] [openmcl-devel] atomic-incf

David L. Rager ragerdl at cs.utexas.edu
Mon Oct 19 03:41:13 UTC 2009


Great, thanks!

On Sun, Oct 18, 2009 at 7:09 PM, Gary Byers <gb at clozure.com> wrote:

>
>
> On Sun, 18 Oct 2009, David L. Rager wrote:
>
>  Hi,
>> One more hopefully quick question about atomic-incf/atomic-decf:
>>
>> Can the return value of an atomic-incf/decf be relied upon as the value
>> right after that particular incf occurrs?  Given it's a compare and swap,
>> I'm guessing that the answer to this question is "yes".
>>
>> Another way to ask the same question is, if I perform two atomic-incf's in
>> a
>> row (i.e., as simultaneously as the x86 platform will allow me), will one
>> of
>> the atomic-incf calls return the intermediate value, and the other
>> atomic-incf return the newest value?
>>
>> E.g.:
>> A long time ago, in some thread far far away:
>> (defvar *x* 3)
>>
>> In a current time, with two concurrent threads
>>
>> Thread A:
>> (atomic-incf *x*) ; sets *x* to 4, returns 4
>>
>> Thread B:
>> (atomic-incf *x*) ; sets *x* to 5, returns 5
>>
>> Of course if Thread B occurred before Thread A, the B transcript would
>> have
>> "sets *x* to 4, returns 4" and the A transcript would also change to use
>> the
>> number 5.
>>
>>
> Yes; after these two ATOMIC-INCFs complete, *X* will have the value 5,
> and the return value of each ATOMIC-INCF will be the value stored in
> the location at the time that it was successfully stored.  (That may
> or may not be the same as "the contents of *X* by the time that you
> can observe it.")
>
> The code behaves something like:
>
> (loop
>  (let* ((old-value (contents-of loc))
>         (new-value (1+ old-value))
>         (win nil))
>   (with-bus-locked
>     (when (= (contents-of loc) old-value)  ; this is just compare-and-swap
>       (setf (contents-of loc) new-value
>             win t)))
>   (if win (return new-value))))
>
> where WITH-BUS-LOCKED is supposed to suggest some sort of hardware-imposed
> atomicity and memory-ordering (the "lock" prefix on x86, cache-line
> reservations
> and memory barriers on PPC, etc.)
>
> This is a pretty obscure thing to note, but note that the compare-and-swap
> doesn't quite guarantee that the location hasn't changed: it's possible
> that
> it was incremented and decremented between the time it was read (at the top
> of the loop) and the time that it was tested by compare-and-swap.  In a lot
> of cases this doesn't matter; in some cases, it might.  (I think that
> that's
> known as the "ABA" problem, where a memory location's contents change from
> A to B and back to A; I know that it comes up, but don't remember canonical
> examples that demonstrate cases where it matters.)
>
>
>  Hopefully between my two versions of the question, my real question is
>> clear.
>>
>> Thanks,
>> David
>>
>>
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