so, lock-free counter can be something like:

atomicIncr

| oldValue |

[
oldValue := self atomicSwapCounterWithDummy.
oldValue == dummy ] whileTrue: [ Processor yield ].

^ counter := oldValue + 1

atomicSwapCounterWithDummy

| old |

old := counter.

counter := dummy.

^ old

so you use dummy to block anyone from setting the new value unless you set it.

dummy can be any object (not a number in your care),

and lastly, once you assign a new value, you effectively release a "lock"
:)

Processor yield is mainly attempt to avoid busy-waiting.

On 3 April 2014 01:28, Igor Stasenko <siguctua@gmail.com> wrote:

On 3 April 2014 00:11, Sven Van Caekenberghe <sven@stfx.eu> wrote:

Hi,

Is it possible to have a simple lock-free atomic counter in Pharo 3.0 ?

nextId
�� ^ idCounter := idCounter + 1

Or is it still possible that two process entering this code can mess things up ?

#+ is a message send. So technically, if you will be interrupted at the point of
returning a result from it, then somebody else could run #nextId without notice,
as result you will get 2 processes returning same value from #nextId.

(and increasing numbers of concurrent processes will produce even more surprising results :)
��
I vaguely remember a discussion about that long ago...

assignment is atomic.. the one which i use in atomic queue impl., but it is hidden, undocumented VM implementation detail :) e.g.:

oldA := a.
a := newA.
..
actually can be any series of it, as long as nothing else there (no message sends)

x:= a.
y:=b.
z := c.
w := e.

will be performed atomically.

��
TIA,

Sven

--
Best regards,
Igor Stasenko.

--
Best regards,
Igor Stasenko.