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.