Just for the sake of discussion, you try to prevent interruptions by using assignments, right ? But you still need #== which seems like a (potential) message send, which brings us back to the other arguments in this thread. Furthermore, the dummy value must be different for each of the processes/threads entering, no ?
On 03 Apr 2014, at 01:34, Igor Stasenko <siguc...@gmail.com> wrote: > 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 <siguc...@gmail.com> wrote: > > > > On 3 April 2014 00:11, Sven Van Caekenberghe <s...@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.
