On 4 April 2014 18:31, Sven Van Caekenberghe <s...@stfx.eu> wrote: > 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 ? > > dummy is placeholder used solely to detect that counter is 'locked' via #== comparison.. can be any object not really matters if you share it among processes or not, since it carries no state.
#== , whileTrue: .. as well as any other message sends potentially is subject of being interrupted.. but there's nothing wrong with it, as long as two assignments (in a row) don't have chance to be interrupted. > 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. > > > -- Best regards, Igor Stasenko.
