hi! is it safe to use `TransferMode.UNSAFE` when we properly synchronise everything using posix mute...

# kotlin-native

[lukaville]

hi! is it safe to use TransferMode.UNSAFE when we properly synchronise everything using posix mutexes and we don’t have references to the objects in the passing worker?

[lukaville]

for example here:

val worker = Worker.start()
    worker.execute(
        mode = TransferMode.UNSAFE,
        producer = { Unit },
        job = {
            val a = mutableListOf<Int>()
            val worker2 = Worker.start()
            worker2.execute(
                mode = TransferMode.UNSAFE,
                producer = { a },
                job = { list ->
                    sleep(1)
                    GC.collect()
                    sleep(1)
                    list.add(1)
                    print(list)
                }
            )
        }
    )

is it guaranteed that list a will not be GCed after worker1 finished?

[kpgalligan]

Why would you?

[kpgalligan]

The safe/unsafe is about the producer and Unit

[kpgalligan]

I’d say “yes” but would want to double check and don’t have time today…

[kpgalligan]

Oh wait, the second unsafe no way

[kpgalligan]

The outer one probably wouldn’t matter

[lukaville]

so it means that list a will be collected by GC when the outer worker is finished?

[kpgalligan]

No. The GC doesn’t really work the way you think it does. Reference counting on non-frozen state is not atomic. You’ll have memory crashes and/or leaks (potentially).

[kpgalligan]

https://medium.com/@kpgalligan/kotlin-native-stranger-threads-ep-1-1ccccdfe0c99

[kpgalligan]

https://medium.com/@kpgalligan/kotlin-native-stranger-threads-ep-2-208523d63c8f

[kpgalligan]

I talk about unsafe in those and why you can’t do that

[Arkadii Ivanov]

Is there any good chance that memory model will allow sharing?

[kpgalligan]

You can share frozen state. You cannot share “regular” mutable state. You can pass mutable state, but that will be syntactically complex in the case you’ve presented.

[kpgalligan]

You can also use mutable frozen collections, but if performance is a concern, be advised it’ll be different than you’re used to. https://github.com/touchlab/Stately

[Arkadii Ivanov]

Thanks for the readings. Do you know if there are any known plans to update the memory model so it will allow mutable state sharing?

[kpgalligan]

That’s a long discussion. I doubt it, except in limited cases. It was designed for safer concurrency, so they’d need to “give up” on that plan. My thoughts: https://medium.com/@kpgalligan/saner-concurrency-74b0bf8ed446