Is there multiplatform lock in Kotlin?

From Kotlin/Native Concurrent documentation (here):

Concurrency in Kotlin/Native

Kotlin/Native runtime doesn't encourage a classical thread-oriented concurrency model with mutually exclusive code blocks and conditional variables, as this model is known to be error-prone and unreliable. Instead, we suggest a collection of alternative approaches, allowing you to use hardware concurrency and implement blocking IO. Those approaches are as follows, and they will be elaborated on in further sections:

• Workers with message passing
• Object subgraph ownership transfer
• Object subgraph freezing
• Object subgraph detachment
• Raw shared memory using C globals
• Coroutines for blocking operations (not covered in this document)

It seems that locks are not exposed in Kotlin/Native by design. There are implementations (see Lock.kt), however that class is marked internal.

However, there is a multi-platform implemenation of locks in KTOR (very limited doc, source code). It is public, but marked with @InternalApi, which may affect its stability.

You might also be interested in this KotlinLang discussion thread: Replacement for synchronized

There is no lock nor synchronized in Kotlin common. Kotlin's approach is to use immutable data. You can add your own expect AtomicReference in common and actual implementations in JVM Native, it will help a lot. Also keep in mind that coroutines in Native are single threaded at the moment. Also you can't share mutable state between threads in Native.