Locking
Locking features such as synchronized, volatile, atomic, reentrant are implemented using different features of the kernel and CPU. All the locking features are implemented in the Asynchronous Queue Synchronizer (AQS) class of Java.
If we use the Futex feature of Kernel directly, there is no ordered guaranteed. If multiple threads are waiting for a lock and if such a lock is released, then kernel schedules any one of the blocked threads again.
How threads block and release each other?
When the thread holding the lock calls the unlock method, as part of this unlock method, the next thread in the queue is scheduled using Futex. It's important to build a mental model that there is no central thread that's controlling multiple threads.
Barging is a scenario where a new thread comes in while the thread 2 is in the process of being freed and scheduled. The third thread will run CAS on the main lock address and will acquire the lock successfully.
Reentrant Locking
Reentrant locking is the case where the same thread tries to acquire the same lock again while it already owns the lock. In this case, AQS checks if the thread asking the lock is already the owner of that specific lock and allows it.