On Fri, 8 Nov 2024 11:56:08 GMT, Quan Anh Mai <qa...@openjdk.org> wrote:
> Hi,
>
> This patch fixes the deadlock in `TestMemorySession#testAcquireCloseRace`.
> The lock-step spin lock is implemented as with `lock` being an
> `AtomicInteger`:
>
> // Keep the 2 threads operating on the same scope
> int k = lock.getAndAdd(1) + 1;
> while (k != i * 2) {
> Thread.onSpinWait();
> k = lock.get();
> }
>
> Given the initial condition:
>
> Thread 1: i = 0
> Thread 2: i = 0
> lock: -2
>
> The `lock` then undergoes the following operations:
>
>
>
> Thread 1 Thread 2 lock value
> getAndAdd(1) -1
> getAndAdd(1) 0 -> Thread 2 then continues
> its next iteration, its i value is now 1
> getAndAdd(1) 1 -> Thread 2 reaches the next
> iteration before thread 1 has a chance to read the value 0
> get() 1 -> Thread 1 now cannot
> proceed because it missed the value 0
> get() 1 -> Thread 2 now cannot
> proceed because lock can never reach 2
>
>
> The solution is to not rely on the exact value of the lock but instead
> whether the lock has passed the expected value.
>
> Testing: I have run this test several hundreds times and got no failure while
> without this patch I encountered a timeout every approximately 30 times.
>
> Please take a look, thanks a lot.
I think I also prefer Maurizio's version.
-------------
PR Comment: https://git.openjdk.org/jdk/pull/21976#issuecomment-2465114542
