Real-world Pinning/ReentrantLock deadlock

[Alan Bateman]

On 25/06/2024 02:13, Danny Thomas wrote:
> Hi folks,
>
> We ran into a real-world deadlock with virtual threads and Micrometer, 
> Brave/Zipkin. In short, there are two paths to 
> CountBoundedQueue.offer[1] for finishing a span. RealSpan.finish[2] 
> has a synchronized block, RealScopedSpan.finish[3] doesn't. If a 
> virtual thread using RealScopedSpan is next line for the lock, but all 
> carriers are currently occupied by pinned VTs in RealSpan.finish, the 
> application will deadlock:
>
> https://gist.github.com/DanielThomas/dddd850f7e491cac3a2dd734978f4267
>
> Looking forward to the locking improvements!
>

Yes, if a j.u.concurrent lock is sometimes acquired by threads holding a 
monitor and at other times by threads that aren't holding a monitor then 
deadlock is possible when all carriers are pinned and the selected 
successor is unmounted. Have you, or your team, tried the early access 
build [1] with the changes for object monitors yet?

-Alan

[1] https://jdk.java.net/loom/