Java 21 clinit deadlock

[Simone Bordet]

Hello,

We switched the Jetty builds to Java 21 a while ago, and they randomly
fail with a hard deadlock during class initialization.

We tried to understand if we were doing something wrong, but the code
compiles fine, and at first glance there seems to be nothing wrong
with it, but it may well be a popular Java puzzler that I am not aware
of :)

We did not see these failures with Java 20, and we do not see them with Java 17.

I was wondering if something changed in Java 21 to cause this deadlock?

I hope this is the right mailing list for this problem.
If not, please let me know.

I am an OpenJDK author so I can open an issue about this, but I don't
have a reproducer, since it seems a race condition happening randomly.

Below you can find more details about this deadlock.

Thanks!

----

Taking a thread dump we see one thread in this state:

java.lang.Thread.State: RUNNABLE
at org.eclipse.jetty.http.HttpFields.build(org.eclipse.jetty.http at 12.0.2-SNAPSHOT/HttpFields.java:76)
- waiting on the Class initialization monitor for
org.eclipse.jetty.http.MutableHttpFields
at org.eclipse.jetty.http.HttpFields.<clinit>(org.eclipse.jetty.http at 12.0.2-SNAPSHOT/HttpFields.java:67)
"ForkJoinPool-1-worker-1" #23 [1457161] daemon prio=5 os_prio=0
cpu=320.77ms elapsed=8870.98s allocated=18414K defined_classes=928
tid=0x00007f6ec0c6a6f0 nid=1457161 waiting on condition
[0x00007f6e91ffb000]

And other threads are in this state (or similar -- the bottom frame is
different but still triggering the clinit of HttpFields):

java.lang.Thread.State: RUNNABLE
at org.eclipse.jetty.http.HttpTester.parseResponse(org.eclipse.jetty.http at 12.0.2-SNAPSHOT/HttpTester.java:274)
- waiting on the Class initialization monitor for
org.eclipse.jetty.http.HttpFields
at org.eclipse.jetty.http.HttpTester.parseResponse(org.eclipse.jetty.http at 12.0.2-SNAPSHOT/HttpTester.java:261)
"ForkJoinPool-1-worker-2" #24 [1457162] daemon prio=5 os_prio=0
cpu=136.15ms elapsed=8870.96s allocated=3303K defined_classes=340
tid=0x00007f6e4c01ddb0 nid=1457162 waiting on condition
[0x00007f6e91efc000]

Class hierarchy:

MutableHttpFields implements HttpFields.Mutable
HttpFields.Mutable extends HttpFields

Full thread dump:
https://gist.github.com/olamy/7e883adcf51b2410337abfa775a79a0b

Code:
https://github.com/eclipse/jetty.project/blob/jetty-12.0.1/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java

-- 
Simone Bordet
---
Finally, no matter how good the architecture and design are,
to deliver bug-free software with optimal performance and reliability,
the implementation technique must be flawless.   Victoria Livschitz