Question about JDK-8205878 (tolerating missing JNI Detach)

[David Holmes]

Hi Thomas,

On 18/11/2023 1:42 am, Thomas Stüfe wrote:
> Hi,
> 
> the AIX folks have problems with 
> runtime/jni/terminatedThread/TestTerminatedThread.java. I am trying to 
> understand some details and would be happy for pointers.
> 
> The way I understand TestTerminatedThread.java and the RFR discussion 
> for 8205878 [1], the test seems to deliberately omit 
> JNI_DetachCurrentThread to simulate a JNI coding error, right?

Right.

> It joins 
> the thread, causing the OS to clean out all associated resources. The 
> pthread_t, kernel thread id, stack, etc all become invalid. The test 
> then nudges the VM in various ways to shake out problems relating to the 
> continued use of these resources.
> 
> Is my understanding correct, or am I missing something?

That is correct.

> If I got this right so far, is this not inherently unstable?

Not sure if "unstable" is the right word but yes it can have issues.

> What 
> happens if the associated resources get reused by the libc? pthread_t 
> could be a pointer to a struct or a slot index into a table, and get 
> reused by a different thread. The kernel thread id could be reused too.

It is an interesting question, but beyond this test what happens with 
real code if that were the case? We can't detect it. We will just have 
an "orphan" Thread that we can query in various ways hence ...

... the test is just a "canary" to see if the VM encounters any 
problematic scenarios when the various API's are applied to a thread 
that terminated without detaching, and which the VM can handle more 
robustly.

It turned out that other than the original CPU time issue, nothing bad 
is observed on Linux, BSD/macxOS in general. We did have one case on 
Linux PPC [1] were we saw something unexpected and had to adjust the 
test. It may be that we need something for AIX too? Or we can skip it on 
AIX if necessary.

Cheers,
David

[1] https://bugs.openjdk.org/browse/JDK-8211931



> Thanks, Thomas
> 
> [1] 
> https://mail.openjdk.org/pipermail/hotspot-runtime-dev/2018-July/029022.html <https://mail.openjdk.org/pipermail/hotspot-runtime-dev/2018-July/029022.html>