Initialize recycled continuations in createContinuation before usage

[babsingh]

Currently, recycled continuations are reset and initialized in
freeContinuation at the end of their use. There are also gaps where
the recycled continuation might retain old values and not be properly
reset.

The above approach leads to corruption and segfaults as seen in
the runJavaThread failure reported in #16728.

To fix the above crash, all continuations (new and recycled) are
initialized and reset in createContinuation just before the
continuation begins execution.

Related: #16728

[babsingh]

jenkins test sanity.functional,sanity.openjdk zlinux,win,aix jdk21

[tajila]

There are also gaps where
the recycled continuation might retain old values and not be properly
reset.

How do the gaps arise? Were the recycled continuations not being fully reset?

[babsingh]

How do the gaps arise? Were the recycled continuations not being fully reset?

Yes, continuations not being fully reset is part of the reason. In freeContinuation -> recycleContinuation, memset for the continuation was missing, and we were not setting all the continuation fields that are being set in createContinuation. Also, few fields in the continuation->stackObject were not being reset after the old stacks were freed. The most likely assumption for this approach was that those fields either stay unchanged or don't impact a continuation's functioning.

I also tried to mirror the continuation init code from createContinuation in freeContinuation -> recycleContinuation. This didn't help. There is more at play, which I couldn't figure out. The missing piece is AIX specific because the current implementation works on all other platforms.

Further, the runJavaThread crash started to occur more frequently after we reduced the stack increment size from 16KB (default) to 2KB in openj9-openjdk-jdk21#82. The crash was previously hidden behind the OutOfMemoryError, and showed up once stack grow was triggered more often.

[babsingh]

JDK21 win sanity.openjdk build is taking too much time. Since all other builds have passed, it might not be worth to wait for its completion.

Ran a 100x Skynet grinder (25x on 4 different AIX machines): https://openj9-jenkins.osuosl.org/job/Grinder/3148. Ran the grinder with -Xgcpolicy:optthruput since #18637 wasn't merged when the PR builds were launched.

@pshipton Can you please review/merge this PR?

[babsingh]

Backports

• (0.42) Initialize recycled continuations in createContinuation before usage #18657
• (0.43) Initialize recycled continuations in createContinuation before usage #18658

[tajila]

I had noticed that recycled stacks where appear beneath current stacks, in some cases the stack walker would walk both. Also, the carrier sp would change between vthread mount and unmount. I think some of that can be explained by not resetting all the fields.

Given the issue is not reproduceable, and the proposed change is correct, I say we go ahead with this.

[babsingh]

https://openj9-jenkins.osuosl.org/job/Test_openjdk21_j9_sanity.openjdk_x86-64_windows_Personal/20/

JDK21 win sanity.openjdk build job was aborted by the infra scripts because the timeout limit was hit.

16:39:25  Cancelling nested steps due to timeout
16:39:25  Sending interrupt signal to process

[tajila]

No failures in windows testing