8298376: ZGC: thaws stackChunk with stale oops

[stefank]

During the recent ZGC + Loom rewrites JDK-8296875 a bug was introduced, which causes Loom to thaw stackChunks with stale oops.

Before that change we made sure that only the GC performed load barriers on the cont oop in stackChunk, so that we could use the oop state as an indication of the age of the stackChunk. That information was then used in ZCollectedHeap::requires_barriers.

In Generational ZGC we changed the implementation of requires_barriers, so that the rest of the JVM didn't need to have special-handling of the cont oop. However, when upstreaming the changes in JDK-8296875 we purposefully didn't update requires_barriers, but we incorrectly brought over the removal of the special-handling of the cont oop.

To limit the fix for this bug, I propose that we restore the special-handling of the cont oop.

I've tested this with a reproducer running Fuzz.java + ZGC.

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Progress

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Issue

• JDK-8298376: ZGC: thaws stackChunk with stale oops

Reviewers

• Erik Österlund (@fisk - Reviewer) ⚠️ Review applies to bb7183bf
• Patricio Chilano Mateo (@pchilano - Reviewer) ⚠️ Review applies to bb7183bf

Reviewing

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[bridgekeeper]

👋 Welcome back stefank! A progress list of the required criteria for merging this PR into pr/11582 will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[stefank]

/label add hotspot

[openjdk]

@stefank
The hotspot label was successfully added.

[mlbridge]

Webrevs

• 01: Full - Incremental (da98a785)
• 00: Full (bb7183bf)

[fisk]

Looks good.

[pchilano]

Looks good, thanks for adding the comment.

[openjdk]

⚠️ @stefank This pull request contains merges that bring in commits not present in the target repository. Since this is not a "merge style" pull request, these changes will be squashed when this pull request in integrated. If this is your intention, then please ignore this message. If you want to preserve the commit structure, you must change the title of this pull request to Merge <project>:<branch> where <project> is the name of another project in the OpenJDK organization (for example Merge jdk:master).

[openjdk]

@stefank This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8298376: ZGC: thaws stackChunk with stale oops

Reviewed-by: eosterlund, pchilanomate

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[coleenp]

Should this be fixed for JDK 20?

[stefank]

Should this be fixed for JDK 20?

Yes. I'll backport this after pushing to 21.

[stefank]

/integrate

[openjdk]

Going to push as commit ed8a212.
Since your change was applied there have been 4 commits pushed to the master branch:

• 2e801e1: 8298475: Remove JVM_ACC_PROMOTED_FLAGS
• 9ee5037: 8298636: Fix return value in WB_CountAliveClasses and WB_GetSymbolRefcount
• ceca4fc: 8298264: Merge OffsetTableContigSpace into TenuredSpace
• d32d6c0: 8297791: update _max_classes in node type system

Your commit was automatically rebased without conflicts.

[openjdk]

@stefank Pushed as commit ed8a212.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

[stefank]

/backport jdk20

[openjdk]

@stefank the backport was successfully created on the branch stefank-backport-ed8a2120 in my personal fork of openjdk/jdk20. To create a pull request with this backport targeting openjdk/jdk20:master, just click the following link:

➡️ Create pull request

The title of the pull request is automatically filled in correctly and below you find a suggestion for the pull request body:

Hi all,

This pull request contains a backport of commit ed8a2120 from the openjdk/jdk repository.

The commit being backported was authored by Stefan Karlsson on 14 Dec 2022 and was reviewed by Erik Österlund and Patricio Chilano Mateo.

Thanks!

If you need to update the source branch of the pull then run the following commands in a local clone of your personal fork of openjdk/jdk20:

$ git fetch https://github.com/openjdk-bots/jdk20 stefank-backport-ed8a2120:stefank-backport-ed8a2120
$ git checkout stefank-backport-ed8a2120
# make changes
$ git add paths/to/changed/files
$ git commit --message 'Describe additional changes made'
$ git push https://github.com/openjdk-bots/jdk20 stefank-backport-ed8a2120