8332506: SIGFPE In ObjectSynchronizer::is_async_deflation_needed()

[fbredber]

This PR solves a division by zero problem, that according to the bug report happened in ObjectSynchronizer::is_async_deflation_needed(). As it turns out it really happened in the inlined monitors_used_above_threshold() function. The problematic line looked like this:

size_t monitor_usage = (monitors_used * 100LL) / ceiling;

Unfortunately the ceiling value was increased every time there where too many deflations without any progress. This whould eventually lead to an overflow in the ceiling value, and in unlucky circumstances, it would become zero. Thus causing a division by zero crash.

This PR makes sure not to increase the ceiling value if monitor_usage is below the MonitorUsedDeflationThreshold.
It also makes sure the ceiling value is never zero, and does not wrap around.

Tested okay in tier1-5 and test/hotspot/jtreg/runtime/Monitor/MonitorUsedDeflationThresholdTest.java

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8332506: SIGFPE In ObjectSynchronizer::is_async_deflation_needed() (Bug - P3)(⚠️ The fixVersion in this issue is [24] but the fixVersion in .jcheck/conf is 25, a new backport will be created when this pr is integrated.)

Reviewers

• Coleen Phillimore (@coleenp - Reviewer)
• David Holmes (@dholmes-ora - Reviewer)

Reviewing

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Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/22815/head:pull/22815
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Update a local copy of the PR:
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$ git pull https://git.openjdk.org/jdk.git pull/22815/head

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$ git pr checkout 22815

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[bridgekeeper]

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

[openjdk]

@fbredber 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:

8332506: SIGFPE In ObjectSynchronizer::is_async_deflation_needed()

Reviewed-by: coleenp, dholmes

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 87 new commits pushed to the master branch:

• 021c476: 8347148: [BACKOUT] AccessFlags can be u2 in metadata
• ddb5881: 8329549: Remove FORMAT64_MODIFIER
• 098afc8: 8339113: AccessFlags can be u2 in metadata
• e413fc6: 8347127: CTW fails to build after JDK-8334733
• 9702acc: 8175709: DateTimeFormatterBuilder.appendZoneId() has misleading JavaDoc
• 030149f: 8334644: Automate javax/print/attribute/PageRangesException.java
• c8a9dd3: 8346609: Improve MemorySegment.toString
• 4d8fb80: 8347038: [JMH] jdk.incubator.vector.SpiltReplicate fails NoClassDefFoundError
• cf3e48e: 8346965: Multiple compiler/ciReplay test fails with -XX:+SegmentedCodeCache
• e5f0c19: 8345041: IGV: Free Placement Mode in IGV Layout
• ... and 77 more: https://git.openjdk.org/jdk/compare/d50b725ac0f228f5b04dd68c123c7f6b263c0d02...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

[openjdk]

@fbredber The following label will be automatically applied to this pull request:

• hotspot-runtime

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

[mlbridge]

Webrevs

• 03: Full - Incremental (99a31a2f)
• 02: Full - Incremental (502766b7)
• 01: Full - Incremental (452fb584)
• 00: Full (16c3fecd)

[coleenp]

I have question about the math. This is a good find and the change looks reasonable to fix the problem. Maybe we can use this change and investigate further improvements later, but I'd like to know what you think about this first.

[coleenp]

I walked through the math and this looks good. This solves both problems of the ceiling being incremented too often and the overflow problem because of the first problem.

[dholmes-ora]

I remain concerned that this both addresses the potential overflow, and also changes related logic about the ceiling/threshold in a way that I do not understand. It would help if there was a clear definition of what the "ceiling" means and how it relates to list->max() and list->count().

[mlbridge]

Mailing list message from David Holmes on hotspot-runtime-dev:

FYI for the mailing list records, this comment was deleted in the PR.

David

On 7/01/2025 10:56 am, David Holmes wrote:

[dholmes-ora]

Okay, I think I grok the changes now.

Thanks for your patience.

[coleenp]

I like the new comment and the other new comments in this patch.

[fbredber]

Thanks for the review.
/integrate

[openjdk]

Going to push as commit cbabc04.
Since your change was applied there have been 92 commits pushed to the master branch:

• 9872421: 8347094: Inline CollectedHeap::increment_total_full_collections
• b741f3f: 8343629: More MergeStore benchmark
• b0c935c: 8347047: Cleanup action passed to MemorySegment::reinterpret keeps old segment alive
• bcefab5: 8342468: Improve API documentation for java.lang.classfile.constantpool
• 40f0a39: 8343342: java/io/File/GetXSpace.java fails on Windows with CD-ROM drive
• 021c476: 8347148: [BACKOUT] AccessFlags can be u2 in metadata
• ddb5881: 8329549: Remove FORMAT64_MODIFIER
• 098afc8: 8339113: AccessFlags can be u2 in metadata
• e413fc6: 8347127: CTW fails to build after JDK-8334733
• 9702acc: 8175709: DateTimeFormatterBuilder.appendZoneId() has misleading JavaDoc
• ... and 82 more: https://git.openjdk.org/jdk/compare/d50b725ac0f228f5b04dd68c123c7f6b263c0d02...master

Your commit was automatically rebased without conflicts.

[openjdk]

@fbredber Pushed as commit cbabc04.

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

[fbredber]

/backport :jdk24

[openjdk]

@fbredber the backport was successfully created on the branch backport-fbredber-cbabc045-jdk24 in my personal fork of openjdk/jdk. To create a pull request with this backport targeting openjdk/jdk:jdk24, 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 cbabc045 from the openjdk/jdk repository.

The commit being backported was authored by Fredrik Bredberg on 8 Jan 2025 and was reviewed by Coleen Phillimore and David Holmes.

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/jdk:

$ git fetch https://github.com/openjdk-bots/jdk.git backport-fbredber-cbabc045-jdk24:backport-fbredber-cbabc045-jdk24
$ git checkout backport-fbredber-cbabc045-jdk24
# make changes
$ git add paths/to/changed/files
$ git commit --message 'Describe additional changes made'
$ git push https://github.com/openjdk-bots/jdk.git backport-fbredber-cbabc045-jdk24

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