8386287: GenShen: assert(starts_object(card_index)) failed: Can't get last start because no object starts here

[earthling-amzn]

This was a somewhat exotic bug. After JDK-8376839, Shenandoah registers the starts for all promoted objects in a separate phase after evacuation completes and all PLABs are retired. The sequence of events in this assertion failure were as follows:

1. evacuation is running
2. a mutator fails to allocate a humongous object
3. evacuation is cancelled
4. because it was a humongous allocation failure, Shenandoah runs a full gc instead of a degenerated cycle
5. an old region holding recently evacuated objects is pinned when the full gc starts
6. the pinned region will not be compacted and will contain unmarked (dead) objects
7. during update refs, the full gc tries to coalesce and fill these dead objects
8. the coalesce and fill for the region also tries to update the card table object registrations
9. this assertion fails because none of the promoted objects have been registered yet

At the end of a full GC, all of the regions are scanned and the remembered set is completely rebuilt, so step 7 above was unnecessary. In this PR, we simply skip it.

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Issue

• JDK-8386287: GenShen: assert(starts_object(card_index)) failed: Can't get last start because no object starts here (Bug - P4)(⚠️ The fixVersion in this issue is [27-pool, 28] but the fixVersion in .jcheck/conf is 28, a new backport will be created when this pr is integrated.)

Reviewers

• Kelvin Nilsen (@kdnilsen - Reviewer)
• Xiaolong Peng (@pengxiaolong - Committer)
• Rui Li (@rgithubli - Author) Review applies to c0fdc948

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[bridgekeeper]

👋 Welcome back wkemper! 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]

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

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

8386287: GenShen: assert(starts_object(card_index)) failed: Can't get last start because no object starts here

Reviewed-by: kdnilsen, xpeng, ruili

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 25 new commits pushed to the master branch:

• b123df7: 8385119: Unify boolean value normalization
• e965fcf: 8386124: Test serviceability/sa/TestG1HeapRegion.java failed: Address of G1HeapRegion does not match
• a47f362: 8382063: Jtreg test javax/swing/SwingWorker/TestDoneBeforeDoInBackground.java fails
• ... and 22 more: https://git.openjdk.org/jdk/compare/3fd70f8d38fadf8874849ffc87a1f0ed11d10392...master

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

[openjdk]

@earthling-amzn The following labels will be automatically applied to this pull request:

• hotspot-gc
• shenandoah

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[mlbridge]

Webrevs

• 01: Full - Incremental (b3a02a20)
• 00: Full (c0fdc948)

[kdnilsen]

Thanks for this fix. Good analysis.

[pengxiaolong]

Thanks for the fix!

[earthling-amzn]

/integrate

[openjdk]

Going to push as commit 1251526.
Since your change was applied there have been 25 commits pushed to the master branch:

• b123df7: 8385119: Unify boolean value normalization
• e965fcf: 8386124: Test serviceability/sa/TestG1HeapRegion.java failed: Address of G1HeapRegion does not match
• a47f362: 8382063: Jtreg test javax/swing/SwingWorker/TestDoneBeforeDoInBackground.java fails
• ... and 22 more: https://git.openjdk.org/jdk/compare/3fd70f8d38fadf8874849ffc87a1f0ed11d10392...master

Your commit was automatically rebased without conflicts.

[openjdk]

@earthling-amzn Pushed as commit 1251526.

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