8339579: ZGC: Race results in only one of two remembered sets being cleared

[jsikstro]

#20821 introduced a fix that skips clearing of a potential remembered set in the young collection in favor of doing it only in the old collection.

The code responsible for clearing the remset (ZRememberedSet::clear_all()) is dependent on the _current variable in ZRememberedSet not being altered in between the two "clear_*" calls in ZRememberedSet::clear_all(). If the remembered set is being cleared in the old collection, and successfully clears the current remset with clear_current(), a young collection might then flip/swap the current/previous remembered sets by changing the _current value in ZRememberedSet. This would mean that the call to clear_previous() would clean the same bitmap again, resulting in one of the two bitmaps not being cleared at all. This will later crash in an assert checking if both bitmaps are empty/clear when the page is being freed.

Code in question:

void ZRememberedSet::clear_all() {
  clear_current();
  clear_previous();
}

This PR makes sure that clearing is done independently of what the _current value is, by accessing the two bitmaps directly.

Tested with tier5, where the fails/crashes occured before this fix, and a reproducer of the crash as well.

---

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-8339579: ZGC: Race results in only one of two remembered sets being cleared (Bug - P3)

Reviewers

• Stefan Karlsson (@stefank - Reviewer)
• Stefan Johansson (@kstefanj - Reviewer)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/20869/head:pull/20869
$ git checkout pull/20869

Update a local copy of the PR:
$ git checkout pull/20869
$ git pull https://git.openjdk.org/jdk.git pull/20869/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 20869

View PR using the GUI difftool:
$ git pr show -t 20869

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/20869.diff

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[bridgekeeper]

👋 Welcome back jsikstro! 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]

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

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8339579: ZGC: Race results in only one of two remembered sets being cleared

Reviewed-by: stefank, sjohanss

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At the time when this comment was updated there had been 3 new commits pushed to the master branch:

• a505a1d: 8337951: Test sun/security/validator/samedn.sh CertificateNotYetValidException: NotBefore validation
• 6be9272: 8338591: Improve performance of MemorySegment::copy
• cb9f5c5: 8339290: Optimize ClassFile Utf8EntryImpl#writeTo

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[openjdk]

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

• hotspot-gc

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[mlbridge]

Webrevs

• 00: Full (9ea8903a)

[stefank]

Looks good!

[kstefanj]

Looks good.

[jsikstro]

/integrate

[openjdk]

@jsikstro
Your change (at version 9ea8903) is now ready to be sponsored by a Committer.

[stefank]

/sponsor

[openjdk]

Going to push as commit ab656c3.
Since your change was applied there have been 3 commits pushed to the master branch:

• a505a1d: 8337951: Test sun/security/validator/samedn.sh CertificateNotYetValidException: NotBefore validation
• 6be9272: 8338591: Improve performance of MemorySegment::copy
• cb9f5c5: 8339290: Optimize ClassFile Utf8EntryImpl#writeTo

Your commit was automatically rebased without conflicts.

[openjdk]

@stefank @jsikstro Pushed as commit ab656c3.

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

[jsikstro]

Thank you for the reviews! @stefank @kstefanj