8388449: GenShen: Degenerated cycle could be skipped

[earthling-amzn]

Not clean, but not difficult to resolve conflicts. Fixes a serious issue in which Shenandoah could skip a degenerated cycle, or observe an unacceptable cancellation reason during shutdown.

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• JDK-8388809 needs maintainer approval
• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• JDK-8388449 needs maintainer approval

Issues

• JDK-8388449: GenShen: Degenerated cycle could be skipped (Bug - P2 - Requested)
• JDK-8388809: Shenandoah: Control thread may observe incorrect cancellation reason (Bug - P4 - Requested)

Reviewers

• Kelvin Nilsen (@kdnilsen - no project role)
• Aleksey Shipilev (@shipilev - Reviewer)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk25u-dev.git pull/691/head:pull/691
$ git checkout pull/691

Update a local copy of the PR:
$ git checkout pull/691
$ git pull https://git.openjdk.org/jdk25u-dev.git pull/691/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 691

View PR using the GUI difftool:
$ git pr show -t 691

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk25u-dev/pull/691.diff

Using Webrev

Link to Webrev Comment

[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]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

This backport pull request has now been updated with issues from the original commit.

[mlbridge]

Webrevs

• 00: Full (18140f8e)

[kdnilsen]

Thanks for this backport.

[shipilev]

Looks fine, thanks.

[openjdk]

⚠️ @earthling-amzn This change is now ready for you to apply for maintainer approval. This can be done directly in each associated issue or by using the /approval command.

[earthling-amzn]

/approval request We observed an assertion failure in which Shenandoah's generational mode attempted to skip running a degenerated cycle after an allocation failure. This is not optional for Shenandoah and could result in heap corruption. The change here replaces an unconditional exchange with an exchange that protects allocation failures from being overwritten by other threads. There is also a related change that replaces multiple reads of a mutable variable with a single atomic read.

[openjdk]

@earthling-amzn
8388449: The approval request has been created successfully.
8388809: The approval request has been created successfully.