8365994: ZGC: Incorrect type signature in ZMappedCache comparator

[jsikstro]

Hello,

The comparator with two IntrusiveRBNode* in ZMappedCache has the incorrent type signature. This prevents IntrusiveRBTree from using the comparator as intended during validation, resulting in it using the fallback verify function, which always returns true. This could mask potential issues in the tree structure.

Testing:

• Manually placing an assert in the fallback verify function to see if it is used. It is no longer used with this patch.
• Oracle's tier 1-2

---

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-8365994: ZGC: Incorrect type signature in ZMappedCache comparator (Bug - P4)

Reviewers

• Casper Norrbin (@caspernorrbin - Committer)
• Axel Boldt-Christmas (@xmas92 - Reviewer)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/26904/head:pull/26904
$ git checkout pull/26904

Update a local copy of the PR:
$ git checkout pull/26904
$ git pull https://git.openjdk.org/jdk.git pull/26904/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 26904

View PR using the GUI difftool:
$ git pr show -t 26904

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/26904.diff

Using Webrev

Link to Webrev Comment

[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.

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:

8365994: ZGC: Incorrect type signature in ZMappedCache comparator

Reviewed-by: cnorrbin, aboldtch

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 20 new commits pushed to the master branch:

• 45726a1: 8365052: G1: Remove G1CollectionSet::groups() accessors
• 5cc8673: 8365765: thread.inline.hpp includes the wrong primary header file
• 57434c7: 8365976: G1: Full gc should mark nmethods on stack
• ... and 17 more: https://git.openjdk.org/jdk/compare/e1c58f858a64853c2d454fd00a84455ca6700055...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]

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

• hotspot-gc

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.

[caspernorrbin]

Looks good!

[mlbridge]

Webrevs

• 00: Full (cb42883f)

[xmas92]

Good catch.

[albertnetymk]

I'd expect cmp to return int; otherwise, the cmp name is rather misleading. I wonder if rbtree can be changed so that all cmp methods return int instead.

[xmas92]

I tend to agree, or rename it to less.

And I think cmp should even return a more strongly typed class, rather than int to discourage code like return b - a; as you have to be very careful to not introduce underflow arithmetic bugs. Maybe even take inspiration from operator<=>

[jsikstro]

I agree with both of you that there is room for improvement here.

I've talked with @caspernorrbin, who is the author of the tree, and we've agreed that he will address this feedback in a follow-up. I'll go ahead and integrate this change so that the comparator we have in ZGC is aligned with the (current) type signature and we verify the tree correctly.

[jsikstro]

Thank you for the reviews! @caspernorrbin @xmas92
/integrate

[openjdk]

Going to push as commit 3641c32.
Since your change was applied there have been 36 commits pushed to the master branch:

• 2ae3ea2: 8366035: Simplify CPUTimeCounters::publish_gc_total_cpu_time
• 68abf76: 8366105: Update link to the external RuleBasedBreakIterator documentation
• 5013d69: 8365633: Incorrect info is reported on hybrid CPU
• ... and 33 more: https://git.openjdk.org/jdk/compare/e1c58f858a64853c2d454fd00a84455ca6700055...master

Your commit was automatically rebased without conflicts.

[openjdk]

@jsikstro Pushed as commit 3641c32.

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