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8268290: Improve LockFreeQueue<> utility #4379

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@kimbarrett kimbarrett commented Jun 6, 2021

Please review this change to the LockFreeQueue utility class.

The LockFreeQueue originated as an implementation detail of
G1DirtyCardQueueSet, and was recently refactored into a public utility
class. In that refactoring it retained some limitations that were
acceptable in its original context, but may be problematic as a general

In particular, under some conditions a thread was not be able to pop the
last element in the queue, due to interference by a concurrent operation.
And this state will persist, so retrying the pop operation won't help until
the interfering thread had made sufficient progress. This was mitigated by
making the API more complex to provide notice to the client that the queue
may be in this state.

But it turns out we can do somewhat better, eliminating one of the
limitations, which is the point of this change. We introduce a
pseudo-object used as an end of queue marker. We can use the transition of
the last element's next value from the end marker to NULL by a pop operation
as a claim on the element, allowing the losing thread to recognize, retry,
and make progress.

This queue still has the limitation that an in-progress push/append may
prevent popping elements. Because of this, the class is being renamed to
NonblockingQueue. The old name suggests stronger guarantees than actually

The PR has two commits, the first for the functional changes, the second for
the renaming. The github diffs don't seem to be recognizing the renaming of
the source files as a rename, instead treating the old files as deleted and
the new files as added. The first commit by itself is probably more useful
for reviewing the functional changes.

mach5 tier1-5


  • Change must not contain extraneous whitespace
  • Commit message must refer to an issue
  • Change must be properly reviewed




Using git

Checkout this PR locally:
$ git fetch pull/4379/head:pull/4379
$ git checkout pull/4379

Update a local copy of the PR:
$ git checkout pull/4379
$ git pull pull/4379/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 4379

View PR using the GUI difftool:
$ git pr show -t 4379

Using diff file

Download this PR as a diff file:

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@bridgekeeper bridgekeeper bot commented Jun 6, 2021

👋 Welcome back kbarrett! 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.

@kimbarrett kimbarrett marked this pull request as ready for review Jun 6, 2021
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@openjdk openjdk bot commented Jun 6, 2021

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

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@openjdk openjdk bot added hotspot rfr labels Jun 6, 2021
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@mlbridge mlbridge bot commented Jun 6, 2021


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@walulyai walulyai left a comment


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@tschatzl tschatzl left a comment

I think this is good.

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@openjdk openjdk bot commented Jun 22, 2021

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

ℹ️ This project also has non-automated pre-integration requirements. Please see the file for details.

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

8268290: Improve LockFreeQueue<> utility

Reviewed-by: iwalulya, tschatzl

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@openjdk openjdk bot added the ready label Jun 22, 2021
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@kimbarrett kimbarrett commented Jun 22, 2021

Thanks @walulyai and @tschatzl for reviews.

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@kimbarrett kimbarrett commented Jun 22, 2021


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@openjdk openjdk bot commented Jun 22, 2021

Going to push as commit 0c693e2.

@openjdk openjdk bot closed this Jun 22, 2021
@openjdk openjdk bot added integrated and removed ready rfr labels Jun 22, 2021
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@openjdk openjdk bot commented Jun 22, 2021

@kimbarrett Pushed as commit 0c693e2.

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

@kimbarrett kimbarrett deleted the lfqueue branch Jun 22, 2021
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@caoman caoman commented Jun 25, 2021

Thank you for this fix. I like how append() can just store to &_head on CAS failure.

Minor comments:
Is it recommended to add SpinPause() in pop()'s while loop body?
This sentence above append() in inline.hpp could be removed: "it is an invariant that the old tail's "next" value is NULL".
I can make a PR for these separately.

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@walulyai walulyai commented Jun 25, 2021

Minor comments:
Is it recommended to add SpinPause() in pop()'s while loop body?

I think with a try_pop method, if one needs to have a SpinPause, they can re-implement the retry-loop with the SpinPause and try_pop instead of having this in a generic pop().

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@kimbarrett kimbarrett commented Jun 25, 2021

I don't think pop() needs SpinPause() in its body. It's not really an empty spin. It only loops on a cmpxchg failure. Someone might want to use try_pop() and SpinYield or something similar in a highly contended case, but I'd rather leave that to the specific case to parameterize.

You are correct that this comment should be removed: "it is an invariant that the old tail's "next" value is NULL". Oops, sorry I missed that.

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