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8367646: [GenShen] Control thread may overwrite gc cancellation cause set by mutator #27662

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8367646: [GenShen] Control thread may overwrite gc cancellation cause set by mutator #27662

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@earthling-amzn earthling-amzn commented Oct 6, 2025 •
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I believe the following events could lead to this assertion failure:

  1. Control thread reads the heap's gc cancellation cause as shenandoah_concurrent_gc
  2. Mutator thread has an allocation failure and sets the heap's gc cancellation cause to shenandoah_alloc_failure
  3. Control thread uses stale value from 1 and decides to unconditionally clear the cancellation cause
  4. Mutator thread assert that gc is still cancelled

The proposed fix here has the control thread use a CAS operation to only clear the gc if the existing value is shenandoah_concurrent_gc. This will stop the control thread from erroneously changing the value if a mutator has already set it to shenandoah_alloc_failure. A mutator thread may still have an allocation failure after the control thread has cleared the cancellation, but this is normal and expected.

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  • JDK-8367646: [GenShen] Control thread may overwrite gc cancellation cause set by mutator (Bug - P4)

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bridgekeeper bot commented Oct 6, 2025

👋 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.

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

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openjdk bot commented Oct 6, 2025

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

  • hotspot-gc
  • shenandoah

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing lists. If you would like to change these labels, use the /label pull request command.

@openjdk openjdk bot added the rfr Pull request is ready for review label Oct 6, 2025
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kdnilsen
kdnilsen approved these changes Oct 6, 2025
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There's a typo in description of the issue: s/if/is/

Have we been able to confirm through testing that this resolves the previously observed assertion failure?

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I was never able to reproduce it. The crash was observed on somewhat exotic hardware. I'm fairly confident in the fix because in all other cases the cancellation cause is only cleared on a safepoint. That is to say, this code is the only place the collector clears the cancellation cause concurrently with mutators.

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@kdnilsen kdnilsen kdnilsen approved these changes

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