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8256040: Shenandoah: Allow NULL referent in ShenandoahReferenceProcessor::should_discover() #1117

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@rkennke rkennke commented Nov 9, 2020

Testing showed the following assert:

# Internal Error (/home/rkennke/src/openjdk/jdk/src/hotspot/share/oops/compressedOops.inline.hpp:54), pid=711923, tid=712264
# assert(!is_null(v)) failed: narrow oop value can never be zero

V [] bool ShenandoahReferenceProcessor::should_discover<narrowOop>(oop, ReferenceType) const+0x404
V [] bool ShenandoahReferenceProcessor::discover<narrowOop>(oop, ReferenceType, unsigned int)+0x4b
V [] ShenandoahReferenceProcessor::discover_reference(oop, ReferenceType)+0x138
V [] bool InstanceRefKlass::try_discover<narrowOop, ShenandoahMarkRefsMetadataClosure>(oop, ReferenceType, ShenandoahMarkRefsMetadataClosure*)+0x96
V [] void InstanceRefKlass::oop_oop_iterate_discovery<narrowOop, ShenandoahMarkRefsMetadataClosure, AlwaysContains>(oop, ReferenceType, ShenandoahMarkRefsMetadataClosure*, AlwaysContains&) [clone .constprop.696]+0x3d
V [] void InstanceRefKlass::oop_oop_iterate_ref_processing<narrowOop, ShenandoahMarkRefsMetadataClosure, AlwaysContains>(oop, ShenandoahMarkRefsMetadataClosure*, AlwaysContains&)+0x81
V [] void OopOopIterateDispatch<ShenandoahMarkRefsMetadataClosure>::Table::oop_oop_iterate<InstanceRefKlass, narrowOop>(ShenandoahMarkRefsMetadataClosure*, oop, Klass*)+0x1ee
V [] void ShenandoahConcurrentMark::do_task<ShenandoahMarkRefsMetadataClosure>(Padded<BufferedOverflowTaskQueue<ShenandoahMarkTask, (MEMFLAGS)5, 131072u>, 128ul>*, ShenandoahMarkRefsMetadataClosure*, unsigned short*, ShenandoahMarkTask*)+0x7be
V [] void ShenandoahConcurrentMark::mark_loop_work<ShenandoahMarkRefsMetadataClosure, true>(ShenandoahMarkRefsMetadataClosure*, unsigned short*, unsigned int, TaskTerminator*)+0x3ce

We have this code in ShRefProc::should_discover():
oop referent = CompressedOops::decode_not_null(heap_oop);

However, the referent can legally be NULL. We even treat NULL specially in is_inactive().


  • hotspot_gc_shenandoah
  • tier1+UseShenandoahGC+ShenandoahVerify (which originally showed the problem)


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


Linux x64 Linux x86 Windows x64 macOS x64
Build ✔️ (5/5 passed) ✔️ (2/2 passed) ✔️ (2/2 passed) ✔️ (2/2 passed)
Test (tier1) ✔️ (9/9 passed) ✔️ (9/9 passed) ✔️ (9/9 passed) ✔️ (9/9 passed)


  • JDK-8256040: Shenandoah: Allow NULL referent in ShenandoahReferenceProcessor::should_discover()



$ git fetch pull/1117/head:pull/1117
$ git checkout pull/1117

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@bridgekeeper bridgekeeper bot commented Nov 9, 2020

👋 Welcome back rkennke! 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 openjdk bot added the rfr label Nov 9, 2020
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@openjdk openjdk bot commented Nov 9, 2020

@rkennke 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 hotspot-gc shenandoah labels Nov 9, 2020
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@mlbridge mlbridge bot commented Nov 9, 2020


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@shipilev shipilev left a comment

Looks fine and trivial.

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@openjdk openjdk bot commented Nov 9, 2020

@rkennke 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:

8256040: Shenandoah: Allow NULL referent in ShenandoahReferenceProcessor::should_discover()

Reviewed-by: shade

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 no new commits pushed to the master branch. If another commit should be pushed before you perform the /integrate command, your PR will be automatically rebased. If you prefer to avoid any potential automatic rebasing, please check the documentation for the /integrate command for further details.

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@openjdk openjdk bot added the ready label Nov 9, 2020
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@rkennke rkennke commented Nov 10, 2020


@openjdk openjdk bot closed this Nov 10, 2020
@openjdk openjdk bot added integrated and removed ready rfr labels Nov 10, 2020
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@openjdk openjdk bot commented Nov 10, 2020

@rkennke Since your change was applied there have been 10 commits pushed to the master branch:

  • 01567b5: 8256036: Shenandoah: MethodHandles adapters section overflows after JDK-8255762
  • 4bc065c: 8255782: Turn UseTLAB and ResizeTLAB from product_pd to product, defaulting to "true"
  • b5d78af: 8254863: Delete code leftover from old fixes
  • 8066b33: 8253905: Update sanity test suite to not place windows at (0,0)
  • f71f9dc: 8255949: AArch64: Add support for vectorized shift right and accumulate
  • 1332ba3: 8256039: Shenandoah: runtime/stringtable/ fails
  • 11431b1: 4619330: All built-in java.awt.color.ColorSpace fields should be specified as such
  • 17f04fc: 8254078: DataOutputStream is very slow post-disabling of Biased Locking
  • 79b7909: 8255980: G1 Service thread register_task can be used after shutdown
  • dd8e4ff: 8255711: Fix and unify hotspot signal handlers

Your commit was automatically rebased without conflicts.

Pushed as commit a38dd53.

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

openjdk-notifier bot referenced this issue Nov 10, 2020
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hotspot-gc integrated shenandoah
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