8364159: Shenandoah assertions after JDK-8361712

[tstuefe]

In ShenandoahAsserts::extract_klass_safely, we attempt to carefully extract the Klass for an object and trip over our own feet doing this. We load the Markword twice: once to read "is_marked" and get the forwardee, once to extract the narrowKlass.

If between the first and the second load we did evacuate the object concurrently, this happens:

T1 : fwd = SheandoahForwarding::get_forwardee_raw_unchecked(obj) => obj unmarked => (fwd == obj)
T2 : (installs forwarding pointer in obj)
T1: read markword from fwd again, (fwd == obj) => reads markword from forwarded obj, interprets the zeros in the upper part of the forwardee address as narrowKlass == 0, asserts

The solution is - as ShenandoahForwarding::klass() already does - to load the MW from original obj only once. Patch does that.

---

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Issue

• JDK-8364159: Shenandoah assertions after JDK-8361712 (Bug - P3)

Reviewers

• Matthias Baesken (@MBaesken - Reviewer)
• Aleksey Shipilev (@shipilev - Reviewer)

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[openjdk]

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

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

8364159: Shenandoah assertions after JDK-8361712

Reviewed-by: mbaesken, shade

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At the time when this comment was updated there had been 34 new commits pushed to the master branch:

• 7af3cd9: 8156755: [TESTBUG] Fix gc/g1/humongousObjects/objectGraphTest/TestObjectGraphAfterGC.java
• 4da1c9d: 8364196: G1: Fix typo in "cset_groud_gid" local variable in G1FlushHumongousCandidateRemSets
• 302508d: 8364249: G1: Fix some comments about "maximum_collection"
• ... and 31 more: https://git.openjdk.org/jdk/compare/e307b5cca870f7ea9b79b7a9d4e6f7f762c27aa4...master

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@tstuefe The following labels will be automatically applied to this pull request:

• hotspot-gc
• shenandoah

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[tstuefe]

MacOS error unrelated

[mlbridge]

Webrevs

• 00: Full (f2cba46b)

[MBaesken]

Thanks for fixing this, I checked for similar code locations but could not find any (hope I did not overlook something).
The fix fixes our various asserts we saw across jtreg HS tests when enabling CompressedObjectHeaders by default,

[tstuefe]

Thanks @MBaesken ! My own tests are also clean now.

Any shenandoah devs willing to give me a quick review?

[shipilev]

Yes, please integrate. Seeing failure in current testing as well.

[tstuefe]

/integrate

[openjdk]

Going to push as commit 317dacc.
Since your change was applied there have been 35 commits pushed to the master branch:

• a6fb87d: 8364042: UnsafeMemoryAccess will not work with AOT cached code stubs
• 7af3cd9: 8156755: [TESTBUG] Fix gc/g1/humongousObjects/objectGraphTest/TestObjectGraphAfterGC.java
• 4da1c9d: 8364196: G1: Fix typo in "cset_groud_gid" local variable in G1FlushHumongousCandidateRemSets
• ... and 32 more: https://git.openjdk.org/jdk/compare/e307b5cca870f7ea9b79b7a9d4e6f7f762c27aa4...master

Your commit was automatically rebased without conflicts.

[openjdk]

@tstuefe Pushed as commit 317dacc.

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