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8259063: Possible deadlock with vtable/itable creation vs concurrent class unloading #103

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@fisk fisk commented Jan 11, 2021

There is an offender of consistent lock ordering: the creation of itable/vtable stubs. It happens when an inline cache is transitioning to megamorphic. Way earlier in the call stack, we grab the nmethod lock for IC patching, and then when a megamorphic transition happens and there is no stub created yet for the given vtable/itable index (rather uncommon), a new vtable/itable stub is created. This creation of a stub takes the CodeCache lock. We already deal with failure to create the stubs today, by making the inline cache "clean" instead, essentially deferring the transition to megamorphic to a subsequent call. We deal with that today to handle the code cache running out of memory, making it temporarily impossible to transition to megamorphic until memory has been freed up. My patch rides on that logic, so that we can try_lock() the CodeCache_lock instead. If we fail to take the lock, then it is arguably a bad time to do this megamorphic transition, regardless of deadlocks, as we might have to wait quite a while. So if we fail to take the lock, I just return NULL saying we couldn't create a vtable/itable stub at this time, signalling we defer the transition to later, as we would also do if we couldn't create the stub due to memory exhaustion. This solves the deadlock situation.

Eric Caspole who reported the bug has tried this patch, and it solved the problem. I also tried it with the same test myself, with the same successful results. I also ran it through tier1-5 as it is touching inline cache code which can be a bit subtle.


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  • JDK-8259063: Possible deadlock with vtable/itable creation vs concurrent class unloading



$ git fetch pull/103/head:pull/103
$ git checkout pull/103

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👋 Welcome back eosterlund! 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 Jan 11, 2021
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@openjdk openjdk bot commented Jan 11, 2021

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

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pliden approved these changes Jan 12, 2021
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@pliden pliden left a comment

Looks good to me!

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@openjdk openjdk bot commented Jan 12, 2021

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

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8259063: Possible deadlock with vtable/itable creation vs concurrent class unloading

Reviewed-by: pliden, neliasso

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At the time when this comment was updated there had been 6 new commits pushed to the master branch:

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  • a7e5da2: 8258384: AArch64: SVE verify_ptrue fails on some tests
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  • d60a937: 8259028: ClassCastException when using custom filesystem with wrapper FileChannel impl
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@neliasso neliasso left a comment

I like the solution - simple and without adding extra complexity or lock juggling.

It's a good just skipping the call to handle_full_code_cache. The VTableBlobs are not the thing usually filling up the code cache - and they can be placed in any codeheap. And even if we skip it - no harm done - there will be other allocations that will trigger the call.

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@fisk fisk commented Jan 12, 2021

Thanks for the reviews @neliasso and @pliden.

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@fisk fisk commented Jan 13, 2021


@openjdk openjdk bot closed this Jan 13, 2021
@openjdk openjdk bot added integrated and removed ready rfr labels Jan 13, 2021
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@openjdk openjdk bot commented Jan 13, 2021

@fisk Since your change was applied there have been 18 commits pushed to the master branch:

  • 6bb6093: 8259657: typo in generated HELP page prevents localization
  • 5567530: 8258272: LoadVectorMaskedNode can't be replaced by zero con
  • a99df45: 8259560: Zero m68k: "static assertion failed: align" after JDK-8252049
  • efc36be: 8258985: Parallel WeakProcessor may use too few threads
  • 417e1d1: 8259061: C2: assert(found) failed: memory-writing node is not placed in its original loop or an ancestor of it
  • 793017d: 8259601: AArch64: Fix reinterpretX2D match rule issue
  • 15dd8f3: 8259275: JRuby crashes while resolving invokedynamic instruction
  • 1cf2378: 8259353: VectorReinterpretNode is incorrectly optimized out
  • 17b4db3: 8259636: Check for buffer backed by shared segment kicks in in unexpected places
  • 5f9cd72: 8259645: Revert JDK-8245956 JavaCompiler still uses File API instead of Path API in a specific case
  • ... and 8 more:

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