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8258380: [JVMCI] don't clear InstalledCode reference when unloading JVMCI nmethods #1777

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@tkrodriguez tkrodriguez commented Dec 15, 2020

We have been getting crashes running our unit tests in our gates with JDK15. The crashes are mostly in nmethod::is_unloaded because IsUnloadingBehaviour::current() is null. It seemed like there was some inconsistency in the notion of is_unloading for JVMCI nmethods that was leading to the crashes. I noticed that the is_unloading path rescans the nmethod using a closure to determine if it's unloading and since make_unloaded will clear the InstalledCode reference at the end if you ran the closure again it might return false since the oop is now null instead of being an unmarked oop. There's no point in the clearly the oop in this path anyway since the nmethod is dead and leaving the oop around seem to solve this crash. We're only see this crash in 15 but I don't see why it couldn't occur in 11 or later.


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  • JDK-8258380: [JVMCI] don't clear InstalledCode reference when unloading JVMCI nmethods



$ git fetch pull/1777/head:pull/1777
$ git checkout pull/1777

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@openjdk openjdk bot added the rfr label Dec 15, 2020
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@openjdk openjdk bot commented Dec 15, 2020

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

  • hotspot-compiler

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

@openjdk openjdk bot added the hotspot-compiler label Dec 15, 2020
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@tkrodriguez tkrodriguez commented Dec 15, 2020

@fisk any thoughts on this bug?

fisk approved these changes Dec 15, 2020
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@fisk fisk left a comment

Right. It is definitely assumed that the transition to is_unloading() is completely monotonic. And it seems like the clearing could violate that indeed.
Worth noticing is that with concurrent class unloading, the make_unloaded operation is split into two: 1. unlinking, 2. deleting stuff. This operation would go under unlinking. Hence, it would need to check for is_unloading instead of is_unloaded, as the unlinking happens before the nmethod becomes unloaded. But we can cross that bridge when we get there I suppose. Looks good.

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@openjdk openjdk bot commented Dec 15, 2020

@tkrodriguez 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:

8258380: [JVMCI] don't clear InstalledCode reference when unloading JVMCI nmethods

Reviewed-by: eosterlund

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At the time when this comment was updated there had been 1 new commit pushed to the master branch:

  • 3ab1dfe: 8257828: SafeFetch may crash if invoked in non-JavaThreads

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@vnkozlov vnkozlov left a comment

As noted in JBS report this fix needs to be re-based to JDK 16 repo (with new PR) and pushed there.
It will be automatically forward ported into JDK 17 later.

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@tkrodriguez tkrodriguez commented Dec 15, 2020

Moved to openjdk/jdk16#25

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hotspot-compiler ready rfr
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