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

[tkrodriguez]

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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Progress

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Issue

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

Reviewers

• Erik Österlund (@fisk - Reviewer)

Download

$ git fetch https://git.openjdk.java.net/jdk pull/1777/head:pull/1777
$ git checkout pull/1777

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[openjdk]

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[mlbridge]

Webrevs

• 00: Full (8c65667)

[tkrodriguez]

@fisk any thoughts on this bug?

[fisk]

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.

[openjdk]

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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]

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.

[tkrodriguez]

Moved to openjdk/jdk16#25