When investigating an intermittent NPE with an Oracle internal test on AArch64, I noticed that the NPE is actually a SIGSEGV in the code emitted by `MethodHandles::jump_to_lambda_form` when trying to load the `MemberName::method` field because the `MemberName` we retrieved from `LambdaForm::vmentry` is null:
https://github.com/openjdk/jdk/blob/f0374a0bc181d0f2a8c0aa9aa032b07998ffaf60/src/hotspot/cpu/aarch64/methodHandles_aarch64.cpp#L141-L143 The JVM translates SIGSEGVs happening in method handle intrinsics to NPEs, assuming that they are implicit NPEs due to a null receiver: https://github.com/openjdk/jdk/blob/f0374a0bc181d0f2a8c0aa9aa032b07998ffaf60/src/hotspot/share/runtime/sharedRuntime.cpp#L979-L982 After digging around in the MethodHandle implementation that I'm not too familiar with, I found this suspicious code in `MethodHandle::updateForm`: https://github.com/openjdk/jdk/blob/36314a90c15e2ab2a9b32c2e471655c1b07d452c/src/java.base/share/classes/java/lang/invoke/MethodHandle.java#L1881-L1883 The `Unsafe.fullFence` was added by [JDK-8059877](https://bugs.openjdk.org/browse/JDK-8059877) and replaced a `// ISSUE: Should we have a memory fence here?` [comment](https://github.com/openjdk/jdk/commit/224c42ee4d4c3027d1f8f0d8b7ecf6646e9418c3#diff-5a4b2f817a4273eacf07f3ee24782c58c8ff474c6d790f72298e906837c5543aL1442). If the intention was to [prevent publishing a partially initialized object](https://wiki.sei.cmu.edu/confluence/display/java/TSM03-J.+Do+not+publish+partially+initialized+objects), I think it was added to the wrong place (paging @iwanowww). In the failing scenario, we concurrently trigger LambdaForm customization for a VarHandle invoker: at java.base/java.lang.invoke.MethodHandle.updateForm(MethodHandle.java:1883) at java.base/java.lang.invoke.MethodHandle.customize(MethodHandle.java:1856) at java.base/java.lang.invoke.MethodHandle.maybeCustomize(MethodHandle.java:1846) at java.base/java.lang.invoke.Invokers.maybeCustomize(Invokers.java:632) at java.base/java.lang.invoke.Invokers.checkCustomized(Invokers.java:626) The problem is that another thread can observe `newForm` which via the `MethodHandle::form` field before the store to `vmentry` in `prepare()` (see [here](https://github.com/openjdk/jdk/blob/36314a90c15e2ab2a9b32c2e471655c1b07d452c/src/java.base/share/classes/java/lang/invoke/LambdaForm.java#L821)) is visible and therefore observe null. We need a StoreStore barrier to prevent stores that initialize the newForm from being reordered with the subsequent store to the `MethodHandle::form` field that would make it accessible by other threads. I removed the `fullFence` which should not be needed. Unfortunately, my new tests seems to trigger another issue (even if I leave the `fullFence` in): java.lang.AssertionError at java.base/java.lang.invoke.LambdaForm.arityCheck(LambdaForm.java:1006) at TestLambdaFormCustomization.lambda$test$0(TestLambdaFormCustomization.java:46) at java.base/java.lang.Thread.run(Thread.java:1576) It's this assert: https://github.com/openjdk/jdk/blob/36314a90c15e2ab2a9b32c2e471655c1b07d452c/src/java.base/share/classes/java/lang/invoke/LambdaForm.java#L1010 UPDATE: After discussing with @JornVernee, it seems that my test using `-Djava.lang.invoke.MethodHandle.COMPILE_THRESHOLD=1` was the issue since that flag triggers some untested code paths. I removed that flag from the test, verified that it still triggers the original issue and leave it to the method handle maintainers to address this issue separately. Thanks to @eme64 for taking a first step at simplifying the original test. Best regards, Tobias ------------- Commit messages: - Use putReferenceRelease - Change from driver to main - Updated copyright year - Don't use COMPILE_THRESHOLD=1 in the test. Verified that it still reproduces - Removed fullFence - 8340812: LambdaForm customization via MethodHandle::updateForm is not thread safe Changes: https://git.openjdk.org/jdk/pull/21160/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21160&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8340812 Stats: 73 lines in 2 files changed: 70 ins; 1 del; 2 mod Patch: https://git.openjdk.org/jdk/pull/21160.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/21160/head:pull/21160 PR: https://git.openjdk.org/jdk/pull/21160
