On Tue, 24 Sep 2024 14:18:58 GMT, Tobias Hartmann <thartm...@openjdk.org> wrote:
> 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 r... Thanks again for the review. I'll open this once my testing passed. ------------- PR Comment: https://git.openjdk.org/jdk/pull/21160#issuecomment-2371646077
