RFR: 8337753: Target class of upcall stub may be unloaded [v5]

[Jorn Vernee]

On Fri, 6 Sep 2024 17:51:15 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> As discussed in the JBS issue:
>> 
>> FFM upcall stubs embed a `Method*` of the target method in the stub. This `Method*` is read from the `LambdaForm::vmentry` field associated with the target method handle at the time when the upcall stub is generated. The MH instance itself is stashed in a global JNI ref. So, there should be a reachability chain to the holder class: `MH (receiver) -> LF (form) -> MemberName (vmentry) -> ResolvedMethodName (method) -> Class<?> (vmholder)`
>> 
>> However, it appears that, due to multiple threads racing to initialize the `vmentry` field of the `LambdaForm` of the target method handle of an upcall stub, it is possible that the `vmentry` is updated _after_ we embed the corresponding `Method`* into an upcall stub (or rather, the latest update is not visible to the thread generating the upcall stub). Technically, it is fine to keep using a 'stale' `vmentry`, but the problem is that now the reachability chain is broken, since the upcall stub only extracts the target `Method*`, and doesn't keep the stale `vmentry` reachable. The holder class can then be unloaded, resulting in a crash.
>> 
>> The fix I've chosen for this is to mimic what we already do in `MethodHandles::jump_to_lambda_form`, and re-load the `vmentry` field from the target method handle each time. Luckily, this does not really seem to impact performance.
>> 
>> <details>
>> <summary>Performance numbers</summary>
>> x64:
>> 
>> before:
>> 
>> Benchmark             Mode  Cnt   Score   Error  Units
>> Upcalls.panama_blank  avgt   30  69.216 ± 1.791  ns/op
>> 
>> 
>> after:
>> 
>> Benchmark             Mode  Cnt   Score   Error  Units
>> Upcalls.panama_blank  avgt   30  67.787 ± 0.684  ns/op
>> 
>> 
>> aarch64:
>> 
>> before:
>> 
>> Benchmark             Mode  Cnt   Score   Error  Units
>> Upcalls.panama_blank  avgt   30  61.574 ± 0.801  ns/op
>> 
>> 
>> after:
>> 
>> Benchmark             Mode  Cnt   Score   Error  Units
>> Upcalls.panama_blank  avgt   30  61.218 ± 0.554  ns/op
>> 
>> </details>
>> 
>> As for the added TestUpcallStress test, it takes about 800 seconds to run this test on the dev machine I'm using, so I've set the timeout quite high. Since it runs for so long, I've dropped it from the default `jdk_foreign` test suite, which runs in tier2. Instead the new test will run in tier4.
>> 
>> Testing: tier 1-4
>
> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
> 
>   remove PC save/restore on s390

src/hotspot/share/prims/upcallLinker.cpp line 160:

> 158: 
> 159:   assert(entry->method_holder()->is_initialized(), "no clinit barrier");
> 160:   CompilationPolicy::compile_if_required(mh_entry, CHECK_0);

Note that this call to `compile_if_required` doesn't make sense here, since the target method can change (through a race). But also: we already call `compile_if_required` for the target of a method handle in `CallInfo::set_common`, which we reach when resolving the target `vmentry` (a `MemberName`).

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PR Review Comment: https://git.openjdk.org/jdk/pull/20479#discussion_r1750172834