RFR: 8287788: Implement a better allocator for downcalls [v8]

[Maurizio Cimadamore]

On Thu, 23 Jan 2025 13:07:03 GMT, Matthias Ernst <duke at openjdk.org> wrote:

> Ah no, the allocate fails of course:
> 
> ```
> java.lang.WrongThreadException: Attempted access outside owning thread
> 	at java.base/jdk.internal.foreign.MemorySessionImpl.wrongThread(MemorySessionImpl.java:322)
> 	at java.base/jdk.internal.misc.ScopedMemoryAccess$ScopedAccessError.newRuntimeException(ScopedMemoryAccess.java:114)
> 	at java.base/jdk.internal.foreign.MemorySessionImpl.checkValidState(MemorySessionImpl.java:217)
> 	at java.base/jdk.internal.foreign.SegmentFactories.allocateSegment(SegmentFactories.java:181)
> 	at java.base/jdk.internal.foreign.ArenaImpl.allocateNoInit(ArenaImpl.java:52)
> 	at java.base/jdk.internal.foreign.ArenaImpl.allocate(ArenaImpl.java:57)
> 	at java.base/jdk.internal.foreign.ArenaImpl.allocate(ArenaImpl.java:31)
> 	at java.base/java.lang.foreign.SegmentAllocator.allocate(SegmentAllocator.java:644)
> 	at java.base/jdk.internal.foreign.abi.BufferStack$PerThread.<init>(BufferStack.java:73)
> 	at java.base/jdk.internal.foreign.abi.BufferStack$1.initialValue(BufferStack.java:50)
> 	at java.base/jdk.internal.foreign.abi.BufferStack$1.initialValue(BufferStack.java:47)
> 	at java.base/java.lang.ThreadLocal.setInitialValue(ThreadLocal.java:224)
> 	at java.base/java.lang.ThreadLocal.get(ThreadLocal.java:193)
> 	at java.base/java.lang.ThreadLocal.getCarrierThreadLocal(ThreadLocal.java:180)
> 	at java.base/java.lang.System$1.getCarrierThreadLocal(System.java:2237)
> 	at java.base/jdk.internal.misc.CarrierThreadLocal.get(CarrierThreadLocal.java:39)
> 	at java.base/jdk.internal.foreign.abi.BufferStack.pushFrame(BufferStack.java:61)
> 	at TestBufferStack.lambda$stress$1(TestBufferStack.java:113)
> ```

I was under the impression (which appears incorrect) that `initialValue` would be called by the carrier thread as well. So, we basically have a mismatch where `initialValue` is called with thread A, but `terminate` is called on thread B ?

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PR Review Comment: https://git.openjdk.org/jdk/pull/23142#discussion_r1927067860