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

[Matthias Ernst]

On Thu, 23 Jan 2025 14:26:23 GMT, Maurizio Cimadamore <mcimadamore 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$1(TestBufferStack.java:113)
>
>> Ah no, the allocate fails of course:
>> 
>> ```
>> java.lang.WrongThreadException: Attempted access outside owning thread
>> 	at java.base/jdk.internal.foreign.MemorySessionImpl.wrongThread(MemorySessionImpl.java:322)
>> 	at java.base/jdk.internal.misc.ScopedMemoryAccess$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 ?

Exactly. initialValue happens on-demand, not upfront, i.e. in the very moment the VT calls get().
Whereas terminate happens as the last action of the CT.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23142#discussion_r1927071961