RFR: 8344332: (bf) Migrate DirectByteBuffer to use java.lang.ref.Cleaner [v8]

[Brent Christian]

On Wed, 22 Jan 2025 17:50:52 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> DirectByteBuffers are still using old `jdk.internal.ref.Cleaner` implementation. That implementation carries a doubly-linked list, and so makes DBB suffer from the same issue fixed for generic `java.lang.ref.Cleaner` users with [JDK-8343704](https://bugs.openjdk.org/browse/JDK-8343704). See the bug for the reproducer.
>> 
>> We can migrate DBBs to use `java.lang.ref.Cleaner`.
>> 
>> There are two pecularities during this rewrite.
>> 
>> First, the old ad-hoc `Cleaner` implementation used to exit the VM when cleaning action failed. I presume it was to avoid memory leak / accidental reuse of the buffer. I moved the relevant block to `Deallocator` directly. Unfortunately, I cannot easily test it.
>> 
>> Second is quite a bit hairy. Old DBB cleaning code was hooked straight into `Reference` processing loop. This was possible because we could infer that the weak references we are processing were DBB cleaning actions, since old `Cleaner` was the only use of this code. With standard `Cleaner`, we have lost this association, and so we cannot really do this from the reference processing loop. With the patched version, we now rely on normal `Cleaner` thread to do cleanups for us. Because of this, there is a new outpacing opportunity window where reference processing might have been over, but cleaner thread has not reacted yet.
>> 
>> Tests show that DirectBufferAlloc tests are still surviving this, possibly due to exponential sleep-backoff already built in. See the reclamation path in `Bits.unreserveMemory`: https://github.com/openjdk/jdk/blob/c207cc7e705d3f449f2387324d86cfb31ce40c44/src/java.base/share/classes/java/nio/Bits.java#L106-L186
>> 
>> Additional testing:
>>  - [x] Linux x86_64 server fastdebug, `java/nio java/io`
>>  - [x] Linux AArch64 server fastdebug, `java/nio java/io`
>>  - [ ] Linux x86_64 server fastdebug, `all`
>>  - [ ] Linux AArch64 server fastdebug, `all`
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Alan's review

src/java.base/share/classes/java/nio/BufferCleaner.java line 31:

> 29: import java.lang.ref.Cleaner.Cleanable;
> 30: 
> 31: class BufferCleaner {

It looks like `BufferCleaner` is not meant to be instantiated. A private constructor (that throws?) would make that clear.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22165#discussion_r1926125201