RFR: 8344332: (bf) Migrate DirectByteBuffer to use java.lang.ref.Cleaner [v7]

[Alan Bateman]

On Wed, 22 Jan 2025 15:58:14 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:
> 
>   Remove vestigial reference to waitForReferenceProcessing in tests

I've done a pass over the latest version and I think it's okay but want to do another pass on tomorrow, only because this is change code in a very critical area.

src/java.base/share/classes/java/nio/Bits.java line 160:

> 158:                 } catch (InterruptedException e) {
> 159:                     interrupted = true;
> 160:                     break;

I'm not sure that bailing out to throw OOME when interrupted is the right thing to do here. If ByteBuffer.allocateDIrect is called with the interrupt status set, or someone interrupts a thread while allocating on this slow path, then it would be surprising to have it throw OOME when it might have succeeded (with the interrupt status set).

src/java.base/share/classes/java/nio/BufferCleaner.java line 48:

> 46:     }
> 47: 
> 48:     public static class Canary implements Runnable {

Confusing for Canary to be public, it's not accessible outside of this package.

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PR Comment: https://git.openjdk.org/jdk/pull/22165#issuecomment-2607840604
PR Review Comment: https://git.openjdk.org/jdk/pull/22165#discussion_r1925699621
PR Review Comment: https://git.openjdk.org/jdk/pull/22165#discussion_r1925700380