RFR: 8324972: (bf) Make DirectByteBuffer.Deallocator idempotent [v3]

[Per Minborg]

On Wed, 31 Jan 2024 17:58:04 GMT, Mark Sheppard <msheppar at openjdk.org> wrote:

>> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Swith to using Unsafe
>
> src/java.base/share/classes/java/nio/Direct-X-Buffer.java.template line 99:
> 
>> 97:         public void run() {
>> 98:             // Ensure idempotency (paranoia)
>> 99:             if (Unsafe.getUnsafe().compareAndSetInt(this, INVOKED_OFFSET, 0, 1)) {
> 
> a question for my edification: is this compound statement suitably atomic to ensure synchronzied serial access to freeMemory block ?  There's no danger of concurrent interleaved execution resulting in inconsistent view of the member variable via the reflective  access ?

The CAS operation establishes a happens-before relation between the CAS operation itself and any following operations (such as `freeMemory()`). Also, only one thread will succeed in changing the value from zero to one.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/17647#discussion_r1482677523