RFR: 8370203 - Add jcmd AOT.end_recording diagnostic command

[Mat Carter]

On Tue, 28 Oct 2025 09:33:28 GMT, Andrew Dinn <adinn at openjdk.org> wrote:

>> Add jcmd AOT.end_recording diagnostic command.  When this command is issued, a targeted JVM that is currently recording AOT information will stop recording.  Existing functionality is preserved: when stopped the JVM will create the required artifacts based on the execution mode.  Conveniently as the application running on the JVM has not stopped (as was previously the only way to stop recording), the application will resume execution after the artifacts have been generated.
>> 
>> The command will report back to the user one of the following messages depending on the state of the JVM:
>> 
>> - Error! Not a recording run
>> - Error! Not recording
>> - Recording ended successfully
>> - Error! Failed to end recording
>> 
>> It follows that issues the command to a JVM that is recording, twice in succession, should (baring internal errors) would produce the following two responses:
>> 
>> - Recording ended successfully
>> - Error! Not recording
>> 
>> Passes tier1 on linux (x64) and windows (x64)
>
> src/hotspot/share/cds/aotMetaspace.cpp line 963:
> 
>> 961: bool AOTMetaspace::is_recording_preimage_static_archive() {
>> 962:   if (CDSConfig::is_dumping_preimage_static_archive()) {
>> 963:       return _preimage_static_archive_dumped == 0;
> 
> Do we need to use an acquiring load here? i.e. can the read here and the cmpxchg below happen in different threads?
> n.b. I'm not just thinking about the behaviour when this patch makes the DCmd available but also what happens when we supplement it with the MXBean interface to end recordings.

I think it's okay as ending the recording is controlled by the cmpxchg, even if two threads think the recording is still going on, only one call to end the recording will work, and if the threads both check whether the recording has completed they will both see that it has (regardless of which thread 'won')

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PR Review Comment: https://git.openjdk.org/jdk/pull/27965#discussion_r2470342427