RFR: 8291555: Implement alternative fast-locking scheme [v29]

[David Holmes]

On Fri, 24 Mar 2023 07:00:35 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Merge remote-tracking branch 'origin/JDK-8291555-v2' into JDK-8291555-v2
>>  - Set condition flags correctly after fast-lock call on aarch64
>
>> The lock-stack is grown when needed.
> 
> Could you update the description of the PR with the latest approach please - others are unlikely to read all the comments to realize this has changed.

> @dholmes-ora
> 
> > Is this thread-safe?
> 
> I don't think so, but would the stacklock variant owning_thread_from_monitor_owner not suffer from the same problem?

@tstuefe Yes but that code has already had its thread-safety properties determined (presumably) long ago. Checking whether an address is within a thread's stack is pretty thread-safe. The new code needs to ensure that iteration with `contains` is safe in the face of a concurrent push/pop/remove.

Or  it may be these functions are only called at a safepoint? If so there should be an assert in that case, so I presume that is not the case.

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PR Comment: https://git.openjdk.org/jdk/pull/10907#issuecomment-1482707476