RFR: 8314225: SIGSEGV in JavaThread::is_lock_owned [v2]

[David Holmes]

On Fri, 26 Jan 2024 21:34:44 GMT, Kevin Walls <kevinw at openjdk.org> wrote:

>> JavaThread's _monitor_chunks member is temporary storage used by deoptimization.
>> When other threads inspect it using JavaThread::monitor_chunks(), if it is non-null that means a deoptimization is in progress, and the value will be removed shortly.
>> 
>> There are a few places where we attempt to follow the MonitorChunk*, but that would only be valid if deopt is in progress, and only safe if we could know the deopt is not going to complete.  But that the deopt will complete, and will free the MonitorChunks and clear the value.  So this is rare but there is a race and a risk of following a MonitorChunk* as it gets freed, and crashing.
>
> Kevin Walls has updated the pull request incrementally with one additional commit since the last revision:
> 
>   ThreadsListHandle required for Handshake

AFAICS the `BasicLock` code has been basically unchanged for many years (Java 5 at least) and the `markword` has never pointed to a `MonitorChunk`.

But it does concern me that we don't seem to be able to come up with the reason that `is_lock_owned` searches the monitor chunk list. It is effectively "day one" code (Oct 1997).

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PR Comment: https://git.openjdk.org/jdk/pull/17566#issuecomment-1923045089