RFR: 8319773: Avoid inflating monitors when installing hash codes for LM_LIGHTWEIGHT [v10]

[Daniel D. Daugherty]

On Fri, 17 Nov 2023 07:19:51 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> Do we inflate when the VMThread is doing JVM/TI tagging?
>
> LM_LEGACY and LM_MONITOR will. LM_LIGHTWEIGHT technically may. If deflation finishes between reading the mark word in FastHashCode and reading the mark word in `inflate`. It seems like a rare enough case that it does not need to be handled separately. The following change would avoid inflation all together.
> 
>     // An async deflation can race after the inflate() call and before we
>     // can update the ObjectMonitor's header with the hash value below.
> +   if (LockingMode == LM_LIGHTWEIGHT) {
> +     assert(mark.has_monitor(), "must be");
> +     monitor =  mark.monitor();
> +   } else {
> -   monitor = inflate(current, obj, inflate_cause_hash_code);
> +     monitor = inflate(current, obj, inflate_cause_hash_code);
> +   }
>     // Load ObjectMonitor's header/dmw field and see if it has a hash.
> 
> 
> Maybe I should change it to this. Given that there has been confusion here.
> My ideal solution would be to separate the implementations for the different locking modes all together, all of these functions are littered with if (LockingMode == X).

So what did you decide to do here?

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PR Review Comment: https://git.openjdk.org/jdk/pull/16603#discussion_r1409704775