RFR: 8299673: Simplify object pinning interactions with string deduplication

[Erik Österlund]

On Wed, 11 Jan 2023 04:43:54 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> When raw char* String contents are exposed to JNI code, we 
>> 
>> 1. Load the string.value and pin it 
>> 2. Run native code 
>> 3. Load the string.value and unpin it 
>> 
>> Given this sequence we would be in trouble if between 1 and 3, string deduplication changed the value object. Then the pinning and unpinning wouldn't be balanced. 
>> 
>> The current approach for dealing with this is to have a bunch of code to guard against deduplication. An alternative simpler solution is to just change step 3 to pass in the same value. We already have enough information available to do that. Then the pinning and unpinning is also balanced, and we don't need to have any special interactions with string deduplication and can decouple these orthogonal concerns.
>> 
>> It's worth noting though that the contract of pin_object now makes it explicit that pinned objects must not be recycled, even if not otherwise reachable. That seems to come naturally for region based pinning, but is worth keeping in mind. The exposed char* might be the only thing referencing the string value when string dedup happens concurrently.
>
> Looks good.

Thank you for the reviews, @kimbarrett and @stefank!

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PR: https://git.openjdk.org/jdk/pull/11923