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

[Roman Kennke]

On Tue, 28 Mar 2023 16:00:34 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Please also verify against over- and underflow, and better than just null checks check that every oop really is an oop. I added this to my code:
>> 
>>   assert((_offset <=  end_offset()), "lockstack overflow: _offset %d end_offset %d", _offset, end_offset());
>>   assert((_offset >= start_offset()), "lockstack underflow: _offset %d end_offset %d", _offset, start_offset());
>>   int end = to_index(_offset);
>>   for (int i = 0; i < end; i++) {
>>     assert(oopDesc::is_oop(_base[i]), "index %i: not an oop (" PTR_FORMAT ")", i, p2i(_base[i]));
>> ...
>
> Just realized that my proposal of oop-checking does not work since during GC oop can be moved and will temporarily be invalid.

Checking for is_oop() may not work, because oops may be temporarily invalid, until the GC gets to fix them. This is especially (and probably only) true around oops_do().

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PR Review Comment: https://git.openjdk.org/jdk/pull/10907#discussion_r1152089622