RFR: 8288904: Incorrect memory ordering in UL [v2]

[Johan Sjölén]

On Wed, 22 Jun 2022 12:31:31 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Johan Sjölén has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Comment the loadstore
>
> This fix in itself looks good.
> 
> But it raises a lot of questions for me about how concurrent access is actual controlled here. I can't see what stops a new reader from becoming active after we see _active_readers go to zero?
> 
> Thanks.

@dholmes-ora,

The implementation is lock-free and uses RCU.

This is approximately how it works in pseudo-code:


class LogOutputList {
  LogOutputNode* entry; // All threads gain entry to the linked list through this pointer
}

void LogOutputList::mutate_output_list() {
  // Save the entry pointer
  LogOutputNode* cur = entry;
  // Remove the entry pointer, now no new readers can access this list
  entry = NULL;
  // OK, we can now wait for all readers to exit
  wait_until_no_readers();
  // ... Do things we need to do to cur ...
  // Restore pointer, new readers can occur.
  entry = cur;
} 


There are some details here which I have omitted, but this is the gist of it.

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PR: https://git.openjdk.org/jdk/pull/9225