RFR: 8343840: Rewrite the ObjectMonitor lists [v2]

[Patricio Chilano Mateo]

On Tue, 4 Mar 2025 04:50:34 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> src/hotspot/share/runtime/objectMonitor.cpp line 204:
>> 
>>> 202: //   If the thread (F) that removes itself from the end of the list
>>> 203: //   hasn't got any prev pointer, we just set the tail pointer to
>>> 204: //   null, see 5) and 6) below.
>> 
>> Setting the tail pointer to null would be for the case when this node is also the head, i.e single element. Otherwise we just rebuild the doubly link list, unlink F, and set entry_list_tail to G. In other words, the comment here and below seems to be missing that we have to build the doubly link list when F acquires the monitor, not when F needs to find a successor.
>
> We don't rebuild at this point. The thread that is removing itself just sets tail to null if there is no prev. Later when F exits the monitor it will construct the DLL to find the next successor.

But if there is a previous node (just no previous pointer set) we have to rebuild the list, otherwise G would still be pointing to F. It would be this case: https://github.com/fbredber/jdk/blob/283c2431ec64b0865d4e678913c636732d01658f/src/hotspot/share/runtime/objectMonitor.cpp#L1313

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PR Review Comment: https://git.openjdk.org/jdk/pull/23421#discussion_r1979921706