RFR: 8319797: Recursive lightweight locking: Runtime implementation

[Stefan Karlsson]

On Mon, 13 Nov 2023 01:15:14 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Implements the runtime part of JDK-8319796.
>> The different CPU implementations are/will be created as dependent pull requests.
>> 
>> This enhancement proposes introducing the ability for LM_LIGHTWEIGHT to handle consecutive recursive monitor enter. Limiting the implementation to only consecutive monitor enters allows for more efficient emitted code which only needs to look at the two top most entires on the lock stack to determine what to do in a monitor exit.
>> 
>> A high level overview:
>>   * Locking is still performed on the mark word
>>     * Unlocked (0b01) <=> Locked (0b00)
>>   * Monitor enter on Obj with mark word Unlocked (0b01) is the same
>>     * Transition Obj's mark word Unlocked (0b01) => Locked (0b00)
>>     * Push Obj onto the lock stack
>>     * Success
>>   * Monitor enter on Obj with mark word Locked (0b00) will check the top entry on the lock stack
>>     * If top entry is Obj
>>       * Push Obj on the lock stack
>>       * Success
>>     * If top entry is not Obj
>>       * Inflate and call ObjectMonitor::enter
>>   * Monitor exit on Obj with mark word Locked (0b00) will check the two top entries on the lock stack
>>     * If just the top entry is Obj
>>       * Transition Obj's mark word Locked (0b00) => Unlocked (0b01)
>>       * Pop the entry
>>       * Success
>>     * If both entries are Obj
>>       * Pop the top entry
>>       * Success
>>     * Any other case only occurs for unstructured locking, then just inflate and call ObjectMonitor::exit
>>   * If the monitor has been inflated for object Obj which is owned by the current thread
>>     * All corresponding entries for Obj is removed from the lock stack
>>     * The monitor recursions is set to the number of removed entries - 1
>>     * The owner is changed from anonymous to the thread
>>     * The regular ObjectMonitor::action is called.
>
> src/hotspot/share/runtime/lockStack.inline.hpp line 29:
> 
>> 27: #define SHARE_RUNTIME_LOCKSTACK_INLINE_HPP
>> 28: 
>> 29: #include "runtime/lockStack.hpp"
> 
> Why was this pulled out first?

I moved this include up because this is the mechanism we use to resolve circular dependencies between .inline.hpp files. If you take a look at most other .inline.hpp files you will see the same. I've written a section about this in the style guide:
https://github.com/openjdk/jdk/blob/master/doc/hotspot-style.md
> All .inline.hpp files should include their corresponding .hpp file as the first include line. Declarations needed by other files should be put in the .hpp file, and not in the .inline.hpp file. This rule exists to resolve problems with circular dependencies between .inline.hpp files.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16606#discussion_r1390715758