How to pin/avoid gc for an oop?

[Christian]

Hi Jon,

The STM is to be integrated into the JVM, but the transactional 
execution only affects modifications to the managed memory done by 
bytecode instructions. JVM internal structures are unaffected.

Here an example:

class X {
   int f;

   public static update(X x, int _new) {
      atomic { // tx start
         // (1) update field f of x:

         // undo log maintains a reference to x and old value of f
         // note: x contains a direct reference to the lock

         x.f = _new;

         // (2) assume GC is run
         // Note: The atomic section can be large and blocking,
         // thus, inhibiting a GC run is not an option.

         // (3) End of the atomic section (tx end):
         // 1. In case of abort: restore old value
         // 2. Release lock (obtained via reference to x)
      }
   }
}

How can I assure that the reference to x in the undo log that I made at 
(1) remains valid at (3), even though a GC run may occur at (2)? I 
thought that Handles (share/vm/runtime/handles.hpp) may be an option, 
but I might have misunderstood what they are used for. I can of course 
make the undo log in the managed heap as well, but I hoped that this can 
be done directly in the JVM.

Am 9. März 2017, 21:12, um 21:12, "Jon V." <sybersnake at gmail.com> schrieb:
>Are you trying to build a transactional memory system for the JVM or
>your
>own Java code?
>
>On Thu, Mar 9, 2017 at 2:33 PM Christian <ch-hagedorn at hispeed.ch>
>wrote:
>
>> Hi Jon,
>>
>> I need this for an undo log of a transactional memory system with
>> pessimistic locking/eager version management.
>> Am 9. März 2017, um 19:00, "Jon V." <sybersnake at gmail.com> schrieb:
>>
>> Can you clarify what you mean by Undo and why you think this should
>be
>> done at the VM level so we can better understand the request.
>>
>> If you don't want something to be garbage collected then simply don't
>> dereference it.
>>
>> On Thu, Mar 9, 2017 at 12:38 PM Christian Hagedorn
><ch-hagedorn at hispeed.ch>
>> wrote:
>>
>> Hi,
>>
>>
>>
>> I want to implement an undo functionality and thus need to access an
>oop
>> even when it is out of scope in the Java code. How can I pin such an
>oop to
>> avoid garbage collection of its used memory?
>>
>>
>>
>> Best regards,
>>
>> Christian
>>
>>