Why biased-locking use the different mark word with lightweight-locking

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When one thread hold a bassed-locking. The mark word is thread id. But for lightweight-locking, the mark word is stack pointer to the thread, why not store thread id still?

jvm locking
1 Answers

Thin lock schema assumes that the mark word of a locked object points to a stack slot of the frame that has locked this object. This stack slot stores the original object header (aka displaced header).

    Unlocked:
    [ orig_header | 001 ]       | Stack frame |
                                |             |
    Locked:                     |             |
    [ stack_ptr   | 000 ]       |             |
         |                      |-------------|
          --------------------->| orig_header |
                                |-------------|
                                |             |
                                |             |
                                 -------------

Obviously the stack slot carries more information than a thread ID, since you can derive thread ID from a stack slot, but not vice versa.

Unlike biased schema where unlock operation is effectively a no-op, thin locks need to restore the original header when an object is unlocked. This becomes very simple since the mark word already points to a stack slot with the original value.

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