RFR: 8253064: monitor list simplifications and getting rid of TSM [v4]

[Daniel D.Daugherty]

On Tue, 10 Nov 2020 22:27:06 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> No it is not a ref_count. We got rid of the accurate ref_count field
>> because maintaining it was too slow.
>> 
>> contentions just tells you how many threads have blocked on the
>> slow-path trying to enter the monitor. The fast-paths never touch
>> contentions. The comment on line 360 is accurate.
>
> But contentions is used for more than informing JVMTI, it's used to test whether the monitor is_busy on the slow path.  That's why I wanted the comment to say something like your last sentence, since I spent time trying to understand why the various calls to add_to_contentions(-1) in deflate_monitor earlier.

Ahhh... I think I understand your confusion. This line:

L550: // Deferred decrement for the JT EnterI() that cancelled the async deflation.
L551:  add_to_contentions(-1);

doesn't match up with this line:

L361: add_to_contentions(1);

It matches up with one of these:
  if (try_set_owner_from(DEFLATER_MARKER, Self) == DEFLATER_MARKER) {
    // Cancelled the in-progress async deflation by changing owner from
    // DEFLATER_MARKER to Self. As part of the contended enter protocol,
    // contentions was incremented to a positive value before EnterI()
    // was called and that prevents the deflater thread from winning the
    // last part of the 2-part async deflation protocol. After EnterI()
    // returns to enter(), contentions is decremented because the caller
    // now owns the monitor. We bump contentions an extra time here to
    // prevent the deflater thread from winning the last part of the
    // 2-part async deflation protocol after the regular decrement
    // occurs in enter(). The deflater thread will decrement contentions
    // after it recognizes that the async deflation was cancelled.
    add_to_contentions(1);

The long comments are in the two places where we temporarily
increment contentions to stop the race with the deflater thread
and the shorter comment, e.g., L550, are for where we undo the
temporary increment.

The primary purpose of the contentions field is for JVM/TI and
M&M queries. We just (temporarily) steal it for async deflation
purposes...

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PR: https://git.openjdk.java.net/jdk/pull/642