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

[Daniel D.Daugherty]

On Tue, 10 Nov 2020 23:15:15 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> 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...
>
> Well since it controls async deflation, it should probably get a mention since this comment on its own is not true:
> 
> // Keep track of contention for JVM/TI and M&M queries and control async deflation.
> 
> The field _contentions has a good comment.

The comment for **that** location is correct. That's the location
where we keep track of contended monitors for JVM/TI and M&M.
The comments at the other locations are also correct (very verbose
for two of them, but correct).

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