RFR: 8253902: G1: Starting a new marking cycle before the conc mark thread fully completed causes assertion failure

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that fixes a benign race between giving the signal to start a new concurrent cycle
  and G1ConcurrentMarkThread waiting for that signal?

With the changes from JDK-8240556 there is a small window where after the concurrent mark thread sets itself to idle
(at the end of the `while (wait_for_next_cycle())` loop in `G1ConcurrentMarkThread::run_service()`), a GC pause may
request a new gc, setting the `in-progress` flag, and the concurrent mark thread waiting for a new cycle in
`wait_for_next_cycle()` where this assert fails.

This ordering can be observed in the log message output:

[342.714s][info ][gc] GC(5240) Concurrent Mark Cycle
[342.979s][info ][gc] GC(5240) Pause Remark 75M->75M(132M) 171.410ms
[343.059s][info ][gc] GC(5240) Pause Cleanup 122M->122M(132M) 0.174ms
[343.075s][info ][gc] GC(5241) Pause Young (Concurrent Start) (G1 Humongous Allocation) 123M->123M(132M) 14.023ms <---
sets in-pogress flag [343.078s][info ][gc] GC(5240) Concurrent Mark Cycle 363.855ms <--- occurs before the next
wait_for_next_cycle() call

Previously this problem with the assert did not occur because there has been another "pre-in-progress" state
implemented in a somewhat complicated way with nested states. JDK-8240556 removed that one. The messages could be out
of order before that change too.

The suggested fix is to remove this assert.

Testing: a few thousand times for that test (not to try to observe the assert which is gone now, but double-check if
there is any deadlock etc hiding), tier1-5

-------------

Commit messages:
 - Remove wrong assert, initial commit

Changes: https://git.openjdk.java.net/jdk/pull/501/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=501&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253902
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/501.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/501/head:pull/501

PR: https://git.openjdk.java.net/jdk/pull/501