RFR: 8266557: assert(SafepointMechanism::local_poll_armed(_handshakee)) failed: Must be [v6]

[Robbin Ehn]

> Please consider this change-set which address the issue on hand.
> 
> I identified two problems:
> 
> - is_locked() uses the _owner field which is unordered (no storestore|storeload) on store-side.
> Fixed by leaving the handshakee being processed in queue until completed.
> And remove looping, since if ever the queue is empty the handshakee may processed.
> If ever want to loop again, we must make sure queue is not empty before removing the processed handshake.
> But there is, at this moment, no performance benefit to that, so I chosse the simple, easy to reason about version. (some crazy stress test can see a difference)
> 
> Note that I'll do a follow-up and make is_locked() ifdef ASSERT only.
> 
> - have_non_self_executable_operation() only provide correct acquire if first in queue matched, if second item matched it could be re-orderd with reading the poll state.
> Fixed by adding a loadload.
> 
> I could at first reproduce by checking _active_handshaker in update_poll (~1/50) and an increase in the test time by ten.
> (real reprod ~1/400 with increased test time)
> If we are updating the poll there should not be an active handshaker.
> The above fixed the issue.
> But after a rebase when I was trying to pin point the issue I could no longer reproduce even without any changes.
> 
> Added Atomic store/load to _active_handshaker since it may be concurrently loaded (it may only be used to ask if current thread is active handshaker).
> 
> Passes stressing relevant test on aarch64 and t1-7.
> 
> Thanks, Robbin

Robbin Ehn has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains ten additional commits since the last revision:

 - Merge branch 'master' into handshakee
 - Merge branch 'master' into handshakee
 - Added comment
 - Small review updates
 - Merge branch 'master' into handshakee
 - Merge branch 'master' into handshakee
 - Small update
 - Merge branch 'master' into handshakee
 - Fix

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3973/files
  - new: https://git.openjdk.java.net/jdk/pull/3973/files/b0b22231..7277d7a6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3973&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3973&range=04-05

  Stats: 11266 lines in 295 files changed: 8827 ins; 1635 del; 804 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3973.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3973/head:pull/3973

PR: https://git.openjdk.java.net/jdk/pull/3973