RFR: 8306767: Concurrent repacking of extra data in MethodData is potentially unsafe [v19]

[Emanuel Peter]

> As explained in a [comment below](https://github.com/openjdk/jdk/pull/16840#issuecomment-1833529561), we have to ensure that reading/writing/cleaning the extra data all needs to be guarded by the `extra_data_lock`, and that no safepoint should happen while holding that lock, so that the lock is not broken.
> 
> I introduced `check_extra_data_locked`, where I check that we hold the lock, and if we are a java thread (only those ever safepoint), that we currently are in a `NoSafepointVerifier` scope, hence we verify that no safepoint will be taken.
> 
> I placed `check_extra_data_locked` in all the places where we access the extra data, and then placed locks and no-safepoint-verifiers at the call-site of those places.
> 
> I also needed to change the rank of `extra_data_lock` to `nosafepoint` and set the `Mutex::_no_safepoint_check_flag` when taking the lock. Otherwise I could not take the lock from a VM thread.
> 
> **Complications with ttyl**
> There were a few places in printing code, where did `ttyLocker ttyl;`, and then in that scope we would access the extra data. Now that I introduced locking with `extra_data_lock`, this ran into asserts which check the lock ranks: `ttyl` has a very low rank, and `extra_data_lock` a rather high lock. Hence, we cannot lock `extra_data_lock` inside a `ttyl` scope.
> 
> If we were to simply remove the `ttyl` locking, then the many print lines inside that scope might be interrupted and another thread can insert other printing in between. To avoid that, I now first buffer all lines in a `stringStream`, and then print that buffered stream to `tty` all at once, which means no other printing can be injected in between.
> 
> **Testing**
> Testing: tier1-3 and stress.

Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:

  improved comment for Roland

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16840/files
  - new: https://git.openjdk.org/jdk/pull/16840/files/78a2cdb6..4dbfe9a7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16840&range=18
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16840&range=17-18

  Stats: 7 lines in 1 file changed: 4 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16840.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16840/head:pull/16840

PR: https://git.openjdk.org/jdk/pull/16840