RFR: 8268778: CDS check_excluded_classes needs DumpTimeTable_lock

[Ioi Lam]

On Tue, 15 Jun 2021 17:07:50 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> Fixing a mistake in https://git.openjdk.java.net/jdk/pull/4322
> 
> `SystemDictionaryShared::check_excluded_classes()` iterates over `_dumptime_table`, so we must hold the `DumpTimeTable_lock` first. Otherwise concurrent class unloading may be modifying the `_dumptime_table` while we are still iterating it.
> 
> I also removed an outdated comment.
> 
> (This fix should be backported to JDK 17, but I want to have some mileage on it in the JDK 18 repo first.)

mach5 tiers1-4 passed.
Thanks @calvinccheung and @yminqi for review.

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

PR: https://git.openjdk.java.net/jdk/pull/4494