RFR: 8273695: Safepoint deadlock on VMOperation_lock

[Daniel D.Daugherty]

On Tue, 21 Sep 2021 13:17:43 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

> We should not do any processing in SM::should_process().
> The query is used to determine if we need to process safepoint/handshakes and with this change StackWaterMark.
> When locking a Mutex which may be acquired in such processing, we must release that Mutex before we can start processing, otherwise we can deadlock.
> 
> This change adds a method to determine if StackWaterMarkSet::on_safepoint() will do any processing.
> In that case there are poll is armed, we do not allow suspend handshakes and there is no safepoint and no non-suspend handshakes, we still return true if StackWaterMarkSet needs processing.
> Thus the code querying should release any such Mutex and call process SM::process_if_requested().
> 
> The cross_modify_fence() do not have any such state, so we still need to emit that before returning false if poll is armed.
> 
> Passes t1-t4 and local stressing.

@robehn - There are a couple of other bugs that are considered duplicates
of this bug. Some of those track failures in Tier8 so it would be a good idea
to do testing up through Tier8.

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PR: https://git.openjdk.java.net/jdk/pull/5613