RFR: 8273695: Safepoint deadlock on VMOperation_lock

[Robbin Ehn]

On Tue, 21 Sep 2021 16:15:07 GMT, Daniel D. Daugherty <dcubed 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.
>
> src/hotspot/share/runtime/safepointMechanism.inline.hpp line 81:
> 
>> 79:   // 1: A suspend handshake targets the JavaThread.
>> 80:   // 2: A safepoint takes place.
>> 81:   // 3: The JavaThread wakes-up from blocked and do not allow suspend.
> 
> typo: s/do not/does not/

Fixed

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