On Sat, 10 Apr 2021 13:56:41 GMT, Daniel D. Daugherty <dcu...@openjdk.org>
wrote:
>> Hi Dan,
>>
>> thanks for picking up my question!
>>
>>> `HandshakeState::suspend()` is a synchronous handshake and adding the
>>> handshake to the queue is lock free, but the execution of the synchronous
>>> handshake itself requires a `HandshakeState::claim_handshake()` call which
>>> does acquire the lock in question.
>>
>> My point would be that the attempt to execute the synchronous handshake for
>> suspending a thread that is just about to call HandshakeState::thread_exit()
>> cannot make progress (blocks) while the target thread is not safe
>> (_thread_in_vm).
>>
>> A synchronous handshake requires the target thread to be in a safe state for
>> the
>> requester to execute the handshake operation. When executing
>> HandshakeState::thread_exit() the suspendee is _thread_in_vm. And the
>> requester
>> will find it to be `_not_safe` when calling
>> `possibly_can_process_handshake()`
>> before calling `HandshakeState::claim_handshake()` or when calling
>> `can_process_handshake()` afterwards. In both cases try_process() returns
>> with
>> failure _not_safe and the lock is not held.
>>
>> ++
>> 546 if (!possibly_can_process_handshake()) {
>> 547 // JT is observed in an unsafe state, it must notice the handshake
>> itself
>> 548 return HandshakeState::_not_safe;
>> 549 }
>> 550
>> 551 // Claim the mutex if there still an operation to be executed.
>> 552 if (!claim_handshake()) {
>> 553 return HandshakeState::_claim_failed;
>> 554 }
>> 555
>> 556 // If we own the mutex at this point and while owning the mutex we
>> 557 // can observe a safe state the thread cannot possibly continue
>> without
>> 558 // getting caught by the mutex.
>> 559 if (!can_process_handshake()) {
>> 560 _lock.unlock();
>> 561 return HandshakeState::_not_safe;
>> 562 }
>>
>> So isn't being unsafe sufficient to sync with suspend requests?
>
> Interesting point that I didn't pick up from your previous comment.
> Thanks for making it more clear for me. I need to mull on it for a bit.
Technically you are correct.
I have tested to remove this is previously version and all tests passes fine.
The reason I kept it is because the suspender have identified a thread for
suspension and deemed it suspendable, so we play nice.
To know why suspend failed the suspender must check if thread is exiting after
a failed suspend. (originally I had a bug here which caused me to wrongfully
introduce this from the beginning)
I'll remove it, since it simplifies the code and David's comments about this
code is now out-of-line can be fixed.
-------------
PR: https://git.openjdk.java.net/jdk/pull/3191
