RFR: 8242427: JVMTI frame pop operations should use Thread-Local Handshakes
David Holmes
david.holmes at oracle.com
Wed Sep 2 06:29:44 UTC 2020
Hi Serguei,
On 2/09/2020 4:11 pm, serguei.spitsyn at oracle.com wrote:
> Hi Yasumasa,
>
> It seems to me your update for sync with the JvmtiThreadState_lock is
> incorrect.
> Let me explain it.
> The original design was that the functions is_frame_pop, set_frame_pop,
> clear_frame_pop and clear_to_frame_pop are always called either on the
> current thread or in a VMop.
> There are 3 levels of these functions: in JvmtiEnvThreadState,
> JvmtiEventController and JvmtiEventControllerPrivate.
> You already found the JvmtiThreadState_lock is grabbed in the
> JvmtiEventController versions of these functions.
> It is for MT-safety of the recompute_thread_enabled() which can be
> called not only on current thread and VMop.
Right, but now that we use a handshake, not a VMop, we have no safepoint
to guarantee MT-safety and so we have to use the lock to ensure that.
David
-----
> So, I think adding MutexLocker's to the jvmtiEnv.cpp and jvmtiExport.cpp
> is not needed:
>
> + MutexLocker mu(JvmtiThreadState_lock);
> + if (java_thread == JavaThread::current()) {
> + state->update_for_pop_top_frame();
> + } else {
> + UpdateForPopTopFrameClosure op(state);
>
> . . .
>
> + MutexLocker mu(JvmtiThreadState_lock);
> if (java_thread == JavaThread::current()) {
> int frame_number = state->count_frames() - depth;
> state->env_thread_state(this)->set_frame_pop(frame_number);
> } else {
> - VM_SetFramePop op(this, state, depth);
> - VMThread::execute(&op);
> - err = op.result();
> + SetFramePopClosure op(this, state, depth);
> + bool executed = Handshake::execute_direct(&op, java_thread);
> + err = executed ? op.result() : JVMTI_ERROR_THREAD_NOT_ALIVE;
>
> . . .
>
> + MutexLocker mu(JvmtiThreadState_lock);
> ets->clear_frame_pop(cur_frame_number);
>
>
> Instead, they have to be restored in the JvmtiEventController functions:
>
> void
> JvmtiEventController::set_frame_pop(JvmtiEnvThreadState *ets, JvmtiFramePop fpop) {
> - MutexLocker mu(SafepointSynchronize::is_at_safepoint() ? NULL : JvmtiThreadState_lock);
> + assert_lock_strong(JvmtiThreadState_lock);
> JvmtiEventControllerPrivate::set_frame_pop(ets, fpop);
> }
>
>
> void
> JvmtiEventController::clear_frame_pop(JvmtiEnvThreadState *ets, JvmtiFramePop fpop) {
> - MutexLocker mu(SafepointSynchronize::is_at_safepoint() ? NULL : JvmtiThreadState_lock);
> + assert_lock_strong(JvmtiThreadState_lock);
> JvmtiEventControllerPrivate::clear_frame_pop(ets, fpop);
> }
>
>
> void
> JvmtiEventController::clear_to_frame_pop(JvmtiEnvThreadState *ets, JvmtiFramePop fpop) {
> - MutexLocker mu(SafepointSynchronize::is_at_safepoint() ? NULL : JvmtiThreadState_lock);
> + assert_lock_strong(JvmtiThreadState_lock);
> JvmtiEventControllerPrivate::clear_to_frame_pop(ets, fpop);
> }
>
>
> Thanks,
> Serguei
>
>
> On 9/1/20 21:34, serguei.spitsyn at oracle.com wrote:
>> Hi Yasumasa,
>>
>>
>> On 9/1/20 21:17, Yasumasa Suenaga wrote:
>>> Hi David,
>>>
>>> On 2020/09/02 13:13, David Holmes wrote:
>>>> Hi Yasumasa,
>>>>
>>>> On 31/08/2020 7:10 pm, Yasumasa Suenaga wrote:
>>>>> Hi David,
>>>>>
>>>>> I uploaded new webrev. Could you review again?
>>>>>
>>>>> http://cr.openjdk.java.net/~ysuenaga/JDK-8242427/webrev.04/
>>>>>
>>>>> This webrev includes two changes:
>>>>>
>>>>> 1. Use assert_lock_strong() for JvmtiThreadState_lock
>>>>> http://hg.openjdk.java.net/jdk/submit/rev/c85f93d2042d
>>>>>
>>>>> 2. Check return value from execute_direct() with assert()
>>>>> http://hg.openjdk.java.net/jdk/submit/rev/8746e1651343
>>>>
>>>> The message for the assertion:
>>>>
>>>> assert(executed, "Direct handshake failed. Target thread is still
>>>> alive?");
>>>>
>>>> should be phrased:
>>>>
>>>> assert(executed, "Direct handshake failed. Target thread is not
>>>> alive?");
>>>>
>>>> otherwise it sounds like the expectation is that it should not be
>>>> alive.
>>>>
>>>> Other changes fine.
>>>>
>>>> No need to see updated webrev.
>>>
>>> Thanks for your review!
>>> I will fix them before pushing.
>>
>> Please, hold on.
>> I'm still reviewing this.
>> It is not clear yet if sync with the JvmtiThreadState_lock is fully
>> correct.
>>
>> Thanks,
>> Serguei
>>
>>>
>>>
>>> Yasumasa
>>>
>>>
>>>> Thanks,
>>>> David
>>>> -----
>>>>
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Yasumasa
>>>>>
>>>>>
>>>>> On 2020/08/31 15:22, Yasumasa Suenaga wrote:
>>>>>> Hi David,
>>>>>>
>>>>>> On 2020/08/31 14:43, David Holmes wrote:
>>>>>>> Hi Yasumasa,
>>>>>>>
>>>>>>> On 28/08/2020 1:01 pm, Yasumasa Suenaga wrote:
>>>>>>>> Hi David,
>>>>>>>>
>>>>>>>> On 2020/08/28 11:04, David Holmes wrote:
>>>>>>>>> Hi Yasumasa,
>>>>>>>>>
>>>>>>>>> On 28/08/2020 11:24 am, Yasumasa Suenaga wrote:
>>>>>>>>>> Hi David,
>>>>>>>>>>
>>>>>>>>>> On 2020/08/27 15:49, David Holmes wrote:
>>>>>>>>>>> Sorry I just realized I reviewed version 00 :(
>>>>>>>>>
>>>>>>>>> Note that my comments on version 00 in my earlier email still
>>>>>>>>> apply.
>>>>>>>>
>>>>>>>> I copied here your comment on webrev.00:
>>>>>>>>
>>>>>>>>>>>> I see. It is a pity that we have now lost that critical
>>>>>>>>>>>> indicator that shows how this operation can be nested within
>>>>>>>>>>>> another operation. The possibility of nesting is even more
>>>>>>>>>>>> obscure with JvmtiEnvThreadState::reset_current_location.
>>>>>>>>>>>> And the fact it is now up to the caller to handle that case
>>>>>>>>>>>> explicitly raises some concern - what will happen if you
>>>>>>>>>>>> call execute_direct whilst already in a handshake with the
>>>>>>>>>>>> target thread?
>>>>>>>>
>>>>>>>> I heard deadlock would be happen if execute_direct() calls in
>>>>>>>> direct handshake. Thus we need to use active_handshaker() in
>>>>>>>> this change.
>>>>>>>
>>>>>>> Okay. This is something we need to clarify with direct handshake
>>>>>>> usage information. I think it would be preferable if this was
>>>>>>> handled in execute_direct rather than the caller ... though it
>>>>>>> may also be the case that we need the writer of the handshake
>>>>>>> operation to give due consideration to nesting ...
>>>>>>
>>>>>> Agree, I also prefer to check whether caller is in direct
>>>>>> handshake in execute_direct().
>>>>>> But I think this is another enhancement because we need to change
>>>>>> the behavior of execute_direct().
>>>>>>
>>>>>>
>>>>>>>>>>>> Further comments:
>>>>>>>>>>>>
>>>>>>>>>>>> src/hotspot/share/prims/jvmtiEnvThreadState.cpp
>>>>>>>>>>>>
>>>>>>>>>>>> 194 #ifdef ASSERT
>>>>>>>>>>>> 195 Thread *current = Thread::current();
>>>>>>>>>>>> 196 #endif
>>>>>>>>>>>> 197 assert(get_thread() == current || current ==
>>>>>>>>>>>> get_thread()->active_handshaker(),
>>>>>>>>>>>> 198 "frame pop data only accessible from same
>>>>>>>>>>>> thread or direct handshake");
>>>>>>>>>>>>
>>>>>>>>>>>> Can you factor this out into a separate function so that it
>>>>>>>>>>>> is not repeated so often. Seems to me that there should be a
>>>>>>>>>>>> global function on Thread:
>>>>>>>>>>>> assert_current_thread_or_handshaker() [yes unpleasant name
>>>>>>>>>>>> but ...] that will allow us to stop repeating this code
>>>>>>>>>>>> fragment across numerous files. A follow up RFE for that
>>>>>>>>>>>> would be okay too (I see some guarantees that should
>>>>>>>>>>>> probably just be asserts so they need a bit more checking).
>>>>>>>>
>>>>>>>> I filed it as another RFE:
>>>>>>>> https://bugs.openjdk.java.net/browse/JDK-8252479
>>>>>>>
>>>>>>> Thanks.
>>>>>>>
>>>>>>>>
>>>>>>>>>>>> 331 Handshake::execute_direct(&op, _thread);
>>>>>>>>>>>>
>>>>>>>>>>>> You aren't checking the return value of execute_direct, but
>>>>>>>>>>>> I can't tell where _thread was checked for still being alive ??
>>>>>>>>>>>>
>>>>>>>>>>>> ---
>>>>>>>>>>>>
>>>>>>>>>>>> src/hotspot/share/prims/jvmtiEventController.cpp
>>>>>>>>>>>>
>>>>>>>>>>>> 340 Handshake::execute_direct(&hs, target);
>>>>>>>>>>>>
>>>>>>>>>>>> I know this is existing code but I have the same query as
>>>>>>>>>>>> above - no return value check and no clear check that the
>>>>>>>>>>>> JavaThread is still alive?
>>>>>>>>
>>>>>>>> Existing code seems to assume that target thread is alive, frame
>>>>>>>> operations (e.g. PopFrame()) should be performed on live thread.
>>>>>>>> And also existing code would not set any JVMTI error and cannot
>>>>>>>> propagate it to caller. So I do not add the check for thread state.
>>>>>>>
>>>>>>> Okay. But note that for PopFrame the tests for isAlive and
>>>>>>> is-suspended have already been performed before we do the
>>>>>>> execute_direct; so in that case we could simply assert that
>>>>>>> execute_direct returns true. Similarly for other cases.
>>>>>>
>>>>>> Ok, I will change as following in next webrev:
>>>>>>
>>>>>> ```
>>>>>> bool result = Handshake::execute_direct(&hs, target);
>>>>>> guarantee(result, "Direct handshake failed. Target thread is still
>>>>>> alive?");
>>>>>> ```
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> Yasumasa
>>>>>>
>>>>>>
>>>>>>>>>>>> Do we know if the existing tests actually test the nested
>>>>>>>>>>>> cases?
>>>>>>>>
>>>>>>>> I saw some error with assertion for JvmtiThreadState_lock and
>>>>>>>> safepoint in vmTestbase at first, so I guess nested call would
>>>>>>>> be tested, but I'm not sure.
>>>>>>>>
>>>>>>>>
>>>>>>>>>>> I have concerns with the added locking:
>>>>>>>>>>>
>>>>>>>>>>> MutexLocker mu(JvmtiThreadState_lock);
>>>>>>>>>>>
>>>>>>>>>>> Who else may be holding that lock? Could it be our target
>>>>>>>>>>> thread that we have already initiated a handshake with? (The
>>>>>>>>>>> lock ranking checks related to safepoints don't help us
>>>>>>>>>>> detect deadlocks between a target thread and its handshaker.
>>>>>>>>>>> :( )
>>>>>>>>>>
>>>>>>>>>> I checked source code again, then I couldn't find the point
>>>>>>>>>> that target thread already locked JvmtiThreadState_lock at
>>>>>>>>>> direct handshake.
>>>>>>>>>
>>>>>>>>> I'm very unclear exactly what state this lock guards and under
>>>>>>>>> what conditions. But looking at:
>>>>>>>>>
>>>>>>>>> src/hotspot/share/prims/jvmtiEnv.cpp
>>>>>>>>>
>>>>>>>>> Surely the lock is only needed in the direct-handshake case and
>>>>>>>>> not when operating on the current thread? Or is it there
>>>>>>>>> because you've removed the locking from the lower-level
>>>>>>>>> JvmtiEventController methods and so now you need to take the
>>>>>>>>> lock higher-up the call chain? (I find it hard to follow the
>>>>>>>>> call chains in the JVMTI code.)
>>>>>>>>
>>>>>>>> We need to take the lock higher-up the call chain. It is
>>>>>>>> suggested by Robbin, and works fine.
>>>>>>>
>>>>>>> Okay. It seems reasonably safe in this context as there is little
>>>>>>> additional work done while holding the lock.
>>>>>>>
>>>>>>>>
>>>>>>>>>>> It is far from clear now which functions are reachable from
>>>>>>>>>>> handshakes, which from safepoint VM_ops and which from both.
>>>>>>>>>>>
>>>>>>>>>>> ! assert(SafepointSynchronize::is_at_safepoint() ||
>>>>>>>>>>> JvmtiThreadState_lock->is_locked(), "Safepoint or must be
>>>>>>>>>>> locked");
>>>>>>>>>>>
>>>>>>>>>>> This can be written as:
>>>>>>>>>>>
>>>>>>>>>>> assert_locked_or_safepoint(JvmtiThreadState_lock);
>>>>>>>>>>>
>>>>>>>>>>> or possibly the weak variant of that. ('m puzzled by the
>>>>>>>>>>> extra check in the strong version ... I think it is intended
>>>>>>>>>>> for the case of the VMThread executing a non-safepoint VMop.)
>>>>>>>>>>
>>>>>>>>>>> JvmtiEventController::set_frame_pop(),
>>>>>>>>>>> JvmtiEventController::clear_frame_pop() and
>>>>>>>>>>> JvmtiEventController::clear_to_frame_pop() are no longer
>>>>>>>>>>> called at safepoint, so I remove safepoint check from
>>>>>>>>>>> assert() in new webrev.
>>>>>>>>>
>>>>>>>>> You should use assert_lock_strong for this.
>>>>>>>>
>>>>>>>> I will do that.
>>>>>>>
>>>>>>> Thanks,
>>>>>>> David
>>>>>>> -----
>>>>>>>
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>>
>>>>>>>> Yasumasa
>>>>>>>>
>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>> David
>>>>>>>>>
>>>>>>>>>> webrev:
>>>>>>>>>> http://cr.openjdk.java.net/~ysuenaga/JDK-8242427/webrev.03/
>>>>>>>>>> diff from previous webrev:
>>>>>>>>>> http://hg.openjdk.java.net/jdk/submit/rev/2a2c02ada080
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Thanks,
>>>>>>>>>>
>>>>>>>>>> Yasumasa
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>> Thanks,
>>>>>>>>>>> David
>>>>>>>>>>> -----
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> On 27/08/2020 4:34 pm, David Holmes wrote:
>>>>>>>>>>>> Hi Yasumasa,
>>>>>>>>>>>>
>>>>>>>>>>>> On 27/08/2020 9:40 am, Yasumasa Suenaga wrote:
>>>>>>>>>>>>> Hi David,
>>>>>>>>>>>>>
>>>>>>>>>>>>> On 2020/08/27 8:09, David Holmes wrote:
>>>>>>>>>>>>>> Hi Yasumasa,
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> On 26/08/2020 5:34 pm, Yasumasa Suenaga wrote:
>>>>>>>>>>>>>>> Hi Patricio, David,
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Thanks for your comment!
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> I updated webrev which includes the fix which is
>>>>>>>>>>>>>>> commented by Patricio, and it passed submit repo. So I
>>>>>>>>>>>>>>> switch this mail thread to RFR.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> JBS: https://bugs.openjdk.java.net/browse/JDK-8242427
>>>>>>>>>>>>>>> webrev:
>>>>>>>>>>>>>>> http://cr.openjdk.java.net/~ysuenaga/JDK-8242427/webrev.00/
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> I understand David said same concerns as Patricio about
>>>>>>>>>>>>>>> active handshaker. This webrev checks active handshaker
>>>>>>>>>>>>>>> is current thread or not.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> How can the current thread already be in a handshake with
>>>>>>>>>>>>>> the target when you execute this code?
>>>>>>>>>>>>>
>>>>>>>>>>>>> EnterInterpOnlyModeClosure might be called in handshake
>>>>>>>>>>>>> with UpdateForPopTopFrameClosure or with SetFramePopClosure.
>>>>>>>>>>>>>
>>>>>>>>>>>>> EnterInterpOnlyModeClosure is introduced in JDK-8238585 as
>>>>>>>>>>>>> an alternative in VM_EnterInterpOnlyMode.
>>>>>>>>>>>>> VM_EnterInterpOnlyMode returned true in
>>>>>>>>>>>>> allow_nested_vm_operations(). Originally, it could have
>>>>>>>>>>>>> been called from other VM operations.
>>>>>>>>>>>>
>>>>>>>>>>>> I see. It is a pity that we have now lost that critical
>>>>>>>>>>>> indicator that shows how this operation can be nested within
>>>>>>>>>>>> another operation. The possibility of nesting is even more
>>>>>>>>>>>> obscure with JvmtiEnvThreadState::reset_current_location.
>>>>>>>>>>>> And the fact it is now up to the caller to handle that case
>>>>>>>>>>>> explicitly raises some concern - what will happen if you
>>>>>>>>>>>> call execute_direct whilst already in a handshake with the
>>>>>>>>>>>> target thread?
>>>>>>>>>>>>
>>>>>>>>>>>> I can't help but feel that we need a more rigorous and
>>>>>>>>>>>> automated way of dealing with nesting ... perhaps we don't
>>>>>>>>>>>> even need to care and handshakes should always allow nested
>>>>>>>>>>>> handshake requests? (Question more for Robbin and Patricio.)
>>>>>>>>>>>>
>>>>>>>>>>>> Further comments:
>>>>>>>>>>>>
>>>>>>>>>>>> src/hotspot/share/prims/jvmtiEnvThreadState.cpp
>>>>>>>>>>>>
>>>>>>>>>>>> 194 #ifdef ASSERT
>>>>>>>>>>>> 195 Thread *current = Thread::current();
>>>>>>>>>>>> 196 #endif
>>>>>>>>>>>> 197 assert(get_thread() == current || current ==
>>>>>>>>>>>> get_thread()->active_handshaker(),
>>>>>>>>>>>> 198 "frame pop data only accessible from same
>>>>>>>>>>>> thread or direct handshake");
>>>>>>>>>>>>
>>>>>>>>>>>> Can you factor this out into a separate function so that it
>>>>>>>>>>>> is not repeated so often. Seems to me that there should be a
>>>>>>>>>>>> global function on Thread:
>>>>>>>>>>>> assert_current_thread_or_handshaker() [yes unpleasant name
>>>>>>>>>>>> but ...] that will allow us to stop repeating this code
>>>>>>>>>>>> fragment across numerous files. A follow up RFE for that
>>>>>>>>>>>> would be okay too (I see some guarantees that should
>>>>>>>>>>>> probably just be asserts so they need a bit more checking).
>>>>>>>>>>>>
>>>>>>>>>>>> 331 Handshake::execute_direct(&op, _thread);
>>>>>>>>>>>>
>>>>>>>>>>>> You aren't checking the return value of execute_direct, but
>>>>>>>>>>>> I can't tell where _thread was checked for still being alive ??
>>>>>>>>>>>>
>>>>>>>>>>>> ---
>>>>>>>>>>>>
>>>>>>>>>>>> src/hotspot/share/prims/jvmtiEventController.cpp
>>>>>>>>>>>>
>>>>>>>>>>>> 340 Handshake::execute_direct(&hs, target);
>>>>>>>>>>>>
>>>>>>>>>>>> I know this is existing code but I have the same query as
>>>>>>>>>>>> above - no return value check and no clear check that the
>>>>>>>>>>>> JavaThread is still alive?
>>>>>>>>>>>>
>>>>>>>>>>>> ---
>>>>>>>>>>>>
>>>>>>>>>>>> Do we know if the existing tests actually test the nested
>>>>>>>>>>>> cases?
>>>>>>>>>>>>
>>>>>>>>>>>> Thanks,
>>>>>>>>>>>> David
>>>>>>>>>>>> -----
>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>
>>>>>>>>>>>>> Yasumasa
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>> David
>>>>>>>>>>>>>> -----
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Cheers,
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Yasumasa
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> On 2020/08/26 10:13, Patricio Chilano wrote:
>>>>>>>>>>>>>>>> Hi Yasumasa,
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> On 8/23/20 11:40 PM, Yasumasa Suenaga wrote:
>>>>>>>>>>>>>>>>> Hi all,
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I want to hear your opinions about the change for
>>>>>>>>>>>>>>>>> JDK-8242427.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I'm trying to migrate following operations to direct
>>>>>>>>>>>>>>>>> handshake.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> - VM_UpdateForPopTopFrame
>>>>>>>>>>>>>>>>> - VM_SetFramePop
>>>>>>>>>>>>>>>>> - VM_GetCurrentLocation
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Some operations (VM_GetCurrentLocation and
>>>>>>>>>>>>>>>>> EnterInterpOnlyModeClosure) might be called at
>>>>>>>>>>>>>>>>> safepoint, so I want to use
>>>>>>>>>>>>>>>>> JavaThread::active_handshaker() in production VM to
>>>>>>>>>>>>>>>>> detect the process is in direct handshake or not.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> However this function is available in debug VM only, so
>>>>>>>>>>>>>>>>> I want to hear the reason why it is for debug VM only,
>>>>>>>>>>>>>>>>> and there are no problem to use it in production VM. Of
>>>>>>>>>>>>>>>>> course another solutions are welcome.
>>>>>>>>>>>>>>>> I added the _active_handshaker field to the
>>>>>>>>>>>>>>>> HandshakeState class when working on 8230594 to adjust
>>>>>>>>>>>>>>>> some asserts, where instead of checking for the VMThread
>>>>>>>>>>>>>>>> we needed to check for the active handshaker of the
>>>>>>>>>>>>>>>> target JavaThread. Since there were no other users of
>>>>>>>>>>>>>>>> it, there was no point in declaring it and having to
>>>>>>>>>>>>>>>> write to it for the release bits. There are no issues
>>>>>>>>>>>>>>>> with having it in production though so you could change
>>>>>>>>>>>>>>>> that if necessary.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> webrev is here. It passed jtreg tests
>>>>>>>>>>>>>>>>> (vmTestbase/nsk/{jdi,jdwp,jvmti}
>>>>>>>>>>>>>>>>> serviceability/{jdwp,jvmti})
>>>>>>>>>>>>>>>>> http://cr.openjdk.java.net/~ysuenaga/JDK-8242427/proposal/
>>>>>>>>>>>>>>>> Some comments on the proposed change.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> src/hotspot/share/prims/jvmtiEnvThreadState.cpp,
>>>>>>>>>>>>>>>> src/hotspot/share/prims/jvmtiEventController.cpp
>>>>>>>>>>>>>>>> Why is the check to decide whether to call the handshake
>>>>>>>>>>>>>>>> or execute the operation with the current thread
>>>>>>>>>>>>>>>> different for GetCurrentLocationClosure vs
>>>>>>>>>>>>>>>> EnterInterpOnlyModeClosure?
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> (GetCurrentLocationClosure)
>>>>>>>>>>>>>>>> if ((Thread::current() == _thread) ||
>>>>>>>>>>>>>>>> (_thread->active_handshaker() != NULL)) {
>>>>>>>>>>>>>>>> op.do_thread(_thread);
>>>>>>>>>>>>>>>> } else {
>>>>>>>>>>>>>>>> Handshake::execute_direct(&op, _thread);
>>>>>>>>>>>>>>>> }
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> vs
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> (EnterInterpOnlyModeClosure)
>>>>>>>>>>>>>>>> if (target->active_handshaker() != NULL) {
>>>>>>>>>>>>>>>> hs.do_thread(target);
>>>>>>>>>>>>>>>> } else {
>>>>>>>>>>>>>>>> Handshake::execute_direct(&hs, target);
>>>>>>>>>>>>>>>> }
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> If you change VM_SetFramePop to use handshakes then it
>>>>>>>>>>>>>>>> seems you could reach
>>>>>>>>>>>>>>>> JvmtiEventControllerPrivate::enter_interp_only_mode()
>>>>>>>>>>>>>>>> with the current thread being the target.
>>>>>>>>>>>>>>>> Also I think you want the second expression of that
>>>>>>>>>>>>>>>> check to be (target->active_handshaker() ==
>>>>>>>>>>>>>>>> Thread::current()). So either you are the target or the
>>>>>>>>>>>>>>>> current active_handshaker for that target. Otherwise
>>>>>>>>>>>>>>>> active_handshaker() could be not NULL because there is
>>>>>>>>>>>>>>>> another JavaThread handshaking the same target. Unless
>>>>>>>>>>>>>>>> you are certain that it can never happen, so if
>>>>>>>>>>>>>>>> active_handshaker() is not NULL it is always the current
>>>>>>>>>>>>>>>> thread, but even in that case this way is safer.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> src/hotspot/share/prims/jvmtiThreadState.cpp
>>>>>>>>>>>>>>>> The guarantee() statement exists in release builds too
>>>>>>>>>>>>>>>> so the "#ifdef ASSERT" directive should be removed,
>>>>>>>>>>>>>>>> otherwise "current" will not be declared.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Thanks!
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Patricio
>>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Yasumasa
>>>>>>>>>>>>>>>>
>>
>
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