RFR: 8242427: JVMTI frame pop operations should use Thread-Local Handshakes
serguei.spitsyn at oracle.com
serguei.spitsyn at oracle.com
Wed Sep 2 07:10:18 UTC 2020
Hi David,
On 9/1/20 23:29, David Holmes wrote:
> 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.
Thank you for the comment.
My understanding is that a handshake (at least, direct) is an equivalent
of the current thread.
Is it correct?
Thanks,
Serguei
>
> 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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