RFR: 8242427: JVMTI frame pop operations should use Thread-Local Handshakes
serguei.spitsyn at oracle.com
serguei.spitsyn at oracle.com
Wed Sep 2 08:38:06 UTC 2020
Hi David,
On 9/2/20 00:32, David Holmes wrote:
> Hi Serguei,
>
> On 2/09/2020 5:10 pm, serguei.spitsyn at oracle.com wrote:
>> 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?
>
> A direct handshake operation can be executed by either the target
> thread or the handshaker.
My understanding is that it has to be MT-safe either it is executed by
the target thread or the handshaker.
So, the functions is_frame_pop, set_frame_pop, clear_frame_pop and
clear_to_frame_pop are always executed on the current/target thread or
the handshaker.
However, the concern is about some internal calls from these function,
e.g. recompute_thread_enabled.
It is why the JvmtiThreadState_lock sync is moved from the
JvmtiEventControllerPrivate to be around calls the JvmtiEnvThreadState
functions.
I do not see any particular issues now but the change looks a little bit
risky to me as it is easy to overlook problems.
It is a pity the JvmtiThreadState_lock can not be used inside handshakes.
I wonder if this can be allowed to restore the original sync approach.
We may observe more such problems with handshakes in the future if we
convert more VMops into handshakes.
> Not sure what difference that makes though. The JvmtiThreadState_lock
> is a very coarse-grained locked and presumably needs to be held across
> any operation that might access or update any thread-state whilst
> another thread could be doing the same. It would be better if this
> were per-thread of course but that isn't the way it works AFAIK.
> Are you suggesting the lock is only needed to protect access to the
> same thread's thread-state?
I'm not that concerned that a per-thread lock is not used to protect
thread states.
It seems to be okay unless some performance problems or deadlocks are
encountered.
Thanks,
Serguei
> David
>
>> 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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