RFR: 8242427: JVMTI frame pop operations should use Thread-Local Handshakes
David Holmes
david.holmes at oracle.com
Wed Sep 2 04:13:34 UTC 2020
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,
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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