RFR: 8221207: Redo JDK-8218446 - SuspendAtExit hangs

[David Holmes]

Thanks Dan! webrev updated in place.

I'll wait to push this until after your stress testing has been done.

David

On 22/03/2019 8:17 am, Daniel D. Daugherty wrote:
> On 3/21/19 5:52 PM, David Holmes wrote:
>> Hi Dan,
>>
>> Thanks again for the eagle-eyed analysis ...
>>
>> On 22/03/2019 5:57 am, Daniel D. Daugherty wrote:
>>> On 3/21/19 4:47 AM, David Holmes wrote:
>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8221207
>>>>
>>>> There was one small oversight in the original fix that led to 
>>>> crashes, seen (randomly) in JDI tests. The safepoint check must not 
>>>> happen if the thread-state is already _thread_in_native. I've 
>>>> checked the thread-state on all call paths to confirm that.
>>>>
>>>> Incremental webrev from original fix: 
>>>> http://cr.openjdk.java.net/~dholmes/8221207/webrev.inc/
>>>>
>>>> Full webrev: http://cr.openjdk.java.net/~dholmes/8221207/webrev/
>>>
>>> src/hotspot/share/runtime/thread.hpp
>>>      No comments.
>>>
>>> src/hotspot/share/runtime/thread.cpp
>>>      So here's the crashing stack:
>>>
>>>      V [libjvm.so+0x14b1fee] 
>>> SafepointSynchronize::block(JavaThread*)+0xae
>>>      V [libjvm.so+0x14ba78d] 
>>> SafepointMechanism::block_if_requested_slow(JavaThread*)+0x6d
>>>      V [libjvm.so+0x1637207] 
>>> JavaThread::handle_special_runtime_exit_condition(bool)+0x67
>>>      V [libjvm.so+0x1073b1e] 
>>> JvmtiJavaThreadEventTransition::JvmtiJavaThreadEventTransition(JavaThread*)+0x1ae 
>>>
>>>      V [libjvm.so+0x1069990] 
>>> JvmtiExport::post_class_prepare(JavaThread*, Klass*)+0x1b0
>>>
>>>      so we have a JvmtiJavaThreadEventTransition helper object to
>>>      handle the transition from thread_in_vm -> thread_in_native:
>>>
>>>      src/hotspot/share/prims/jvmtiExport.cpp
>>>
>>>          class JvmtiJavaThreadEventTransition : StackObj {
>>>          private:
>>>            ResourceMark _rm;
>>>            ThreadToNativeFromVM _transition;
>>>            HandleMark _hm;
>>>
>>>          public:
>>>            JvmtiJavaThreadEventTransition(JavaThread *thread) :
>>>              _rm(),
>>>              _transition(thread),
>>>              _hm(thread)  {};
>>>          };
>>>
>>>      so that's just a wrapper around ThreadToNativeFromVM:
>>>
>>>      src/hotspot/share/runtime/interfaceSupport.inline.hpp
>>>
>>>          class ThreadToNativeFromVM : public ThreadStateTransition {
>>>           public:
>>>            ThreadToNativeFromVM(JavaThread *thread) : 
>>> ThreadStateTransition(thread) {
>>>              // We are leaving the VM at this point and going 
>>> directly to native code.
>>>              // Block, if we are in the middle of a safepoint 
>>> synchronization.
>>>              assert(!thread->owns_locks(), "must release all locks 
>>> when leaving VM");
>>> thread->frame_anchor()->make_walkable(thread);
>>>              trans_and_fence(_thread_in_vm, _thread_in_native);
>>>              // Check for pending. async. exceptions or suspends.
>>>              if (_thread->has_special_runtime_exit_condition()) 
>>> _thread->handle_special_runtime_exit_condition(false);
>>>            }
>>>
>>>            ~ThreadToNativeFromVM() {
>>>              trans_from_native(_thread_in_vm);
>>> assert(!_thread->is_pending_jni_exception_check(), "Pending JNI 
>>> Exception Check");
>>>              // We don't need to clear_walkable because it will 
>>> happen automagically when we return to java
>>>            }
>>>          };
>>>
>>>      so trans_and_fence() calls transition_and_fence() which
>>>      does this:
>>>
>>>          static inline void transition_and_fence(JavaThread *thread, 
>>> JavaThreadState from, JavaThreadState to) {
>>>            assert(thread->thread_state() == from, "coming from wrong 
>>> thread state");
>>>            assert((from & 1) == 0 && (to & 1) == 0, "odd numbers are 
>>> transitions states");
>>>            // Change to transition state
>>>            thread->set_thread_state((JavaThreadState)(from + 1));
>>>
>>> InterfaceSupport::serialize_thread_state_with_handler(thread);
>>>
>>>            SafepointMechanism::block_if_requested(thread);
>>>            thread->set_thread_state(to);
>>>
>>> CHECK_UNHANDLED_OOPS_ONLY(thread->clear_unhandled_oops();)
>>>          }
>>>
>>>      So for this use of handle_special_runtime_exit_condition(false),
>>>      a safepoint is already handled by the previous 
>>> transition_and_fence()
>>>      with the thread still in the right thread state. However, if that
>>>      handle_special_runtime_exit_condition() honors a self-suspend
>>>      request and there's another safepoint, then we run the risk of
>>>      the VMThread seeing _thread_blocked during the self-suspend
>>>      phase of the thread and then the thread will go ahead into
>>>      thread_native without stopping for the safepoint.
>>>
>>>      Okay, but do we care? I don't think so. The thread will be off
>>>      in native code and if it returns quickly and the safepoint is
>>>      still active, then ~ThreadToNativeFromVM() should cause the
>>>      thread to block for the safepoint.
>>
>> The way I looked at this is that we are trying to ensure that the 
>> VMThread doesn't see _thread_blocked just before we restore the true 
>> state, and allow the thread to escape the safepoint. In this case the 
>> true state is _thread_in_native, which is safepoint-safe just as 
>> _thread_blocked is, so it doesn't matter if the VMThread sees the true 
>> state or _thread_blocked as the result is the same. Hence for 
>> _thread_in_native we don't need to do the additional safepoint check.
>>
>>>      So this is a long winded way of saying I think the revised
>>>      fix is okay. :-)
>>
>> Okay - thanks :)
>>
>>>      You added this comment for the new if-statement:
>>>
>>>      +// But it's more complicated than that as not all initial 
>>> thread-states are suitable for
>>>      +// doing safepoint checks. Fortunately, _thread_in_native is 
>>> the only unsuitable state we
>>>      +// can encounter based on our two callers.
>>>
>>>      and I'm okay with it.
>>>
>>>      Please consider adding this comment:
>>>
>>>      +  if (state != _thread_in_native) {
>>>           // _thread_in_native will block for a safepoint when it 
>>> transitions back.
>>>           SafepointMechanism::block_if_requested(this);
>>>      +  }
>>
>> It's a correct statement but I'm not sure its really relevant to the 
>> context. But I didn't explain in the code why _thread_in_native is 
>> special. My "But it's more complicated ..." comment doesn't explain 
>> things properly. How about:
>>
>> // However, not all initial-states are allowed when performing a
>> // safepoint check, as we should never be blocking at a safepoint
>> // whilst in those states. Of these 'bad' states only _thread_in_native
>> // is possible when executing this code (based on our two callers).
>> // A thread that is _thread_in_native is already safepoint-safe and so
>> // it doesn't matter whether the VMThread sees the _thread_blocked
>> // state, or the _thread_in_native state, and so we don't need the
>> // explicit safepoint check.
> 
> I like it!
> 
> Dan
> 
> 
>>
>> ?
>>
>> Thanks,
>> David
>> -----
>>
>>> Thumbs up!
>>>
>>> Dan
>>>
>>>
>>>>
>>>> Re-tested in mach5 tiers 1-3 and com/sun/jdi tests (but they passed 
>>>> last time too.).
>>>>
>>>> Thanks,
>>>> David
>>>>
>>>
>