RFR (S): 8222534: VerifyBeforeExit is not honored when System.exit is called

[David Holmes]

Updated webrev: http://cr.openjdk.java.net/~dholmes/8222534/webrev.v2/

The historic setting of the thread state to _thread_in_vm is moved ahead 
of the taking of the heap_lock. That logic has been there since "exit at 
a safepoint" was introduced by JDK-4526887, and there's nothing to tell 
you how we might get there in a different state. To be safe I've left it 
and added a comment.

Thanks,
David

On 4/05/2019 9:45 am, David Holmes wrote:
> Hi Coleen,
> 
> On 4/05/2019 12:56 am, coleen.phillimore at oracle.com wrote:
>> On 4/30/19 11:42 PM, David Holmes wrote:
>>> Hi Coleen,
>>>
>>> On 1/05/2019 7:59 am, coleen.phillimore at oracle.com wrote:
>>>>
>>>> David, Thank you for the writeup of the exit paths.
>>>>
>>>> The reason I added this test with JDK-9074355 is because taking the 
>>>> Heap_lock in destroy_vm prevented a crash in VerifyBeforeExit.
>>>>
>>>> http://cr.openjdk.java.net/~coleenp/2019/8074355.06/webrev/test/hotspot/jtreg/runtime/Shutdown/ShutdownTest.java.html 
>>>>
>>>>
>>>> I wonder if you need to take the Heap_lock in the VM_Exit path as well.
>>>
>>> Yes you are right. Sorry I was focusing on the wrong aspect of that 
>>> discussion. I've updated the v2 webrev in place as java.cpp is now 
>>> back in it. I will also update the exit path document to show the 
>>> taking of the Heap_lock.
>>>
>>> http://cr.openjdk.java.net/~dholmes/8222534/webrev.v2/
>>>
>>> Re-testing in mach5 underway - though it probably won't be testing 
>>> VerifyBeforeExit.
>>
>> I thought this change would enable VerifyBeforeExit with all our 
>> testing because it's trueInDebug and now on the exit path?
> 
> You are right - I forgot about trueInDebug.
> 
>>>
>>> I'm assuming, based on the fact the destroy_JavaVM path doesn't hang, 
>>> that taking the Heap_lock can't succeed until there are no GC 
>>> safepoint ops queued, and conversely once taken no GC safepoint ops 
>>> will queue (or if they do they won't block trying to take the heap 
>>> lock!)?
>>>
>> I believe this is the effect.
>>
>> I'm having trouble with control flow.   In destroy_vm() the Heap_lock 
>> is taken after that JavaThread is removed from the thread list, so we 
>> have a _no_safepoint_check_flag.   In this place, from debugging the 
>> thread is on the threads list, so safepoint checking is ok.
>>
>> http://cr.openjdk.java.net/~dholmes/8222534/webrev.v2/src/hotspot/share/runtime/java.cpp.frames.html 
>>
> 
> Not only okay but essential - you will get deadlocks if you try to take 
> without safepoint check (been there done that :) ). And yes this is just 
> a normal live JavaThread executing normally so it has to check safepoints.
> 
>>   536     VM_Exit op(code);
>>
>> Can this line be after the Heap_lock acquisition or does the Heap_lock 
>> make this operation wait until GC is done to queue?  ie it's this 
>> line, and not this one that we're blocking GC for:
>>
>>   548     VMThread::execute(&op);
> 
> We need the heap_lock before line #548 - which never returns. The 
> declaration of the VM_Exit op and the taking of the heap lock can occur 
> in either order.
> 
>>
>> Also, aside.
>>
>>   546     if (thread->is_Java_thread())
>>   547       ((JavaThread*)thread)->set_thread_state(_thread_in_vm);
>>
>> Not your change but this is surprising.  I wonder why this is?
> 
> VMThread::execute (and subsequent code) will expect it ... oh! Shoot! So 
> will trying to take the heap_lock :( I need to move that higher up. I'm 
> assuming that some of the paths to vm_exit may not have the thread 
> "in_vm" - or at least at some point in time there was such a path. I'll 
> do some archaeology on that.
> 
> Thanks Coleen!
> 
> David
> 
>>
>>> This bug has turned out to be far more complex than originally 
>>> anticipated :) Seems there are numerous reasons why various actions 
>>> are not on the vm_exit() path! I only wish someone had documented why 
>>> they were missing in the first place. So I added a comment about the 
>>> xtty logging problem in java.cpp (now that it's back in the change).
>>>
>>
>> Thank you for all the comments.  This code is full of land mines 
>> without them.
>>
>> Looks good aside from questions.
>>
>> Coleen
>>
>>> Thanks,
>>> David
>>>
>>>>
>>>> Coleen
>>>>
>>>> On 4/29/19 11:54 PM, David Holmes wrote:
>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8222534
>>>>> Webrev: http://cr.openjdk.java.net/~dholmes/8222534/webrev/
>>>>>
>>>>> Stefan noticed that VerifyBeforeExit was not honoured when a Java 
>>>>> application terminates via System.exit.
>>>>>
>>>>> Examination of the exit code sequences revealed four differences 
>>>>> between an exit due to the last non-daemon thread terminating 
>>>>> (handled via jni_DestroyJavaVM) and a call to System.exit() 
>>>>> (handled via JVM_Halt()). There are four missing actions on the 
>>>>> System.exit() path:
>>>>>
>>>>> - No processing of VerifyBeforeExit
>>>>> - No XML logging before exit
>>>>> - No LogConfiguration::finalize()
>>>>> - No IdealGraphPrinter::clean_up()
>>>>>
>>>>> The first three have now been added at the appropriate point. 
>>>>> VerifyBefore exit was the main omission. The compiler team (i.e. 
>>>>> Vladimir K.) indicated they'd also like the XML logging. And the 
>>>>> LogConfiguration::finalize while possibly not essential avoids any 
>>>>> doubt as to whether buffered log output may get lost.
>>>>>
>>>>> The IdealGraphPrinter::cleanup was deemed unnecessary due to the 
>>>>> fact the process is being blown away anyway.
>>>>>
>>>>> The bug report contains a lot of details on the exit sequences 
>>>>> including a side-by-side comparison in the attached pdf, showing 
>>>>> the relative positioning of each action and that the correct order 
>>>>> has been maintained.
>>>>>
>>>>> The vm_exit() code affects a number of "abrupt" exit paths in the 
>>>>> VM, not just JVM_Halt, and this is discussed in the bug report as 
>>>>> well. In short the addition of the missing actions should not cause 
>>>>> any issues.
>>>>>
>>>>> Testing:
>>>>>  - some manual checking of exit paths and whether new code was 
>>>>> executed
>>>>>  - all hotspot/jtreg/gc tests with -XX:+VerifyBeforeExit added
>>>>>  - mach5 tiers 1-3
>>>>>
>>>>> Thanks,
>>>>> David
>>>>
>>