RFR (s): 8250606: Remove unnecessary assertions in ObjectSynchronizer FastHashcode and inflate

Coleen Phillimore coleen.phillimore at oracle.com
Fri Aug 7 00:25:32 UTC 2020



On 8/6/20 7:49 PM, David Holmes wrote:
> On 7/08/2020 9:37 am, Ioi Lam wrote:
>>
>>
>> On 8/6/20 2:46 PM, David Holmes wrote:
>>> Hi Dan,
>>>
>>> On 7/08/2020 4:29 am, Daniel D. Daugherty wrote:
>>>> On 8/5/20 6:31 PM, David Holmes wrote:
>>>>> Hi Dan,
>>>>>
>>>>> Thanks for taking a look at this.
>>>>>
>>>>> On 6/08/2020 4:08 am, Daniel D. Daugherty wrote:
>>>>>> On 8/5/20 3:58 AM, David Holmes wrote:
>>>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8250606
>>>>>>> webrev: http://cr.openjdk.java.net/~dholmes/8250606/webrev/
>>>>>>
>>>>>> src/hotspot/share/runtime/synchronizer.cpp
>>>>>>      L1010:       // Relaxing assertion for bug 6320749.
>>>>>>      L1011:       assert(Universe::verify_in_progress() ||
>>>>>>      L1012: !SafepointSynchronize::is_at_safepoint(),
>>>>>>      L1013:              "biases should not be seen by VM thread 
>>>>>> here");
>>>>>>          This assertion is similar and tagged with the same bug 
>>>>>> ID (6320749)
>>>>>>          as the ones that you removed. Granted that the Biased 
>>>>>> Locking code
>>>>>>          is on the way out, but...
>>>>>>
>>>>>>          I'm fine if you plan to leave this one for Biased 
>>>>>> Locking removal.
>>>>>
>>>>> Yeah I confess I had my biased-locking filter on when looking at 
>>>>> this part of the code. You've actually exposed a bit of a mess. :)
>>>>>
>>>>> It is unclear why the same !at_safepoint assertion was initially 
>>>>> added to the biased-locking revocation chunk of code given the 
>>>>> same assertion would then be executed after it. But I think it is 
>>>>> more to do with the line following the assertion (original code):
>>>>>
>>>>> assert(!SafepointSynchronize::is_at_safepoint(), "biases should 
>>>>> not be seen by VM thread here");
>>>>> BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
>>>>>
>>>>> If we are at a safepoint in the VMThread then 
>>>>> JavaThread::current() will assert! So even after the assertion was 
>>>>> apparently relaxed with the verify_in_progress() check, we could 
>>>>> never have hit this code as the JavaThread::current() would still 
>>>>> assert. So I can (and should) remove the assertion here as well 
>>>>> (CDS dump could reach this in theory), but I also need to correct 
>>>>> this current code:
>>>>>
>>>>> -      BiasedLocking::revoke(hobj, JavaThread::current());
>>>>> +      BiasedLocking::revoke(hobj, self);
>>>>>
>>>>> as re-manifesting the current thread is pointless. That said the 
>>>>> above code (nor the original) also can't execute at a safepoint:
>>>>>
>>>>> void BiasedLocking::revoke(Handle obj, TRAPS) {
>>>>>   assert(!SafepointSynchronize::is_at_safepoint(), "must not be 
>>>>> called while at safepoint");
>>>>
>>>>
>>>> With the assert in JavaThread::current() and with the assert in
>>>> BiasedLocking::revoke(), I don't see how we could ever get away
>>>> with calling FastHashCode() on a biased object at a safepoint.
>>>> It simply should have blown up (with non-release bits).
>>>>
>>>> I'm not yet sure what keeps us from seeing a biased object at
>>>> a safepoint, but there has to be something there. We aren't that
>>>> lucky to have simply missed this...
>>>
>>> Seems we are that lucky in a sense. It seems that the verification 
>>> process that we allow for is not the same as it once was and so we 
>>> don't hit the code that needs the FastHashCode. The only other 
>>> safepoint op that gets involved is CDS dumping and there we also get 
>>> lucky in that none of the objects that get archived are bias-locked. 
>>> I've been using Ioi's test from
>>>
>>> http://cr.openjdk.java.net/~iklam/jdk16/8249276-reset-archive-obj-headers.v02/ 
>>>
>>>
>>> to force the object to be biased but I still can't hit that code 
>>> fragment so something else is still in play.
>>>
>>> David
>>> -----
>>>
>>
>> Oh I feel so dumb. I forgot that we force UseBiasedLocking to false 
>> when CDS is enabled (arguments.cpp)!
>>
>>    if (DumpSharedSpaces) {
>>      // Disable biased locking now as it interferes with the clean up of
>>      // the archived Klasses and Java string objects (at dump time 
>> only).
>>      UseBiasedLocking = false;
>
> Ah! :(
>
>> Anyway, it seems like UseBiasedLocking is not compatible with CDS in 
>> many ways. I have updated the patch to disable the above line. I also 
>> use a more reliably way get a biased-locked object. The test locks on 
>> an interned string, which we know for sure no one else would lock, or 
>> compute identity_hash of it.
>>
>> http://cr.openjdk.java.net/~iklam/jdk16/8249276-reset-archive-obj-headers.v03-with-debugging/ 
>>
>>
>> and when dumping with -XX:+UseBiasedLocking, we crash while doing GC 
>> inside a safepoint:
>>
>> #19 0x00007ffff5bb1a80 in oopDesc::set_mark (this=0xbcbcbcbcbcbcbcbc, 
>> m=...)
>> #20 0x00007ffff5bb0d0b in BiasedLocking::restore_marks ()
>
> Know bug: JDK-8251118
>
>> #21 0x00007ffff5fa3515 in G1FullCollector::complete_collection (...)
>> #22 0x00007ffff5f68b57 in G1CollectedHeap::do_full_collection (...)
>> #23 0x00007ffff5f68bac in G1CollectedHeap::do_full_collection (...)
>> #24 0x00007ffff5dc7aad in CollectedHeap::collect_as_vm_thread (...)
>> #25 0x00007ffff60885f4 in HeapShared::run_full_gc_in_vm_thread ()
>>
>> So it looks like the G1GC doesn't support collection inside the VM 
>> thread with biased-locked objects.
>>
>> Is biased locking going to be removed soon? I think we probably 
>> shouldn't spend too much time on it.
>
> There was some feedback on the disabling that indicated it may still 
> be useful so that decision to proceed with the removal has yet to occur.
>
>> Maybe instead of removing the asserts, just enclose them inside "if 
>> UseBiasedLocking"?
>
> I think I'm going to fix the BL block of code separately. There's no 
> reason I can see why a biased-locked object should be a concern in 
> FastHashCode, we just need to change the revocation code to allow for 
> the safepoint (which is exactly what we do elsewhere).

I'm not sure why you want to do this, since I can't see why anything 
would call this with BiasedLocking on.  But for what it's worth, I had a 
prototype of something else where I changed that code to be:

   if (SafepointSynchronize::is_at_safepoint()) {
     BiasedLocking::revoke_at_safepoint();
   } else {
     BiasedLocking::revoke();
   }

and it worked fine.

Coleen

>
> Thanks,
> David
>
>> Thanks
>> - Ioi
>>
>>
>>>> Dan
>>>>
>>>>>
>>>>> so even larger changes will be needed to make this code work at a 
>>>>> safepoint, and that is going well beyond the intended scope of 
>>>>> this particular bug. :( I'll need to get back to you.
>>>>>
>>>>> I also need to check our GC verify tests to see how we manage to 
>>>>> apparently never encounter a biased-locked object, otherwise we 
>>>>> would have seen this inconsistency in the code.
>>>>>
>>>>> Thanks,
>>>>> David
>>>>> -----
>>>>>
>>>>>
>>>>>> Thumbs up.  Thanks for digging up the history behind these very 
>>>>>> old asserts.
>>>>>
>>>>> Thanks,
>>>>> David
>>>>>
>>>>>> Dan
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> tl;dr I removed some assertions that were deemed unnecessary 
>>>>>>> after interfering with other work. :)
>>>>>>>
>>>>>>> ObjectSynchronizer::inflate had the following assertion added 
>>>>>>> back in JDK 6 under JDK-5030359:
>>>>>>>
>>>>>>> ObjectMonitor * ObjectSynchronizer::inflate (oop object) {
>>>>>>>   // Inflate mutates the heap ...
>>>>>>>   assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
>>>>>>>
>>>>>>> That same change added FastHashcode which had the following 
>>>>>>> assertions:
>>>>>>>
>>>>>>> intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop 
>>>>>>> obj) {
>>>>>>>   assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
>>>>>>>   assert (Self->is_Java_thread(), "invariant") ;
>>>>>>>   assert (((JavaThread *)Self)->thread_state() != 
>>>>>>> _thread_blocked, "invariant") ;
>>>>>>>
>>>>>>> These methods had assertions that they were not executed at a 
>>>>>>> safepoint in case the modification of the markWord could cause 
>>>>>>> interference with GC.
>>>>>>>
>>>>>>> JDK-6320749 then relaxed the assertions to allow for GC 
>>>>>>> verification - which does happen at a safepoint and so obviously 
>>>>>>> not in a JavaThread.
>>>>>>>
>>>>>>> The FastHashCode assertions were further relaxed in JDK-8015086 
>>>>>>> to allow the CDS dump VM safepoint operation to call it. But 
>>>>>>> FastHashCode can also call inflate so that was a mismatch in 
>>>>>>> assertions just waiting to happen - which is how I hit this in a 
>>>>>>> VM that always forces inflation :)
>>>>>>>
>>>>>>> I initially intended to extend the FastHashCode assertion to 
>>>>>>> match inflate, but then a question arose about the safety of the 
>>>>>>> CDS dumping code using FastHashcode which showed that the code 
>>>>>>> is in fact safe. Notwithstanding what the state of the world may 
>>>>>>> have been in the JDK 6 timeframe, there is now no potential 
>>>>>>> interference possible between execution of these methods at an 
>>>>>>> arbitrary safepoint and any concurrent GC use of the markWord. 
>>>>>>> So the original !is_at_safepoint assertion can just be dropped 
>>>>>>> from both methods. That in turn means we either drop the 
>>>>>>> JavaThread assertion or else expand it to be "JavaThread or 
>>>>>>> VMThread" - but that itself seems unnecessarily restrictive: why 
>>>>>>> can't a GC thread use this code for example? So I chose to drop it.
>>>>>>>
>>>>>>> That leaves the "if I'm a JavaThread I'm not _thread_blocked" 
>>>>>>> assertion. I don't see that this really guards against anything 
>>>>>>> useful (a JavaThread should be _thread_in_vm when executing this 
>>>>>>> code ... though there may be leaf calls where the thread is 
>>>>>>> _thread_in_java). So again I just dropped the assertion.
>>>>>>>
>>>>>>> So a long winded justification for dropping a few assertions. :)
>>>>>>>
>>>>>>> Thanks,
>>>>>>> David
>>>>>>>
>>>>>>>
>>>>>>
>>>>
>>



More information about the hotspot-runtime-dev mailing list