RFR(S) 8249276 CDS archived objects must have "neutral" markwords

[Ioi Lam]

Hi David,

As we discussed off-line, I pushed the fix as-it. I'll fix the assert
you mentioned below after 8251460 (Fix the biased-locking code in
ObjectSynchronizer::FastHashCode) is integrated.

Thanks
- Ioi

On 8/11/20 7:20 PM, David Holmes wrote:
> Hi Ioi,
>
> But one further follow up in relation to biased-locking code. This 
> assertion is invalid:
>
>  104 unsigned HeapShared::oop_hash(oop const& p) {
>  105   assert(!p->mark().has_bias_pattern(),
>  106          "this object should never have been locked");  // so 
> identity_hash won't safepoin
>  107   unsigned hash = (unsigned)p->identity_hash();
>  108   return hash;
>  109 }
>
> As I discovered having the "bias pattern" doesn't mean you have been 
> locked, it can mean you're eligible to be biased-locked (ie you are 
> anonymously biased). So if you enable BL for CDS dump (and fix the 
> asserts I'm working on) you will still hit this assertion.
>
> Cheers,
> David
>
> On 11/08/2020 7:31 pm, David Holmes wrote:
>> Hi Ioi,
>>
>> Okay nothing further from me on the test.
>>
>> Thanks,
>> David
>>
>> On 11/08/2020 3:29 pm, Ioi Lam wrote:
>>>
>>>
>>> On 8/6/20 5:55 PM, David Holmes wrote:
>>>> Hi Ioi,
>>>>
>>>> On 6/08/2020 8:26 am, Ioi Lam wrote:
>>>>> Hi David & Coleen,
>>>>>
>>>>> Thanks for the review. I have created a new version:
>>>>>
>>>>> http://cr.openjdk.java.net/~iklam/jdk16/8249276-reset-archive-obj-headers.v02/ 
>>>>>
>>>>
>>>> Functional changes are good.
>>>>
>>>> For the test, as per our offline chat, the Helper class is not 
>>>> being archived. This line:
>>>>
>>>>  66                 TestCommon.testDump(appJar, 
>>>> TestCommon.list(LockDuringDumpApp.class.getName()),
>>>>
>>>> should be
>>>>
>>>>  66                 TestCommon.testDump(appJar, 
>>>> TestCommon.list(appClasses),
>>>>
>>>>
>>>
>>> Hi David,
>>>
>>> (The Helper class was actually archived, because it was loaded by 
>>> LockDuringDumpAgent during dump time, so it was not necessary to 
>>> explicitly include it in the class list. ....)
>>>
>>> Anyway, it wasn't clear to me whether any code (now or in the 
>>> future) in the JVM might synchronize on the java mirrors. So I've 
>>> updated the test case to synchronize on a literal string instead. 
>>> This way I know that only the test, and no one else, will 
>>> synchronize on it.
>>>
>>> I also added comments to explain how the test works.
>>>
>>> http://cr.openjdk.java.net/~iklam/jdk16/8249276-reset-archive-obj-headers.v03/ 
>>>
>>>
>>> Thanks
>>> - Ioi
>>>
>>>
>>>>>
>>>>> I created a new test case (LockDuringDump), and incorporated 
>>>>> David's fix for JDK-8250606 [1] during my testing:
>>>>>
>>>>> + Without my patch, LockDuringDumpApp would crash at runtime, because
>>>>>    we have an invalid markword for the Helper.class object.
>>>>>
>>>>> + Without the JDK-8250606 patch, we would assert during CDS dump 
>>>>> time.
>>>>>
>>>>> I also removed the assert in HeapShared::oop_hash() since it's 
>>>>> irrelevant after JDK-8250606.
>>>>>
>>>>> I intend to push my patch after JDK-8250606 is integrated.
>>>>>
>>>>>
>>>>> -------
>>>>>
>>>>> More comments below
>>>>>
>>>>> On 8/3/20 6:13 PM, David Holmes wrote:
>>>>>> Hi Ioi,
>>>>>>
>>>>>> On 27/07/2020 3:20 pm, Ioi Lam wrote:
>>>>>>> https://bugs.openjdk.java.net/browse/JDK-8249276
>>>>>>> http://cr.openjdk.java.net/~iklam/jdk16/8249276-reset-archive-obj-headers.v01/ 
>>>>>>>
>>>>>>>
>>>>>>> Please review this change (initial patch provided by David 
>>>>>>> Holmes; I rearranged it a little and added more comments).
>>>>>>>
>>>>>>> We reinitialize the markWord of all archived objects to remove 
>>>>>>> any side effect of GC/locking/etc during "java -Xshare:dump".
>>>>>>
>>>>>> Changes look good to me (naturally :) ). A couple of nits:
>>>>>>
>>>>>>    int hash_archived = archived_oop->identity_hash();
>>>>>>    assert(hash_original == hash_archived, "Different hash codes: 
>>>>>> original %x, archived %x", hash_original, hash_archived);
>>>>>>
>>>>>> the has_archived setting should be in ifdef DEBUG.
>>>>>>
>>>>>
>>>>> Fixed
>>>>>
>>>>>
>>>>>> 42 Object.class.wait();
>>>>>>   43                 }
>>>>>>   44                 System.out.println("Huh?? notified??");
>>>>>>
>>>>>> That could be a spurious wakeup. You could just use Thread.sleep 
>>>>>> with a suitably long sleep time. that said I'm not sure how the 
>>>>>> test is intended to operate. If premain returns after starting 
>>>>>> the thread there is no guarantee the thread will start executing 
>>>>>> and lock the object before the dump actually commences. Even the 
>>>>>> sighting of "Let's wait ....." is not sufficient to guarantee 
>>>>>> that unless you move it inside the sync block.
>>>>>>
>>>>>
>>>>> I have fixed the test case so that the "Let's hold the lock" 
>>>>> message is printed after the lock is held, and the lock is never 
>>>>> released afterwards.
>>>>>
>>>>> I also added synchronization to make sure the premain() method 
>>>>> doesn't return until the child thread has held the lock. The log 
>>>>> file shows that theses messages are printed before CDS tries to 
>>>>> load classes from the classlist:
>>>>>
>>>>> [0.005s][info][cds] Allocated shared space: 3221225472 bytes at 
>>>>> 0x0000000800000000
>>>>> inside LockDuringDumpAgent: 
>>>>> jdk.internal.loader.ClassLoaders$AppClassLoader at 707f7052
>>>>> Helper class is initialized
>>>>> Class = class LockDuringDumpApp$Helper
>>>>> Let's hold the lock on Helper.class forever .....
>>>>> Thread has started
>>>>> [0.145s][info][cds] Loading classes to share ...
>>>>>
>>>>>>> (David mentioned in the bug comments about assertions inside the 
>>>>>>> identity_hash() call, but maybe this should be fixed in a 
>>>>>>> different bug?)
>>>>>>
>>>>>> Yes those assertions are being removed under:
>>>>>>
>>>>>> https://bugs.openjdk.java.net/browse/JDK-8250606
>>>>>>
>>>>>
>>>>> Thanks
>>>>> - Ioi
>>>>>
>>>>> -------
>>>>>
>>>>> [1] 
>>>>> https://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2020-August/040985.html 
>>>>>
>>>>>
>>>>>
>>>