RFR: 8212682: Avoid holding Compile_lock when blocking for GC in ObjArrayKlass::allocate_objArray_klass()

dean.long at oracle.com dean.long at oracle.com
Mon Nov 19 18:26:26 UTC 2018

Yes, Reviewed!


On 11/19/18 6:09 AM, Erik Österlund wrote:
> Hi Dean,
> May I interpret your commentary as Reviewed?
> Thanks,
> /Erik
> On 2018-10-24 22:20, dean.long at oracle.com wrote:
>> On 10/24/18 1:52 AM, Erik Österlund wrote:
>>> Hi Dean,
>>> On 2018-10-23 22:27, dean.long at oracle.com wrote:
>>>> Can allocate_objArray_klass() end up calling 
>>>> SystemDictionary::add_to_hierarchy()?  If so, then you'll end up 
>>>> locking Compile_lock anway, but in the wrong order with respect to 
>>>> MultiArray_lock.
>>> No, I can't see that allocate_objArray_klass() ever calls 
>>> SystemDictionary::add_to_hierarchy(). If it did, we would assert 
>>> that the Compile_lock is held; it never re-acquires the lock.
>> OK, I forgot that mutex's can't be locked recursively.
>> I found a clue to the reason for original 1998 change.  This was also 
>> added at the same time, to klassVtable::initialize_vtable():
>>   // We need the compile lock in order to force the super vtables and
>>   // methodOop code pointers to stay constant.
>>   // (Race scenario: compiler thread updates a vtable entry while 
>> we're copying
>>   // down entries from the superclass vtable.)
>>   assert_lock(Compile_lock);
>> By 1999, the vtable code changed a lot and the assert above went 
>> away, so it was probably just an oversight that the extra uses of 
>> Compile_lock weren't removed at the same time.
>> dl
>>> Thanks,
>>> /Erik
>>>> dl
>>>> On 10/23/18 7:59 AM, Erik Österlund wrote:
>>>>> Hi,
>>>>> We occasionally blockingly wait for GC in allocations in 
>>>>> ObjArrayKlass::allocate_objArray_klass(), while holding the 
>>>>> Compile_lock.
>>>>> This is problematic for concurrent class unloading that needs to 
>>>>> hold that lock in the unloading path. This introduces a potential 
>>>>> deadlock situation.
>>>>> After staring enough at this code together with help from Coleen, 
>>>>> I have convinced myself that the Compile_lock is in fact not 
>>>>> needed in this path, and there is nothing to my knowledge it 
>>>>> actually protects here. The vague comment next to the lock 
>>>>> suggests its purpose is to protect vtable stubs. But it doesn't. 
>>>>> There is a VtableStubs_lock for that, and I find no traces if the 
>>>>> Compile_lock having anything to do with that.
>>>>> Coleen helped me trying to trace down where this locking code came 
>>>>> from. It takes us back to before anyone was around, and does not 
>>>>> seem to have changed in the past 2 decades, and its origins are a 
>>>>> bit unclear. The theory is that perhaps vtable stubs used to be 
>>>>> protected by the Compile_lock in ancient times, and the locking 
>>>>> code is still around because nobody dared to touch it.
>>>>> Since both code analysis and mach5 suggest there is no reason for 
>>>>> this lock to be held here, I propose to remove it with this patch.
>>>>> If anyone that has been around for more than 2 decades in HotSpot 
>>>>> happens to know the reason why this locking was introduced, any 
>>>>> commentary around that would be very appreciated.
>>>>> Webrev:
>>>>> http://cr.openjdk.java.net/~eosterlund/8212682/webrev.00/
>>>>> Bug:
>>>>> https://bugs.openjdk.java.net/browse/JDK-8212682
>>>>> Thanks,
>>>>> /Erik

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