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

[Erik Österlund]

Hi Dean,

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.

Thank you for the code archaeology. That really helps a lot. So it still 
looks like this lock doesn't really seem to do anything for us today. 
And perhaps that has been the case for nearly two decades.

Thanks,
/Erik

> 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
>>>
>>
>