RFR: Make object pinning safepoint aware

[Zhengyu Gu]

On 10/15/2017 04:17 AM, Roman Kennke wrote:
> Am 15.10.2017 um 04:28 schrieb Zhengyu Gu:
>> Thread running native code does not usually participate safepoints, 
>> but the write barrier in object pinning code can potential interfere 
>> with safepoints and mutate the heap, so we need to make it safepoint 
>> aware.
>>
>> Webrev: 
>> http://cr.openjdk.java.net/~zgu/shenandoah/obj_pin_safepoint/webrev.00/index.html 
>>
>>
>> Test:
>>   hotspot_gc_shenandoah fastdebug + release on x64 and aarch64.
>>   + new stress test.
>>
>> Thanks,
>>
>> -Zhengyu
> 
> This doesn't seem right.
> 
> Native code (i.e. *outside* a JNI call) does not participate in 
> safepoints. But as soon as we are *inside* a JNI call, i.e. 
> GetPrimitiveArrayCritical, the thread is in VM state. This means it does 
> participate in safepoint in that it cannot reach a safepoint unless we 
> explicitely tell it to. This should be good for us: we will not get any 
> nasty surprises (e.g. change of evac-in-progress flag on the fly) in our 
> write barrier for example.
> 
> Now you introduce a VM->java->VM state transition inside the JNI 
> function. This seems wrong. And what does it help? The thread can now 
> reach a safepoint in the middle of GetPrimitiveArrayCritical: ugh. I 
> doubt very much that this is the right fix. (And besides, there are 
> scoped objects to do that...)
yes, you are right! in_vm should be enough.

Then there is a strange crash:
#
#  Internal Error 
(/home/zhengyu/jdk10/hotspot/src/share/vm/gc/shenandoah/shenandoahHeapRegion.cpp:176), 
pid=17356, tid=17359
#  fatal error: Disallowed transition from Collection Set to Pinned
#

Link: https://paste.fedoraproject.org/paste/jaB5Q6QTDJ-Q64Yg0OM5oA


-Zhengyu




> 
> Can you describe in more detail what the problem is, e.g. you must have 
> seen a crash or wrong behaviour in a testcase or real program? How does 
> it manifest? Then we can look for a solution.
> 
> Roman
>