RFR: Hole in CAS barrier when using traversal heuristics

[Roman Kennke]

Am 25.01.2018 um 18:35 schrieb Zhengyu Gu:
> 
> 
> On 01/25/2018 12:26 PM, Roman Kennke wrote:
>> Am 25.01.2018 um 18:15 schrieb Zhengyu Gu:
>>> I am not complete sure this is right fix. There is hole in CAS 
>>> barrier when using traversal heuristics.
>>>
>>> E.g. Unsafe_CompareAndSetObject() evacuates target and exchange 
>>> object, but not the field, so it may hit assertion in 
>>> ShenandoahBarrier::enqueue().
>>>
>>> I could not come up a reliable reproducer, but I have seen this a few 
>>> time with specjvm ScimarkLU with options:
>>>
>>> "-Xmx1g -Xms1g -XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions 
>>> -XX:+UnlockDiagnosticVMOptions -XX:ShenandoahGCHeuristics=traversal 
>>> -Xlog:gc+stats"
>>>
>>> Webrev: 
>>> http://cr.openjdk.java.net/~zgu/shenandoah/cas_traversal/webrev.00/
>>>
>>>
>>> Thanks,
>>>
>>> -Zhengyu
>>
>> Hi Zhengyu,
>>
>> I am not sure what you mean by 'evacuates target and exchange object,
>> but not the field' .. clearly the target object needs to be evacuated, 
>> because we only write to to-space (write-barrier). Also, the exchange 
>> object needs to be evacuated, to ensure we end up only with to-space 
>> references in fields (storeval-barrier). What do you mean by 
>> evacuation of 'the field' ? The target field is part of the target 
>> object.
> 
> There is an example:
> http://hg.openjdk.java.net/shenandoah/jdk10/file/6183a72bd5c2/src/hotspot/share/prims/unsafe.cpp#l1020 
> 
> 
> the addr points a field in object, that might not be evacuated and I 
> think you do have to enqueue it, as the object may be gray.

addr is a field in p, which is in to-space by the WB a few lines above. 
This should be good. x is the exchange value, also evacuated by the 
storeval barrier. So all should be fine.

What error/assert/crash are you seeing? Is it something in 
SBS::is_safe()? Then it may already be fixed by my subsequent changeset?

It may happen that traversal GC gets cancelled, and then we hit an 
overly strict assert like that.

Roman