RFR: 8166811: Missing memory fences between memory allocation and refinement

[Kim Barrett]

> On Nov 7, 2016, at 5:57 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> Hi,
> 
> On Sat, 2016-10-29 at 19:26 -0400, Kim Barrett wrote:
>>> 
>>> On Oct 25, 2016, at 7:13 PM, Kim Barrett <kim.barrett at oracle.com>
>>> wrote:
>>> 
>>> Please review this change to address missing memory barriers needed
>>> to
>>> ensure ordering between allocation and refinement in G1.
>>> […]
>>> 
>>> CR:
>>> https://bugs.openjdk.java.net/browse/JDK-8166811
>>> 
>>> Webrev:
>>> http://cr.openjdk.java.net/~kbarrett/8166811/webrev.00/
>>> [Based on http://cr.openjdk.java.net/~kbarrett/8166607/webrev.02/]
>>> 
>> -------------------------------------------------------------------
>> ----------- 
>> src/share/vm/gc/g1/g1RemSet.cpp
>>  581   // The region could be young.  Cards for young regions are 
>> dirtied,
>>  582   // so the post-barrier will filter them out.  However, that
>> dirtying
>>  583   // is performed concurrently.  A write to a young object could
>> occur
>>  584   // before the card has been dirtied, slipping past the filter.
>> 
>> This is a rewording of the comment that used to be here.  However, it
>> was not true even before these changes.  As part of JDK-8014555 we
>> mark young region cards with g1_young_card_val().  That's the change
>> set that added the storeload to the post-barrier.
>> 
>> I'm not quite sure what to do about this. The comment is currently
>> wrong.  However, the storeload is considered a problem, and there
>> have been various ideas discussed for eliminating it that might allow
>> us to go back to dirtying young cards.
> 
> Depends on what "dirtying" is supposed to mean in this context -
> setting it to "dirty" or setting it to something non-clean.
> 
> One could replace "dirtied" by something less specific here to make it
> right again.

Good idea.  How about this rewording (using “set to a value”)

  // The region could be young.  Cards for young regions are set to a
  // value that allows the post-barrier to filter them out.  However,
  // that card setting is performed concurrently.  A write to a young
  // object could occur before the card has been set, slipping past
  // the filter.