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

[Kim Barrett]

> On Nov 7, 2016, at 1:36 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
>>> 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.

Oops, no, that isn't right.  (It's been a couple of weeks since I
looked at this, and forgot part of the problem.)

Part of what's wrong with the comment is that we can no longer get to
that point with a young region.  A young region's cards will be either
g1_young_gen or clean, never dirty.  Hence the filtering out of
non-dirty cards a few lines before this comment will have already
discarded a young card before we reach the test this comment is
discussing.  So the whole premise of the comment in question, that the
region could be young, is false.