RFR: 8166811: Missing memory fences between memory allocation and refinement
kim.barrett at oracle.com
Mon Nov 7 19:07:27 UTC 2016
> On Nov 7, 2016, at 1:36 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
>>> 581 // The region could be young. Cards for young regions are
>>> 582 // so the post-barrier will filter them out. However, that
>>> 583 // is performed concurrently. A write to a young object could
>>> 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.
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