Code review request: 6810698: G1: two small bugs in the sparse remembered sets XXXS (updated)

[Tony Printezis]

New webrev here:

http://cr.openjdk.java.net/~tonyp/6810698/webrev.01/

I also resolved a related small issue (and I changed the title of the CR 
to reflect that; I didn't think it was worth opening a second one):

There is an additional issue with the sparse RSets. When we iterate over 
a sparse RSet, we actually use the _next table, instead of the _cur one. 
This creates a race between the _next table being expanded and the 
iteration. The whole point of having two tables is to have a stable one 
(_cur) and one that might be modified (_next) so that we can look at the 
stable one safely during a GC. Iterating over _next is plain wrong. This 
fix, along with the fix related to the _expanded check, seem to have 
resolved all the "missing rem set" issues related to sparse tables.

Tony

Tony Printezis wrote:
> From the CR blurb:
>
> Each sparse RSet has two tables: cur and next. At the beginning, both 
> tables are the same (i.e., the two references _cur and _next point to 
> the same table). When a sparse table needs to be expanded during a GC, 
> the flag _expanded on the table is set, a new table is created 
> (pointed to by _next) and the RSet is added to the expanded list. 
> After the collection (in fact, at the beginning of the next 
> collection), we iterate over the expanded list, we get rid of the _cur 
> tables, and set _cur to point to _next (so the _next table basically 
> replaces the _cur table). Unfortunately, at that time, the _expanded 
> flag is not reset. So, if the sparse table is expanded again, it is 
> not added to the expanded list (that operation is conditional on 
> !_expanded, so that the table is not added twice during the same GC). 
> As a result, some of the sparse tables are left in an inconsistent 
> state, which causes some entries not to be scanned correctly during a GC.
>
> http://cr.openjdk.java.net/~tonyp/6810698/webrev.00/
>
> (one line change!!!)
>
> Tony
>

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