RFR(S): 8039042: G1: Phantom zeros in cardtable
jon.masamitsu at oracle.com
Wed May 14 20:34:14 UTC 2014
Could you use a check on the sparc version to decide
on when to use memset.
On 5/13/2014 3:34 AM, Per Liden wrote:
> Summary: We use memset to initialize the cardtable. On Sparc T4 (and
> later) memset uses some special instructions with the side-effect that
> the memory can temporarily be filled with zeros before the actual
> value is set. These phantom zeros can be observed by concurrent
> readers of this memory, which can trick the G1 post-barrier to
> incorrectly dirty a card in a young region. Please see the bug for a
> more details.
> A similar memset-problem has been observed and fixed before
> (JDK-6948537) when using memset on the BlockOffsetTable. Similar to
> the BlockOffsetTable, I'm using the UseMemSetInBOT flag as an
> indicator of whether it's safe to use memset or not. UseMemSetInBOT is
> not a perfect name for this flag as code other than the BOT might want
> to use it. It's also questionable of this should be a command-line
> flag at all. I've filed a separate RFE, JDK-8042930, to do a bit of
> clean up related to this flag.
> Bug: https://bugs.openjdk.java.net/browse/JDK-8039042
> Webrev: http://cr.openjdk.java.net/~pliden/8039042/webrev.0/
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