RFR(M) JDK-8014555: G1: Memory ordering problem with Conc refinement and card marking

Vladimir Kozlov vladimir.kozlov at oracle.com
Thu Oct 3 16:41:41 PDT 2013

Assembler and compilers changes are good.


On 10/2/13 5:28 AM, Mikael Gerdin wrote:
> Hi
> Please review my fix for the issue discussed in the "G1 question:
> concurrent cleaning of dirty cards" thread on the hotspot-gc-dev mailing
> list.
> I'd like someone from the compiler (and runtime? the interpreter uses
> macroAssembler_*, right?) teams to at least look at the changes to:
> macroAssembler_*.cpp
> c1_Runtime_*.cpp
> graphKit.cpp
> Problem description:
> G1 has a race where the concurrent refinement thread may miss object
> references in a dirty card.
> The problem arises if the CPU re-orders the load of the old card value
> (which G1 checks to determine if it can skip the barrier)
> before the store to the actual object.
> If that occurs the concurrent refinement thread may have set the card to
> "clean" and proceeded to scan the card but the java thread may have seen
> the "dirty" value and skipped the post barrier.
> Suggested fix:
> * Add a memory fence between the store to a java object and the reading
> of the previous card value.
> * Modify the code for handling young regions so that all writes to young
> regions can skip the fence (since it will never be needed for such
> writes). This introduces a new value in the card table for G1 which
> indicates a young region.
> Performance impact:
> * This fix has a negative throughput performance impact of 1-1.5%
> (tested on x86-AMD x86-Intel and SPARC).
> * We may want to try to get rid of this race at some point by
> redesigning G1's post barrier but there is not enough time to do that
> for JDK8.
> Performance numbers for x86 platforms can be seen here:
> http://cr.openjdk.java.net/~mgerdin/8014555/perf.txt
> Unfortunately the JIRA issue is not externally visible, but the major
> parts of the discussions about this are present in the mailing list
> thread. The bug mostly contains my analysis of the crashes which seems
> to have been caused by this bug.
> Bug link: https://bugs.openjdk.java.net/browse/JDK-8014555
> Webrev: http://cr.openjdk.java.net/~mgerdin/8014555/webrev.0

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