7u40 RFR: 8012144 - Missing memory barriers

[David Holmes]

The issue of missing memory barriers in the GC taskqueue code was first 
flagged here:

http://mail.openjdk.java.net/pipermail/hotspot-dev/2013-March/008848.html

and bug 8006971 was opened for it.

There were some further background discussions that I wasn't party to 
and this all somewhat fell between the cracks for a while.

A trimmed version of the original proposed patch was tested internally 
(and I believe by SAP) and that simplified patch is what is being 
proposed now, under 8012144, solely for the 7u40 update. I/we intend to 
revise the original patch for 8006971 and get that resolved properly for 
JDK 8. Meanwhile we have a critical urgency on getting this fixed in 7u40.

The webrev is here:

http://cr.openjdk.java.net/~dholmes/8012144/webrev.hs24/

Primarily it adds a fence for platforms that require it - with platforms 
that don't opting out via the conditional compilation. In this way there 
is no performance impact on those mainline SE platforms. (A correctly 
expressed lock-free algorithm will not have platform specific fences, 
hence the intent to rework this for JDK 8.) Note this is a very low risk 
fix regardless because adding memory barriers will not introduce 
incorrect behaviour.

This patch was not supplied by me, but as a follow on from the original 
work, via John Cuthbertson. So John is one of the contributors (I 
believe Axel Siebenborn is the original contributor - I need an email 
address for the attribution!) and I will actually be a Reviewer.

If anyone has any concerns please raise them quickly as time is very 
much against us here.

Thanks,
David