RFR: 8214556: Crash in DependencyContext::remove_dependent_nmethod still happens

[Erik Österlund]

Hi,

Unfortunately, even after 8213565, there are still crashes in 
DependencyContext::remove_dependent_nmethod.

The claiming mechanism I used for dependency context cleanup used the 
safepoint counter as a clock source for monotonically inrcemented 
cleaning epochs. What I forgot for a moment while fixing this, is that 
VM operations may be coalesced into the same safepoint. If two GC 
operations run in the same safepoint, dependency context cleaning will 
only run for the first GC operation, not any subsequent one (the cleanup 
tasks will appear to have already been claimed), causing stale 
dependency contexts to remain with pointers to unloaded nmethods. This 
would cause very intermittent crashes in subsequent operations on those 
dependency contexts such as the crashes that have been observed.

I fixed this by creating a stand-alone counter incremented each cleanup 
epoch, instead of using the safepoint counter for this.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8214556

Webrev:
http://cr.openjdk.java.net/~eosterlund/8214556/webrev.00/

I have run this through tier1-6 on linux twice, I've also run tier1-3 on 
all platforms twice, and another tier1 on windows. Having said that, 
it's a very intermittent bug... but at least I know this is wrong, can 
cause these crashes, and needs to be fixed.

Thanks,
/Erik