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

[Vladimir Kozlov]

Good.

Thanks,
Vladimir

On 12/3/18 7:29 AM, Erik Österlund wrote:
> 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