[9] RFR(S): 8074553: Crash with assert(!is_unloaded()) failed: should not call follow on unloaded nmethod

[Vladimir Kozlov]

On 3/31/16 6:22 AM, Tobias Hartmann wrote:
> Hi,
>
> please review the following patch:
>
> https://bugs.openjdk.java.net/browse/JDK-8074553
> http://cr.openjdk.java.net/~thartmann/8074553/webrev.00/
>
> While the code cache sweeper processes a nmethod in NMethodSweeper::process_nmethod(), safepoints may happen and the GC may unload the currently processed nmethod. To prevent this, the sweeper uses a NMethodMarker which saves the nmethod in CodeCacheSweeperThread::_scanned_nmethod. The nmethod is then passed to the GC through a CodeBlobClosure in CodeCacheSweeperThread::oops_do() to keep it alive when the GC iterates over all threads.
>
> The problem is that G1 calls nmethods_do() on all threads in the remark phase (see G1RemarkThreadsClosure::do_thread()) which is not overwritten by the sweeper thread. Since the currently processed nmethod is not passed through nmethods_do() by any thread, it is unloaded and we later hit the assert when encountering the nmethod through oops_do().
>
> Mikael Gerdin and Stefan Karlsson (thanks again!) suggested to overwrite nmethods_do() as well in CodeCacheSweeperThread and pass _scanned_nmethod to the closure. I also modified Threads::nmethods_do() to ignore the sweeper thread because we want to avoid marking the _scanned_nmethod as seen on the stack when scanning stacks from the sweeper (the nmethod may already be zombie and only referenced by the sweeper).

I did not get this. If you exclude CodeCacheSweeperThread in Threads::nmethods_do() then 
CodeCacheSweeperThread::nmethods_do() will not be called. What is the point?

Thanks,
Vladimir

>
> Unfortunately, this bug is extremely hard to reproduce (it showed up 18 times since early 2015). I was able to reproduce it only once after thousands of runs and therefore not often enough to verify the fix. However, I'm very confident that this solves the problem.
>
> Tested with JPRT and RBT (running).
>
> Thanks,
> Tobias
>