RFR(S): 8153267: nmethod's exception cache not multi-thread safe

[Christian Thalinger]

> On Apr 1, 2016, at 2:37 AM, Doerr, Martin <martin.doerr at sap.com> wrote:
> 
> Hello everyone,
>  
> we have found a concurrency problem with the nmethod’s exception cache. Readers of the cache may read stale data on weak memory platforms.
>  
> The writers of the cache are synchronized by locks, but there may be concurrent readers: The compiler runtimes use nmethod::handler_for_exception_and_pc to access the cache without locking.
> Therefore, the nmethod's field _exception_cache needs to be volatile and adding new entries must be done by releasing stores. (Loading seems to be fine without acquire because there's an address dependency from the load of the cache to the usage of its contents which is sufficient to ensure ordering on all openjdk platforms.)
>  
> I also added a minor cleanup: I changed nmethod::is_alive to read the volatile field _state only once. It is certainly undesired to force the compiler to load it from memory twice.
>  
> Webrev is here:
> http://cr.openjdk.java.net/~mdoerr/8153267_exception_cache/webrev.00/ <http://cr.openjdk.java.net/~mdoerr/8153267_exception_cache/webrev.00/>
Does it make sense to keep:
   void set_exception_cache(ExceptionCache *ec)    { _exception_cache = ec; }
or would it be safer to always do the store-release even when clearing the cache?

>  
> Please review. I will also need a sponsor.
>  
> Best regards,
> Martin

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