RFR (XS): 8023037 : Race between ciEnv::register_method and nmethod::make_not_entrant_or_zombie

[Roland Westrelin]

Hi Vladimir,

> http://cr.openjdk.java.net/~vlivanov/8023037/webrev.00/
> 
> There's a race between compiler thread during method registration and sweeper: sweeper can invalidate a nmethod which hasn't been installed yet.
> 
> Consider the following scenario:
>  ciEnv::register_method:
>    - new nmethod(...)
> 
>  sweeper:
>    - invalidates newly allocated nmethod and patches VEP to call handle_wrong_method
>    - tries to unlink it, but method()->from_compiled_entry() != verified_entry_point() since nmethod hasn't been installed yet
> 
>  ciEnv::register_method:
>    - installs already invalidated nmethod
> 
> Calling corresponding Java method will lead to infinite loop: VEP of the compiled method calls handle_wrong_method and call site resolution returns the very same compiled method.

Does that mean StressNonEntrant is broken?

> The fix is to grab a lock after nmethod is allocated in the code cache and check that it hasn't been already invalidated by the sweeper before proceeding. Sweeper already synchronizes on a nmethod before invalidating it.

Would another fix be to have the sweeper check that the method was made ready to execute before it attempts to make it not entrant?

i.e. nmethod->method()->code() == nmethod

Roland.