RFR: 8246442: nmethod::can_convert_to_zombie() asserts when not called by the sweeper

[Erik Österlund]

Hi,

nmethod::can_convert_to_zombie() used to be called only by the sweeper, 
after it has checked that
an nmethod is not entrant. Due to the single call site, there was an 
assert checking that the nmethod
should be not entrant (the condition just checked).
However, it is now used in nmethod::post_compiled_method_load_event(), 
where the result of the assert is racy.

When an nmethod is created, the load event will never be able to see the 
nmethod spuriously flipping
to zombie, because there simply are no safepoint polls/transitions 
between the nmethod being created
and the load event firing. However, the jvmti_GenerateEvents can observe 
such spurious flips to zombie.
It is completely harmless, because the whole operation runs under the 
CodeCache_lock, which prevents
further nmethods from being flushed. But they can flip to zombie, which 
is indeed exactly what the
check is design to test. Therefore, this is just a false positive 
assertion failure.

Proposed fix: make the assert only apply when called from the sweeper 
thread.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8246442/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8246442

Thanks,
/Erik