RFR(XS): 8139566 need proper sync for adding default read edges

[serguei.spitsyn at oracle.com]

Please, review a fix for the bug:
https://bugs.openjdk.java.net/browse/JDK-8139566


Webrev:
http://cr.openjdk.java.net/~sspitsyn/webrevs/2016/hotspot/8139566-SyncDefReadEdges.hs1/


There are already two positive (preliminary) reviews from Harold and Lois.


Summary:
   This is a follow-up issue after the fixes for:
https://bugs.openjdk.java.net/browse/JDK-8134950
https://bugs.openjdk.java.net/browse/JDK-8072745

   The webrevs for the issues above are:
http://cr.openjdk.java.net/~sspitsyn/webrevs/2015/hotspot/8134950-JVMTI-jake-ana2.3/ 

http://cr.openjdk.java.net/%7Esspitsyn/webrevs/2015/hotspot/8072745-JVMTI-jake-ana1.5/ 


   The issue is that there can be a race with another thread that checks the
   ModuleEntry::has_default_read_edges() and goes with its 
redefinition/retransformation
   while the upcall to the method 
jdk.internal.module.Modules.transformedByAgent()
   (that adds the default read edges to bootstrap and app class loaders) 
is still in progress.
   The instrumented code can execute while the upcall to 
transformedByAgent has not been completed yet.
   Please, refer to the bug description to get more details.

   The fix is to check the ModuleEntry::has_default_read_edges() in such 
a case.
   The check is added to the ModuleEntry::can_read as it is used in all
   class resolution cases, for instance:

     LinkResolver::check_klass_accessability() ->
       Reflection::verify_class_access() ->
         ModuleEntry::can_read()

     ClassFileParser::check_super_class_access() ->
       Reflection::verify_class_access() ->
         ModuleEntry::can_read()

     ClassFileParser::check_super_interface_access() ->
       Reflection::verify_class_access() ->
         ModuleEntry::can_read()

   Note that the ModuleEntry::_has_default_read_edges bit was added as a 
part
   of the 8144950 to avoid a bad recursion.

   One question is:
   Q1: Can we remove the upcall to the 
jdk.internal.module.Modules.transformedByAgent()
       as it would not be needed anymore? The implementation could me 
simplified.

   A1: I think, the real read edges established via an upcall to the 
Java runtime are still
      needed for the Java level based mechanisms that do not use the 
ModuleEntry::can_read().
      These mechanisms, probably, can tolerate that the "adding to a 
module default read edges"
      signal arrives with some latency.


Thanks,
Serguei