RFR 8185103: TestThreadDumpMonitorContention.java crashed due to SIGSEGV in G1SATBCardTableModRefBS::write_ref_field_pre_work

[harold seigel]

Hi,

Please review this JDK-10 fix for JDK-8185103.  The problem occurred  
because classes were being put on the fixup_module_field_list before 
their mirror field was set.  If a (different) thread called method 
patch_javabase_entries() before the class's mirror field was set then 
this would cause a SIGSEGV because patch_javabase_entries() eventually 
calls obj_field_put() which tries to dereference the class's mirror field.

This change fixes the problem by setting the class's mirror field before 
putting the class on the fixup_module_field_list.

Open Webrev: 
http://cr.openjdk.java.net/~hseigel/bug_8185103/webrev/index.html

JBS Bug: https://bugs.openjdk.java.net/browse/JDK-8185103

The fix was tested with the JCK Lang and VM tests, the JTreg hotspot, 
java/io, java/lang, java/util and other tests, the co-located NSK tests, 
and with JPRT.

Additionally, the fix was tested by temporarily adding a 
naked_short_sleep(50) to method initialize_mirror_fields() shortly after 
it put a class on the fixup_module_field_list.  The sleep was added in 
order to enhance the likelihood of patch_javabase_entries() being called 
before the class's mirror field got set.  Without the fix, the 
TestThreadDumpMonitorContent.java test and the test reported in 
JDK-8183309 <https://bugs.openjdk.java.net/browse/JDK-8183309> reliably 
got the reported SIGSEGVs.  With the fix, the tests passed.

Thanks, Harold