RFR(xs): 8014022: G1: Non Java threads should lock the shared SATB queue lock without safepoint checks.

[Per Liden]

Hi,

Could I please have a couple of reviews on this small fix.

Summary: If a non-Java thread writes to an object field the G1 
pre-barrier can potentially safepoint when acquiring the shared pointer 
queue lock (Shared_SATB_Q_lock). This is problematic if e.g. the 
non-Java thread has joined the STS. The solution is to grab the lock 
without a safepoint check.

Testing: I ran into this bug while doing some (unrelated) experiments 
with G1, where I had a non-JavaThread do calls to oop->obj_field_put(). 
Hotspot doesn't currently have any non-JavaThread which does this, so it 
can't currently be provoked by a test. However, after fixing this in my 
experimental repo I stopped running into this issue.

Webrev: http://cr.openjdk.java.net/~pliden/8014022/webrev.00/

Bug: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8014022

/Per

(btw, Bengt volunteered to sponsor the change)