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

[Per Liden]

Hi Jon,

I only saw this with debug builds of my experimantal repo, where a 
No_Safepoint_Verifier I had noticed the issue. In product builds there 
should be a window for silent oop corruption, but I never managed to 
provoke that.

/Per

On 2013-06-27 19:46, Jon Masamitsu wrote:
> Per,
>
> In a product build what was the symptom of this bug that
> you observed?
>
> Jon
>
> On 6/27/13 6:00 AM, Per Liden wrote:
>> 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)
>