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

[Srinivas Ramakrishna]

Per, this seems reasonable, but can you give an example of a non-Java
thread that executes this code and when? (not knowing enough of the
implementation details of G1, this might be a somewhat naive question,
but bear with me.)

thanks.
-- ramki

On Thu, Jun 27, 2013 at 6:00 AM, Per Liden <per.liden at oracle.com> 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)