RFR: 8244920: Access violation in frames::interpreter_frame_method

[coleen.phillimore at oracle.com]

This makes sense and thanks for the comments in the code for why you're 
doing this.  Thank you for fixing this!
Coleen

On 6/8/20 3:14 AM, Erik Österlund wrote:
> Hi,
>
> When we unpack interpreter frames due to deoptimization, we find 
> ourselves in a situation where the stack space has been allocated,
> and the last_Java_frame has been set, but the contents of the frames 
> has not yet been populated. Any JFR event firing during this
> time will think the stack can be parsed, as the last_Java_frame has 
> been set, and then fail doing so, when running into uninitialized
> stack frames. After ZGC added an event that sometimes fires in load 
> barriers, we suddenly found ourselves in this awkward spot.
>
> I propose to clear the last_Java_frame after the top frame has been 
> acquired, and is no longer needed during the unpacking, so that
> such events will see that we are in fact in a leaf call and should not 
> attempt to sample the stack.
>
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8244920
>
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8244920/webrev.00/
>
> Thanks,
> /Erik