RFR: 8226228: Make Threads_lock an always safepoint checked lock.

[Robbin Ehn]

Hi guys,

Let's pick this up :) Been a while...

I think that we were pretty happy after some explaining !?

Here is full, let's call it v2:
http://cr.openjdk.java.net/~rehn/8226228/v2/webrev/

Passes t1-5.

Thanks, Robbin

On 2019-06-17 13:21, Robbin Ehn wrote:
> Hi all, please review.
> 
> A safepoint checked lock always check for safepoint if the locker is an
> JavaThread. If it's not a JavaThread, the thread are not participating in the
> safepoint protocol thus safepoints are not checked.
> 
> A JavaThread that is not on _the_ ThreadsList is not participating in the 
> safepoint protocol and for most purposes should be treat as NonJavaThread.
> This applies to whether we should check for a safepoint.
> 
> This changeset suggest adding a method for checking this.
> We can use that method in the mutex code to determine if we should treat locker
> as JavaThread or a NonJavaThread.
> 
> The only problematic scenario is when we are an active JavaThread in vmError.cpp 
> but don't want to safepoint, but want the ThreadsList to be stable.
> I choose a best effort approach.
> Alternatives could be stop freeing the backing array if there is an an error or
> letting the thread running the error code become an inactive JavaThread thus
> avoiding safepointing.
> 
> This fix also helps converting Heap_lock to an always lock (Heap_lock needed
> changes not included).
> 
> Issue:
> https://bugs.openjdk.java.net/browse/JDK-8226228
> Code:
> http://cr.openjdk.java.net/~rehn/8226228/webrev/index.html
> 
> Passes t1-3.
> 
> Thanks, Robbin