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

[Robbin Ehn]

Thanks Dan, I'll fix below and push!

/Robbin

On 2019-08-08 21:32, Daniel D. Daugherty wrote:
> On 8/6/19 2:29 AM, Robbin Ehn wrote:
>> 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/
> 
> src/hotspot/share/jfr/periodic/sampling/jfrThreadSampler.cpp
>      No comments.
> 
> src/hotspot/share/prims/jni.cpp
>      L4139:   // This thread will not be safepoint check, since it has
>          nit: s/not be/not do a/
> 
> src/hotspot/share/runtime/handshake.cpp
>      No comments.
> 
> src/hotspot/share/runtime/mutex.cpp
>      L190:   // Safepoint checking logically implies active java_thread.
>          nit: s/java_thread/JavaThread/
> 
> src/hotspot/share/runtime/mutexLocker.cpp
>      No comments.
> 
> src/hotspot/share/runtime/thread.cpp
>      No comments.
> 
> src/hotspot/share/runtime/thread.hpp
>      No comments.
> 
> src/hotspot/share/runtime/threadSMR.cpp
>      No comments.
> 
> src/hotspot/share/runtime/vmOperations.cpp
>      No comments.
> 
> src/hotspot/share/utilities/vmError.cpp
>      You may want this comment:
> 
>      L1794:     Threads_lock->try_lock();
>                 // The VM is going to die so no need to unlock Thread_lock.
> 
> Thumbs up. I only have comment changes/additions so no need for
> a new webrev if you choose to make those changes.
> 
> Dan
> 
> 
> 
>>
>> 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
>>
>