On Mon, 28 Sep 2020 12:00:40 GMT, Albert Mingkun Yang <ay...@openjdk.org> wrote:
>>> Thank you for the comments and diagrams; they make the code much more
>>> digestible. From that diagram, I get the
>>> impression that the watermark is associated with stack pointer, so it
>>> should be 1:1 relation, but `class Thread`
>>> contains multiple watermarks, `StackWatermarks _stack_watermarks;`. I think
>>> some high level description on the relation
>>> between the thread and a list of watermarks belong to that thread could be
>>> beneficial.
>>
>> I added some further comments explaining this.
>>
>>> > The first time it reaches past the last frame it will report true, and
>>> > the second time it will report false.
>>>
>>> Why so? As I see it, once a stream becomes "done", it stays in that state
>>> forever. Am I missing sth?
>>>
>>> ```
>>> inline bool StackFrameStream::is_done() {
>>> return (_is_done) ? true : (_is_done = _fr.is_first_frame(), false);
>>> }
>>> ```
>>
>> When you step into the last frame of the iteration (first frame in the
>> stack), the first time you ask is_done() it will
>> report false, the next time it will report true, despite still being in the
>> same frame. Therefore, the is_done()
>> function is not idempotent, as I need it to be.
>
> I see; thank you for the explanation.
Hi Erik,
I have been playing with this patch for past a few days. Great work!
I found that this patch seems to break an early assumption.
We have a comment in JavaThread::exit() says:
<pre>
// We need to cache the thread name for logging purposes below as once
// we have called on_thread_detach this thread must not access any oops.
</pre>
Then in method :
<pre><code>
void Threads::remove(JavaThread* p, bool is_daemon) {
...
BarrierSet::barrier_set()->on_thread_detach(p);
// Extra scope needed for Thread_lock, so we can check
// that we do not remove thread without safepoint code notice
{ MonitorLocker ml(Threads_lock);
..
}
</code></pre>
It calls barrier's on_thread_detach(), acquires Threads_lock.
The lock acquisition triggers stack processing, that potential accesses oops.
<pre><code>
V [libjvm.so+0x10c6f5e] StackWatermark::start_processing()+0x6a
V [libjvm.so+0x10c77e8] StackWatermarkSet::start_processing(JavaThread*,
StackWatermarkKind)+0x82
V [libjvm.so+0xfd7757] SafepointMechanism::process(JavaThread*)+0x37
V [libjvm.so+0xfd796b]
SafepointMechanism::process_if_requested_slow(JavaThread*)+0x1d
V [libjvm.so+0x4b3683]
SafepointMechanism::process_if_requested(JavaThread*)+0x2b
V [libjvm.so+0xe87f0d]
ThreadBlockInVMWithDeadlockCheck::~ThreadBlockInVMWithDeadlockCheck()+0x5f
V [libjvm.so+0xe86700] Mutex::lock_contended(Thread*)+0x12c
V [libjvm.so+0xe867d8] Mutex::lock(Thread*)+0x96
V [libjvm.so+0xe86823] Mutex::lock()+0x23
V [libjvm.so+0x29b4bc] MutexLocker::MutexLocker(Mutex*,
Mutex::SafepointCheckFlag)+0xe2
V [libjvm.so+0x29b533] MonitorLocker::MonitorLocker(Monitor*,
Mutex::SafepointCheckFlag)+0x29
V [libjvm.so+0x119f2ce] Threads::remove(JavaThread*, bool)+0x56
V [libjvm.so+0x1198a2b] JavaThread::exit(bool, JavaThread::ExitType)+0x905
V [libjvm.so+0x1197fde] JavaThread::post_run()+0x22
V [libjvm.so+0x1193eae] Thread::call_run()+0x230
V [libjvm.so+0xef3e38] thread_native_entry(Thread*)+0x1e4
</code></pre>
This is a problem for Shenandoah, as it flushes SATB buffers during
on_thread_detach() and does not expect to see any
more SATB traffic.
Thanks.
-------------
PR: https://git.openjdk.java.net/jdk/pull/296
