RFR: 8253180: ZGC: Implementation of JEP 376: ZGC: Concurrent Thread-Stack Processing [v6]

[Zhengyu Gu]

On Tue, 29 Sep 2020 14:12:26 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

> > 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:
> > // 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.
> > Then in method :
> > ```
> > 
> > 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);
> > ..
> > }
> > ```
> > 
> > 
> > It calls barrier's on_thread_detach(), acquires Threads_lock.
> > The lock acquisition triggers stack processing, that potential accesses oops.
> > ```
> > 
> > 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
> > ```
> > 
> > 
> > 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.
> 
> What oop are you encountering here? You should have no frames left at this point, and all oops should have been
> cleared. At least that is the theory, and that was why the thread oop moved out from the thread (to enforce that). So I
> am curious what oop you have found to still be around at this point.

They are handles:
<pre><code>
V  [libjvm.so+0x98d693]  chunk_oops_do(OopClosure*, Chunk*, char*)+0xce
V  [libjvm.so+0x98d6d3]  HandleArea::oops_do(OopClosure*)+0x37
V  [libjvm.so+0x1194ad0]  Thread::oops_do_no_frames(OopClosure*, CodeBlobClosure*)+0x88
V  [libjvm.so+0x119b0c8]  JavaThread::oops_do_no_frames(OopClosure*, CodeBlobClosure*)+0x88
</code></pre>

> 
> Anyway, you can try moving the GC hook into the critical section. That should help.

This seems to work.

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PR: https://git.openjdk.java.net/jdk/pull/296