RFR: 8305566: ZGC: gc/stringdedup/TestStringDeduplicationFullGC.java#Z failed with SIGSEGV in ZBarrier::weak_load_barrier_on_phantom_oop_slow_path

[Stefan Karlsson]

On Mon, 24 Apr 2023 08:24:53 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this change to the string deduplication thread to make it a kind
> of JavaThread rather than a ConcurrentGCThread.  There are several pieces to
> this change:
> 
> (1) New class StringDedupThread (derived from JavaThread), separate from
> StringDedup::Processor (which is now just a CHeapObj instead of deriving from
> ConcurrentGCThread).  The thread no longer needs to or supports being stopped,
> like other similar threads.  It also needs to be started later, once Java
> threads are supported.  Also don't need an explicit visitor, since it will be
> in the normal Java threads list.  This separation made the changeover a little
> cleaner to develop, and made the servicability support a little cleaner too.
> 
> (2) The Processor now uses the ThreadBlockInVM idiom to be safepoint polite,
> instead of using the SuspendibleThreadSet facility.
> 
> (3) Because we're using ThreadBlockInVM, which has a different usage style
> from STS, the tracking of time spent by the processor blocked for safepoints
> doesn't really work.  It's not very important anyway, since normal thread
> descheduling can also affect the normal processing times being gathered and
> reported.  So we just drop the so-called "blocked" time and associated
> infrastructure, simplifying Stat tracking a bit.  Also renamed the
> "concurrent" stat to be "active", since it's all in a JavaThread now.
> 
> (4) To avoid #include problems, moved the definition of
> JavaThread::is_active_Java_thread from the .hpp file to the .inline.hpp file,
> where one of the functions it calls also is defined.
> 
> (5) Added servicability support for the new thread.
> 
> Testing:
> mach5 tier1-3 with -XX:+UseStringDeduplication.
> The test runtime/cds/DeterministicDump.java fails intermittently with that
> option, which is not surprising - see JDK-8306712.
> 
> I was never able to reproduce the failure; it's likely quite timing sensitive.
> The fix of changing the type is based on StefanK's comment that ZResurrection
> doesn't expect a non-Java thread to perform load-barriers.

I think this looks sensible to me, though I'm not very familiar with the current StringDedup code, so consider this a partial review only.

src/hotspot/share/gc/shared/stringdedup/stringDedupProcessor.hpp line 29:

> 27: 
> 28: #include "memory/allocation.hpp"
> 29: #include "gc/shared/stringdedup/stringDedup.hpp"

sort order

-------------

Marked as reviewed by stefank (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13607#pullrequestreview-1397500893
PR Review Comment: https://git.openjdk.org/jdk/pull/13607#discussion_r1174989267