RFR(S): 8200374: Add ThreadsSMRSupport::verify_hazard_pointer_scanned() to verify threads_do()

[Daniel D. Daugherty]

The actual relative diff:

$ hg diff
diff -r b1cf49ff8e1c src/hotspot/share/trace/tracingExport.cpp
--- a/src/hotspot/share/trace/tracingExport.cpp    Tue Apr 03 16:57:57 
2018 -0400
+++ b/src/hotspot/share/trace/tracingExport.cpp    Tue Apr 03 17:50:26 
2018 -0400
@@ -36,5 +36,6 @@
  void TracingExport::set_sampler_thread_with_lock(Thread* thread) {
    // Grab Threads_lock to avoid conflicts with Thread-SMR scans.
    MutexLocker ml(Threads_lock);
-  _sampler_thread = thread;
+  // To match with the lock-free load_acquire():
+  OrderAccess::release_store(&_sampler_thread, thread);
  }


The annoying thing is that my plan was to switch from
OrderAccess::release_store_fence() to OrderAccess::release_store()
for this round of testing AND I talked myself into dropping the
release_store() entirely...

Good thing Mach5 is rocking today!

Dan


On 4/3/18 5:45 PM, Daniel D. Daugherty wrote:
> On 4/3/18 5:39 PM, David Holmes wrote:
>> On 4/04/2018 7:25 AM, Daniel D. Daugherty wrote:
>>> Greetings,
>>>
>>> This fix has been revised due to feedback from David H. and Erik O.
>>>
>>> Here's the incremental webrev:
>>>
>>> http://cr.openjdk.java.net/~dcubed/8200374-webrev/2_for_jdk_hs_open.inc/ 
>>>
>>>
>>> And here's the full webrev:
>>>
>>> http://cr.openjdk.java.net/~dcubed/8200374-webrev/2_for_jdk_hs_open.full/ 
>>>
>>>
>>> Summary of the changes relative to 'cr1':
>>>
>>> - Rename TracingExport::get_sampler_thread() to 
>>> sampler_thread_acquire()
>>>    and TracingExport::set_sampler_thread() to 
>>> set_sampler_thread_with_lock().
>>> - set_sampler_thread_with_lock() uses Threads_lock to avoid 
>>> conflicts with
>>>    Thread-SMR scans.
>>
>> Sorry Dan but this is now mixing lock-free and locked in a "broken" way.
>
> Yup... Little voice in the back of my head was telling me
> that... I didn't listen... I'll switch it to:
>
> OrderAccess::release_store(&_sampler_thread, thread);
>
> and retest... :-)
>
> Dan
>
>
>
>> A load_acquire is supposed to always be there to match a 
>> release_store - but we don't have a release_store because we hold the 
>> lock when setting the field. The lock is being used to serialize with 
>> the ThreadSMR scans. If we need an acquire it is because we want to 
>> ensure that we see stores prior to the setting of the sampler_thread 
>> when we read the sampler_thread. If that is the case then we still 
>> need the release_store because we may read the new sampler_thread 
>> value as soon as it is set, but before the unlock occurs - otherwise 
>> prior stores need not be visible. If it is not the case that we need 
>> to see prior stores then we don't need the acquire.
>>
>> David
>> -----
>>
>>> The Mach5 builds-tier1, jdk-tier[1-3], and hs-tier[1-3] run has 
>>> finished
>>> and there are no test failures related to these changes.
>>>
>>> My Solaris-X64 server is finishing the third round of Thread-SMR stress
>>> testing on the 'cr1' version. I'll start a round of stress testing for
>>> this version ('cr2') as soon as I can.
>>>
>>> Thanks, in advance, for any comments, suggestions, or feedback.
>>>
>>> Dan
>>>
>>> On 4/2/18 11:11 AM, Daniel D. Daugherty wrote:
>>>> Greetings,
>>>>
>>>> This fix has been revised due to additional testing and due to
>>>> feedback from David H.
>>>>
>>>> Here's the incremental webrev:
>>>>
>>>> http://cr.openjdk.java.net/~dcubed/8200374-webrev/1_for_jdk_hs_open.inc/ 
>>>>
>>>>
>>>> And here's the full webrev:
>>>>
>>>> http://cr.openjdk.java.net/~dcubed/8200374-webrev/1_for_jdk_hs_open.full/ 
>>>>
>>>>
>>>> My usual Thread-SMR stress testing on my Solaris-X64 is still running;
>>>> so far there has been only one unrelated intermittent test failure. 
>>>> The
>>>> Mach5 builds-tier1, jdk-tier[1-3], and hs-tier[1-3] run is still in
>>>> process...
>>>>
>>>> Thanks, in advance, for any comments, suggestions, or feedback.
>>>>
>>>> Dan
>>>>
>>>>
>>>> On 3/28/18 9:52 PM, Daniel D. Daugherty wrote:
>>>>> Greetings,
>>>>>
>>>>> I have a (mostly) small enhancement for Thread-SMR:
>>>>>
>>>>> JDK-8200374 Add ThreadsSMRSupport::verify_hazard_pointer_scanned() 
>>>>> to verify threads_do()
>>>>> https://bugs.openjdk.java.net/browse/JDK-8200374
>>>>>
>>>>> Here's the webrev URL:
>>>>>
>>>>> http://cr.openjdk.java.net/~dcubed/8200374-webrev/0_for_jdk_hs_open/
>>>>>
>>>>> This is Erik O's improvement on the assertion added by the following
>>>>> bug fix (with some minor tweaking done by me):
>>>>>
>>>>> JDK-8199813 SIGSEGV in ThreadsList::includes()
>>>>> https://bugs.openjdk.java.net/browse/JDK-8199813
>>>>>
>>>>> Summary of the changes:
>>>>>
>>>>> - Replace the assertion that I added in JDK-8199813 with a closure
>>>>>   based function that verifies the threads_do() contract depended
>>>>>   on by Thread-SMR.
>>>>> - Add ThreadsSMRSupport::verify_hazard_pointer_scanned() to verify
>>>>>   that the calling thread's hazard pointer is scanned by 
>>>>> threads_do().
>>>>>   The new function is called from both ThreadsSMRSupport::acquire_*()
>>>>>   functions.
>>>>> - Refactor the non-JavaThread part of Threads::threads_do() into
>>>>>   Threads::non_java_threads_do() so that the non-JavaThread part
>>>>>   can also be called by other threads_do() functions. Yes, the
>>>>>   Threads::threads_do() contract is still to scan every thread in
>>>>>   the system.
>>>>> - Add hooks for a "tracing sampler thread" to be optionally scanned
>>>>>   by Threads::non_java_threads_do().
>>>>>
>>>>> This fix has gone thru a couple of Mach5 builds-tier1, jdk-tier[1-3],
>>>>> and hs-tier[1-3] runs. I've also started my usual 24+ hour Thread-SMR
>>>>> stress testing run on my Solaris-X64 server.
>>>>>
>>>>> Just to be extra sure, I backed out the fix from JDK-8199813 to
>>>>> JavaThread::verify_not_published() (which started this round of
>>>>> Thread-SMR fixes) and we catch the issue in the new function
>>>>> ThreadsSMRSupport::verify_hazard_pointer_scanned().
>>>>>
>>>>> Thanks, in advance, for any comments, suggestions, or feedback.
>>>>>
>>>>> Dan
>>>>>
>>>>>
>>>>
>>>>
>>>
>