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

[David Holmes]

On 3/04/2018 7:08 AM, Daniel D. Daugherty wrote:
> Hi David!
> 
> Thanks for the quick review.
> 
> 
> On 4/2/18 4:34 PM, David Holmes wrote:
>> Hi Dan,
>>
>> Only query, that I overlooked before is:
>>
>>  void TracingExport::set_sampler_thread(Thread * thread) {
>> -  _sampler_thread = thread;
>> +  OrderAccess::release_store_fence(&_sampler_thread, thread);
>>  }
>>
>> why a release_store_fence() rather than simple release_store()?
> 
> I based that choice on this comment:
> 
> src/hotspot/share/runtime/orderAccess.hpp:
> 
> // Conventional usage is to issue a load_acquire for ordered loads.  Use
> // release_store for ordered stores when you care only that prior stores
> // are visible before the release_store, but don't care exactly when the
> // store associated with the release_store becomes visible.  Use
> // release_store_fence to update values like the thread state, where we
> // don't want the current thread to continue until all our prior memory
> // accesses (including the new thread state) are visible to other threads.
> // This is equivalent to the volatile semantics of the Java Memory Model.
> 
> I see the _sampler_thread field as similar to the thread state.
> 
> Since the setter (a generic tracing sampler thread), is going to either
> publish a non-NULL value when the sampler thread starts or is going to
> publish a NULL value when the sampler thread is about to be deleted, I
> figured it was a good idea for the setter to not proceed from either of
> the set points until the value was published and observable by the threads
> that might be doing a get.
> 
> When publishing a non-NULL value:
>    - I don't want the setter to assume that a ThreadsList is safe
>      and start using it while a different thread processing the
>      to-be-deleted list thinks that ThreadsList is freeable.
> 
> When publishing a NULL value:
>    - I don't want the setter thread to delete itself while another
>      thread thinks that the generic tracing sampler thread is still
>      available and query-able.
> 
> Please let me know if I'm being overly cautious.

Very hard to tell. But what you describe seems specific to the semantics 
of ThreadSMR not to a general setting/getting a "sampler thread" 
reference, or even the general semantics of threads_do().

The safety of publishing a NULL is also highly dependent on the overall 
way in which the sampler thread interacts with ThreadSMR - something 
which is completely invisible in this webrev as we don't see that thread.

I'm wondering if the acquire/release-fence semantics should be visible 
in the get/set method names for clarity , as we do elsewhere?

Thanks,
David

> Dan
> 
> 
>>
>> Thanks,
>> David
>>
>> On 3/04/2018 1:11 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
>>>>
>>>>
>>>
>