RFR (S) 5103339: Strengthen NoSafepointVerifier
coleen.phillimore at oracle.com
coleen.phillimore at oracle.com
Wed Aug 7 18:27:49 UTC 2019
On 8/7/19 2:21 PM, Daniel D. Daugherty wrote:
> On 8/7/19 1:52 PM, coleen.phillimore at oracle.com wrote:
>>
>>
>> On 8/7/19 11:55 AM, Daniel D. Daugherty wrote:
>>> Query below...
>>>
>>> On 8/7/19 11:49 AM, coleen.phillimore at oracle.com wrote:
>>>>
>>>> Thanks David,
>>>>
>>>> On 8/6/19 10:36 PM, David Holmes wrote:
>>>>> Hi Coleen,
>>>>>
>>>>> On 7/08/2019 8:43 am, coleen.phillimore at oracle.com wrote:
>>>>>>
>>>>>> Thanks David for reading this.
>>>>>>
>>>>>> On 8/5/19 9:47 PM, David Holmes wrote:
>>>>>>> Hi Coleen,
>>>>>>>
>>>>>>> Could please update the bug explaining why it was re-opened and
>>>>>>> what the intended enhancement is. It was closed in 2005 as not
>>>>>>> necessary because of the unhandled-oop checking.
>>>>>>
>>>>>> Yes, I added a comment.
>>>>>
>>>>> Thanks!
>>>>>
>>>>>>>
>>>>>>> On 6/08/2019 2:00 am, coleen.phillimore at oracle.com wrote:
>>>>>>>> Summary: Add NSV check at possible safepoint transition or
>>>>>>>> places that could take out locks. Consolidate with clearing
>>>>>>>> unhandled oops.
>>>>>>>>
>>>>>>>> This change checks for NoSafepointVerifier no_safepoint_counts
>>>>>>>> at possible safepoints. The starting set is at transitions,
>>>>>>>> and in the
>>>>>>>
>>>>>>> Wouldn't it be better placed in the actual Safepoint checking
>>>>>>> methods rather than callers of those methods?
>>>>>>
>>>>>> See my reply to Robbin. We want to make the checks in places
>>>>>> that conditionally may not safepoint poll, so that timing doesn't
>>>>>> prevent finding when we've violated the NoSafepointVerifier check.
>>>>>
>>>>> Okay.
>>>>>
>>>>>>>
>>>>>>>> "else" clauses where CHECK_UNHANDLED_OOPS were clearing
>>>>>>>> unhandled oops. Some of these were removed because they weren't
>>>>>>>> places with possible safepoints, so were wrong.
>>>>>>>>
>>>>>>>> The unhandled oops clearing and no_safepoint counter check are
>>>>>>>> now done in the same function. MemAllocator ->
>>>>>>>> check_for_valid_allocation_state calls
>>>>>>>> check_for_valid_safepoint_state which calls
>>>>>>>> check_possible_safepoint.
>>>>>>>>
>>>>>>>> Calls to check_possible_safepoint are in DEBUG_ONLY when
>>>>>>>> Thread::current() is called.
>>>>>>>>
>>>>>>>> I had to remove it from the else clause in JvmtiThreadState
>>>>>>>> because it's called from a place that cannot safepoint (see
>>>>>>>> vtableStubs.cpp).
>>>>>>>
>>>>>>> Can you elaborate on that further please. It's not obvious,
>>>>>>> without following call chains, when any code may or may not hit
>>>>>>> a safepoint check.
>>>>>>
>>>>>> No, it isn't. There's quite a large call stack here, but the
>>>>>> CompiledICLocker has an embedded NoSafepointVerifier, so here's
>>>>>> the call stack:
>>>>>>
>>>>>> V [libjvm.so+0x16c5d05] Thread::check_possible_safepoint()+0x55
>>>>>> V [libjvm.so+0x10fb15c]
>>>>>> JvmtiExport::post_dynamic_code_generated_while_holding_locks(char
>>>>>> const*, unsigned char*, unsigned char*)+0x4c
>>>>>> V [libjvm.so+0x17c8fac] VtableStubs::find_stub(bool, int)+0x2ac
>>>>>> V [libjvm.so+0x9dd18e] CompiledIC::set_to_megamorphic(CallInfo*,
>>>>>> Bytecodes::Code, bool&, Thread*)+0x9e
>>>>>> V [libjvm.so+0x1562bb4]
>>>>>> SharedRuntime::handle_ic_miss_helper_internal(Handle,
>>>>>> CompiledMethod*, frame const&, methodHandle, Bytecodes::Code,
>>>>>> CallInfo&, bool&, Thread*)+0x434
>>>>>> V [libjvm.so+0x156d202]
>>>>>> SharedRuntime::handle_ic_miss_helper(JavaThread*, Thread*)+0x372
>>>>>> V [libjvm.so+0x156d7a4]
>>>>>> SharedRuntime::handle_wrong_method_ic_miss(JavaThread*)+0x184
>>>>>>
>>>>>> The function handle_ic_miss_helper_internal has a
>>>>>> CompiledICLocker, which has an embedded NoSafepointVerifier.
>>>>>>
>>>>>> This seems like a lot of code to be protected with a
>>>>>> NoSafepointVerifier, but I assume that at least the find_stub
>>>>>> code shouldn't get cleaned up by a safepoint, so would require it.
>>>>>
>>>>> Okay so because you added the call to jvmti_thread_state() in the
>>>>> vtableStubs code you have to remove the check from the else-clause
>>>>> in jvmti_thread_state(), because it could fail with the new call
>>>>> path.
>>>>
>>>> Yes, but I'm not happy with this so I've reinstated the check in
>>>> the 'else' clause and changed
>>>> post_dynamic_code_generated_while_holding_locks() to simply get the
>>>> jvmti_state directly and not call JvmtiThreadState::state_for().
>>>> It already had an assert that it wasn't null.
>>>
>>> Not calling JvmtiThreadState::state_for() caught my eye. There's a bit
>>> of a protocol involved with safely getting the JvmtiThreadState,
>>> properly
>>> caching it, and dealing with it going away.
>>>
>>> What's the reason for not using JvmtiThreadState::state_for()?
>>
>> This is what JvmtiThreadState::state_for() does:
>>
>> inline JvmtiThreadState* JvmtiThreadState::state_for(JavaThread
>> *thread) {
>> JvmtiThreadState *state = thread->jvmti_thread_state();
>> if (state == NULL) {
>> MutexLocker mu(JvmtiThreadState_lock);
>> // check again with the lock held
>> state = state_for_while_locked(thread);
>> } else {
>> // If the state was be null, then this could have safepointed.
>> thread->check_possible_safepoint();
>> }
>> return state;
>> }
>>
>> If the state returned is non-null it just returns it. There's
>> special code for the state being NULL by checking whether the thread
>> is exiting. When called from VtableStubs::find_stub, it calls
>> post_dynamic_code_generated_while_holding_locks.
>>
>> I actually copied a comment from several jvmtiExport.cpp functions in
>> this:
>>
>> // post a DynamicCodeGenerated event while holding locks in the VM.
>> void
>> JvmtiExport::post_dynamic_code_generated_while_holding_locks(const
>> char* name,
>> address code_begin, address code_end)
>> {
>> // register the stub with the current dynamic code event collector
>> // Can not take safepoint here so can not use state_for to get
>> // jvmti thread state.
>> JvmtiThreadState* state = JavaThread::current()->jvmti_thread_state();
>> // state can only be NULL if the current thread is exiting which
>> // should not happen since we're trying to post an event
>> ...
>
> Thanks for the details. Looks like you have that part covered just fine.
Thanks for double-checking Dan!
Coleen
>
> Dan
>
>
>>
>> Coleen
>>
>>>
>>> Dan
>>>
>>>>
>>>> I'm retesting this and will have an 02 plus incremental next week.
>>>>
>>>>>> I also ran tests with an assert that the jvmti_thread_state() !=
>>>>>> NULL, just to see if it's ever NULL and it wasn't. I would have
>>>>>> liked to keep the else.
>>>>>>>
>>>>>>>> os.cpp ResourceMark needed for debugging.
>>>>>>>
>>>>>>> Interesting catch - I would have expected the RM to be higher up
>>>>>>> the call if needed. How did you detect this?
>>>>>>
>>>>>> Debugging, but I don't remember which thing I was debugging though.
>>>>>>>
>>>>>>>> Tested with tier1 on all Oracle platforms, and tier 1-3 on
>>>>>>>> linux-x64-debug.
>>>>>>>>
>>>>>>>> open webrev at
>>>>>>>> http://cr.openjdk.java.net/~coleenp/2019/5103339.01/webrev
>>>>>>>> bug link https://bugs.openjdk.java.net/browse/JDK-5103339
>>>>>>>
>>>>>>> src/hotspot/share/oops/objArrayKlass.cpp
>>>>>>> src/hotspot/share/oops/typeArrayKlass.cpp
>>>>>>>
>>>>>>> Unclear why the check is predicated on !or_null ??
>>>>>>
>>>>>> If the parameter passed or_null is true, then the function never
>>>>>> safepoints. I added this comment.
>>>>>>
>>>>>> // If passed or_null, this function will not try to safepoint.
>>>>>> if (!or_null) THREAD->check_possible_safepoint();
>>>>>
>>>>> Sorry I'm confused by the placement. We want to call
>>>>> check_possible_safepoint() on code paths that can safepoint, so
>>>>> that we ensure there is no NSV higher up the call chain when we
>>>>> take that path. So notwithstanding where the check_unhandled_oops
>>>>> was, instead of
>>>>>
>>>>> 352 } else {
>>>>> 353 if (!or_null) THREAD->check_possible_safepoint();
>>>>> 354 }
>>>>> 355
>>>>> 356 ObjArrayKlass *ak = ObjArrayKlass::cast(higher_dimension());
>>>>> 357 if (or_null) {
>>>>> 358 return ak->array_klass_or_null(n);
>>>>> 359 }
>>>>> 360 return ak->array_klass(n, THREAD);
>>>>>
>>>>> shouldn't/couldn't we just have:
>>>>>
>>>>> 352 }
>>>>> 353
>>>>> 354
>>>>> 355
>>>>> 356 ObjArrayKlass *ak = ObjArrayKlass::cast(higher_dimension());
>>>>> 357 if (or_null) {
>>>>> 358 return ak->array_klass_or_null(n);
>>>>> 359 }
>>>>> THREAD->check_possible_safepoint();
>>>>> 360 return ak->array_klass(n, THREAD);
>>>>>
>>>>> ?
>>>>
>>>> I couldn't decide if this is better or worse than what I had
>>>> because looking at this, one might question why this call is here,
>>>> and it checks the safepoint twice (already in the MutexLocker
>>>> path). It's a few less lines of code. okay, sure I can change
>>>> it. I didn't find a better refactoring.
>>>>
>>>>> That said, are we certain that the xxx_or_null functions cannot
>>>>> safepoint? They won't try to load a class but surely they could
>>>>> potentially safepoint for other reasons?
>>>>
>>>> I don't see a safepoint in this code, or have found one while
>>>> running the tests. Hopefully the NSV would have found it!
>>>>
>>>> Coleen
>>>>
>>>>>
>>>>>> This is also called with NSV with or_null true. I can't
>>>>>> remember where, I think it was the heap walker.
>>>>>>
>>>>>>>
>>>>>>> ---
>>>>>>>
>>>>>>> src/hotspot/share/code/vtableStubs.cpp
>>>>>>>
>>>>>>> // cause a safepoint in this code that has NSV.
>>>>>>>
>>>>>>> Unclear what "code" has the NSV ??
>>>>>>
>>>>>> See above. I can change the comment to:
>>>>>>
>>>>>> 237 // all locks. Only post this event if a new state is not
>>>>>> required. Creating a new state would
>>>>>> 238 // cause a safepoint and the caller of this code has a
>>>>>> NoSafepointVerifier.
>>>>>
>>>>> Sounds good!
>>>>>
>>>>>>>
>>>>>>> ---
>>>>>>>
>>>>>>> src/hotspot/share/runtime/interfaceSupport.inline.hpp
>>>>>>>
>>>>>>> As mentioned above why not put the check inside
>>>>>>> SafepointMechanism::block_if_requested instead?
>>>>>>
>>>>>> The other callers of block_if_requested already have a
>>>>>> check_possible_safepoint call. The only caller that made sense to
>>>>>> add was transition from or to vm.
>>>>>
>>>>> Got it.
>>>>>
>>>>>>>
>>>>>>> ---
>>>>>>>
>>>>>>> src/hotspot/share/runtime/mutex.cpp
>>>>>>>
>>>>>>> The Mutex constructor cleanup is incidental but okay. (Does make
>>>>>>> me wonder about the whole Monitor construction process though
>>>>>>> ... ClearMonitor doesn't need to be protected any more.)
>>>>>>
>>>>>> This leaked into this change. I'll take it out and post a
>>>>>> different RFR for it. The no-arg constructor Monitor isn't
>>>>>> needed either so ClearMonitor can be deleted.
>>>>>
>>>>> Okay.
>>>>>>>
>>>>>>> ---
>>>>>>>
>>>>>>> src/hotspot/share/runtime/thread.hpp
>>>>>>>
>>>>>>> // NSV checking
>>>>>>> void check_for_valid_safepoint_state(bool
>>>>>>> potential_vm_operation) NOT_DEBUG_RETURN;
>>>>>>> void check_possible_safepoint() NOT_DEBUG_RETURN;
>>>>>>>
>>>>>>> Please expand NSV. Does the comment apply to both methods or
>>>>>>> only the first?
>>>>>>
>>>>>> Both methods. I'll change it to:
>>>>>>
>>>>>> // These functions check conditions on a JavaThread before
>>>>>> possibly going to a safepoint,
>>>>>> // including NoSafepointVerifier.
>>>>>> void check_for_valid_safepoint_state(bool
>>>>>> potential_vm_operation) NOT_DEBUG_RETURN;
>>>>>> void check_possible_safepoint() NOT_DEBUG_RETURN;
>>>>>
>>>>> Okay on the new comment.
>>>>>
>>>>> Thanks,
>>>>> David
>>>>> -----
>>>>>
>>>>>>
>>>>>> Thanks!
>>>>>> Coleen
>>>>>>>
>>>>>>> Thanks,
>>>>>>> David
>>>>>>> -----
>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Coleen
>>>>>>
>>>>
>>>
>>
>
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