RFR: 6498581: ThreadInterruptTest3 produces wrong output on Windows

Yumin Qi yumin.qi at oracle.com
Tue Feb 25 23:40:53 UTC 2014


Thanks. I will change for Windows only --- thanks again.

Yumin

On 2/25/2014 3:37 PM, Karen Kinnear wrote:
> I think Vladimir has a good point - the way I read this, Windows is the only platform
> that has this issue, so making this conditional for Windows makes a lot of sense. Thanks
> for doing this Yumin.
>
> thanks,
> Karen
>
> On Feb 25, 2014, at 5:55 PM, Vladimir Kozlov wrote:
>
>> Repeating my question from other thread.
>>
>> Why you want to remove it on all platforms and not for Windows only?
>>
>> thanks,
>> Vladimir
>>
>> On 2/25/14 2:38 PM, Yumin Qi wrote:
>>> Hi, Karen and all
>>>
>>>    I have modified inline_native_isInterrupted to eliminate second fast
>>> path. Please check webrev at
>>>
>>>    http://cr.openjdk.java.net/~minqi/6498581/webrev01/
>>>
>>>    Also need review from compiler team if the changes is OK.
>>>
>>> Thanks
>>> Yumin
>>>
>>>
>>> On 2/19/2014 1:15 PM, Karen Kinnear wrote:
>>>> Yumin,
>>>>
>>>> The code fix looks good.
>>>>
>>>> The question is whether to leave in the intrinsic or not since it
>>>> could cause inconsistent data
>>>> to be returned.
>>>>
>>>> One way to deal with this would be to modify the intrinsic to only
>>>> optimize the case in
>>>> which the thread is not interrupted, i.e.
>>>> t = Thread.current() && (!TLS._osthread._interrupted)
>>>>
>>>> I think that would remove the inconsistency. Yes, it would only
>>>> optimize a subset, but the
>>>> not interrupted subset is probably the one that makes the difference.
>>>>
>>>> Can you work with the compiler folks to make that modification if it
>>>> makes sense to you?
>>>>
>>>> thanks,
>>>> Karen
>>>>
>>>> On Jan 17, 2014, at 2:01 AM, David Holmes wrote:
>>>>
>>>>> Hi Yumin,
>>>>>
>>>>> No need to bother the compiler folks with this (still cc'd but can be
>>>>> dropped on any reply). We just need to decide if we can tolerate this
>>>>> inconsistency between the intrinsic and non-intrinsic forms.
>>>>> Otherwise we may choose to disable the intrinsic on Windows - I
>>>>> really don't see that it can be adding much value as
>>>>> Thread.currentThread().isInterrupted(false) is hardly a performance
>>>>> critical action. But we would need benchmarking to confirm that.
>>>>>
>>>>> David
>>>>>
>>>>> On 17/01/2014 4:10 PM, Yumin Qi wrote:
>>>>>> Now I got it:
>>>>>>
>>>>>> //------------------------inline_native_isInterrupted------------------
>>>>>> // private native boolean java.lang.Thread.isInterrupted(boolean
>>>>>> ClearInterrupted);
>>>>>> bool LibraryCallKit::inline_native_isInterrupted() {
>>>>>>    // Add a fast path to t.isInterrupted(clear_int):
>>>>>>    //   (t == Thread.current() && (!TLS._osthread._interrupted ||
>>>>>> !clear_int))
>>>>>>    //   ? TLS._osthread._interrupted : /*slow path:*/
>>>>>> t.isInterrupted(clear_int)
>>>>>>    // So, in the common case that the interrupt bit is false,
>>>>>>    // we avoid making a call into the VM.  Even if the interrupt bit
>>>>>>    // is true, if the clear_int argument is false, we avoid the VM
>>>>>> call.
>>>>>>    // However, if the receiver is not currentThread, we must call
>>>>>> the VM,
>>>>>>    // because there must be some locking done around the operation.
>>>>>>
>>>>>>    // We only go to the fast case code if we pass two guards.
>>>>>>    // Paths which do not pass are accumulated in the slow_region.
>>>>>>
>>>>>> this code does not work both before and after the fix then.
>>>>>> Maybe we should not go with fast path then. CC to compiler team.
>>>>>>
>>>>>> Thanks
>>>>>> Yumin
>>>>>>
>>>>>>
>>>>>> On 1/16/2014 3:11 PM, David Holmes wrote:
>>>>>>> On 17/01/2014 7:24 AM, Yumin Qi wrote:
>>>>>>>> Dan,
>>>>>>>>
>>>>>>>>
>>>>>>>> On 1/16/2014 12:48 PM, Daniel D. Daugherty wrote:
>>>>>>>>> On 1/16/14 1:42 PM, Yumin Qi wrote:
>>>>>>>>>> David,
>>>>>>>>>>
>>>>>>>>>>   Thanks for the detail analysis so far and supply suggested fix.
>>>>>>>>>>
>>>>>>>>>> On 1/15/2014 10:21 PM, David Holmes wrote:
>>>>>>>>>>> Hi Yumin,
>>>>>>>>>>>
>>>>>>>>>>> Unfortunately the complexities of this continue to expose
>>>>>>>>>>> themselves :(
>>>>>>>>>>>
>>>>>>>>>>> On 15/01/2014 11:00 AM, Yumin Qi wrote:
>>>>>>>>>>>> Can I have your codereview of the change
>>>>>>>>>>>>
>>>>>>>>>>>> http://cr.openjdk.java.net/~minqi/6498581/webrev00
>>>>>>>>>>>> <http://cr.openjdk.java.net/%7Eminqi/6498581/webrev00/>
>>>>>>>>>>>>
>>>>>>>>>>>> Summary: There is race condition between os::interrupt and
>>>>>>>>>>>> os::is_interrupted on Windows.  See bug comments in detail. When
>>>>>>>>>>>> thread
>>>>>>>>>>>> sleep check if it gets interrupted, it may see interrupted but
>>>>>>>>>>>> not
>>>>>>>>>>>> really interrupted so cause spurious waking up (early return from
>>>>>>>>>>>> sleep). Fix by checking if a real interrupt sent to thread
>>>>>>>>>>>> interrupt
>>>>>>>>>>>> event can prevent such false return. On windows we can get
>>>>>>>>>>>> away the
>>>>>>>>>>>> interrupted field but to keep consistent with other platforms, I
>>>>>>>>>>>> choose
>>>>>>>>>>>> to leave it as it is.
>>>>>>>>>>>>
>>>>>>>>>>>> Contributed-By: David Holmes
>>>>>>>>>>> Not really. :) I suggested using WaitForSingleObject on the
>>>>>>>>>>> interrupt event to detect if the event was signalled or not, and
>>>>>>>>>>> hence whether the thread was interrupted or not. I had
>>>>>>>>>>> envisaged it
>>>>>>>>>>> replacing use of the interrupted field altogether on windows -
>>>>>>>>>>> but I
>>>>>>>>>>> had overlooked the fact that the _interrupted field of osThread is
>>>>>>>>>>> maintained in shared code, and further that there is an intrinsic
>>>>>>>>>>> for Thread.isInterrupted that uses that field!
>>>>>>>>>>>
>>>>>>>>>> I can add myself after you, the main contributor for this fix.
>>>>>>>>>>> The fix you have provided in os::is_interrupted deals nicely with
>>>>>>>>>>> the race between setting/clearing the field and
>>>>>>>>>>> setting/clearing the
>>>>>>>>>>> event. Because of the strong ordering in os::interrupt:
>>>>>>>>>>>
>>>>>>>>>>> 1. osthread->set_interrupted(true);
>>>>>>>>>>> 2. OrderAccess::release();
>>>>>>>>>>> 3. SetEvent(osthread->interrupt_event());
>>>>>>>>>>>
>>>>>>>>>>> we can require both conditions to be met to define that the thread
>>>>>>>>>>> is indeed interrupted. If we have set the field but not the event
>>>>>>>>>>> then is_interrupted will return false without touching the
>>>>>>>>>>> field or
>>>>>>>>>>> the event. If the thread then blocks it will either unblock
>>>>>>>>>>> immediately if setEvent has now been called, or will unblock when
>>>>>>>>>>> setEvent is eventually called. That all seems to work fine.
>>>>>>>>>>>
>>>>>>>>>>> The intrinsic is a fast-path that inlines the access to the
>>>>>>>>>>> _interrupted field, but only for the current thread and only when
>>>>>>>>>>> not trying to also clear the interrupted state (ie only for
>>>>>>>>>>> Thread.currentThread().isInterrupted()**). That seems mostly
>>>>>>>>>>> harmless even if the thread is concurrently halfway through being
>>>>>>>>>>> interrupted (ie the field is set but not the event) as any
>>>>>>>>>>> interrupt
>>>>>>>>>>> responsive operation will at some point call the actual
>>>>>>>>>>> os::is_interrupted() runtime code.
>>>>>>>>>>>
>>>>>>>>>>> The only glitch I see is that it would now be possible for the
>>>>>>>>>>> following code to throw the exception:
>>>>>>>>>>>
>>>>>>>>>>> boolean a = Thread.currentThread().isInterrupted();
>>>>>>>>>>> boolean b = Thread.interrupted();
>>>>>>>>>>> if (a && !b)
>>>>>>>>>>>   throw new Error("inconsistent Thread interrupt state observed");
>>>>>>>>>>>
>>>>>>>>>> Hope no one will write code like this --- but there is other
>>>>>>>>>> possibility that multiple interrupting actions sent to the
>>>>>>>>>> thread so
>>>>>>>>>> such check can not guarantee it returns consistent result. I see
>>>>>>>>>> the
>>>>>>>>>> problem here but think such coding is very rare.  With we return
>>>>>>>>>> real
>>>>>>>>>> status of Event, 'a' get false means the Event is not set yet ----
>>>>>>>>>> and os::is_interrupted returns false is right.
>>>>>>>>> I think you may have missed David's point here. The earlier check:
>>>>>>>>>
>>>>>>>>>     boolean a = Thread.currentThread().isInterrupted();
>>>>>>>>>
>>>>>>>>> is returning 'true' and a later check:
>>>>>>>>>
>>>>>>>>>     boolean b = Thread.interrupted();
>>>>>>>>>
>>>>>>>>> is returning 'false'. If variable 'a' is 'true', then 'b' must
>>>>>>>>> also be 'true' to be consistent.
>>>>>>>>>
>>>>>>>> 1) a = true, b = true
>>>>>>>> a: it will not clear interrupted field. So if we get a 'true', means
>>>>>>>>      interrupted field is true and Event signaled. But from code:
>>>>>>>>
>>>>>>>>    if (interrupted && clear_interrupted) {
>>>>>>>>      osthread->set_interrupted(false);
>>>>>>>>      ResetEvent(osthread->interrupt_event());
>>>>>>>>    } // Otherwise leave the interrupted state alone
>>>>>>>>
>>>>>>>>    We did not do this since clear_interrupted is false.
>>>>>>> You are missing the point. If Thread.currentThread().isInterrupted()
>>>>>>> is compiled at runtime using the intrinisc then it will return true if
>>>>>>> the field is set even if the Event is not. At that point the
>>>>>>> non-intrinsic Thread.interrupted would return false. This would give
>>>>>>> the appearance that the thread was interrupted then not interrupted
>>>>>>> but that is impossible based on the specs for those methods.
>>>>>>>
>>>>>>> David
>>>>>>> -----
>>>>>>>
>>>>>>>
>>>>>>>> b: now we query with clear_interrupted is true.
>>>>>>>>     Note the Event still signaled ( We assume no other interrupt
>>>>>>>> actions
>>>>>>>> from other threads), the os::is_interrupted will return 'true' since
>>>>>>>>     field is 'true' and Event in signaled status.
>>>>>>>>     We do the clear before return 'true' so next if we have call
>>>>>>>> say, c,
>>>>>>>> will get 'false'.
>>>>>>>>
>>>>>>>> 2) a = false b= true
>>>>>>>> There is a situation like
>>>>>>>> a:  get a 'false' ---- the interrupt is going on and SetEvent not
>>>>>>>> called
>>>>>>>> yet, but field already set to interrupted so get a 'false'.
>>>>>>>> b: When b  called, interrupt action done, so  now  field is
>>>>>>>> interrupted
>>>>>>>> and Event  signaled. We get a 'true'.
>>>>>>>>
>>>>>>>> (Assume only one interrupt)
>>>>>>>>
>>>>>>>> But, anyway, a = false and b = true should not be a problem I
>>>>>>>> think. We
>>>>>>>> treat interrupted as both field and Event set.
>>>>>>>> It can not get a is true and b is false in the situation.
>>>>>>>>
>>>>>>>> Thanks
>>>>>>>> Yumin
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>> Dan
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>>> I don't think this is a realistic situation given for the issue to
>>>>>>>>>>> arise you need the interrupting thread to be effectively
>>>>>>>>>>> descheduled
>>>>>>>>>>> after setting the field but before setting the event. But still it
>>>>>>>>>>> is possible and could be observable. I'm inclined to accept this
>>>>>>>>>>> possibility but I think Karen should make that call. I also
>>>>>>>>>>> think we
>>>>>>>>>>> would need to document this possibility just in case it does
>>>>>>>>>>> arise.
>>>>>>>>>>>
>>>>>>>>>>> ** I've no idea why this intrinsic exists as it is a fairly
>>>>>>>>>>> uncommon
>>>>>>>>>>> usage. I guess at some point it was thought that this would be
>>>>>>>>>>> common, but most interrupt responsive code uses Thread.interrupted
>>>>>>>>>>> to clear the interrupt state as well as query it.
>>>>>>>>>>>
>>>>>>>>>> agree.
>>>>>>>>>>
>>>>>>>>>> Thanks
>>>>>>>>>> Yumin
>>>>>>>>>>> Thanks,
>>>>>>>>>>> David
>>>>>>>>>>>
>>>>>>>>>>>> Tests: vm.quick.testlist, specjbb2005, original test case, JPRT
>>>>>>>>>>>>
>>>>>>>>>>>> Thanks
>>>>>>>>>>>> Yumin
>>>>>>>>>>>>
>>>>>>>>>>>>



More information about the hotspot-runtime-dev mailing list