RFR: 8203817: Monitor::try_lock() should not call check_prelock_state()

[Per Liden]

On 05/25/2018 01:47 PM, Per Liden wrote:
> On 05/25/2018 11:45 AM, David Holmes wrote:
>> On 25/05/2018 7:33 PM, Per Liden wrote:
>>> Hi David,
>>>
>>> On 05/25/2018 09:51 AM, David Holmes wrote:
>>>> Hi Erik,
>>>>
>>>> On 25/05/2018 5:17 PM, Erik Osterlund wrote:
>>>>> Hi David,
>>>>>
>>>>> The change Per is proposing would make try_lock perform the same 
>>>>> checks that locking without safepoint checks does.
>>>>
>>>> Yet locking without a safepoint check also ensures 
>>>> _safepoint_check_required != Monitor::_safepoint_check_always. But 
>>>> try_lock does not and can be applied to both kinds of 
>>>> Monitors/Mutexes. Further it also has a stated, but not checked, 
>>>> precondition that the thread is not _thread_in_vm - though that may 
>>>> only be an issue if we were to block.
>>>>
>>>> I'm looking at the checks in check_prelock_state wondering which are 
>>>> only truly needed if we risk blocking? I think most of them, with 
>>>> the exception of is_crash_protected. Are we guaranteed never to run 
>>>> this JFR code on the WatcherThread?
>>>>
>>>> Though I'm also concerned about lock-ranking and deadlocks if we 
>>>> try_lock multiple locks. Notwithstanding the rank/deadlock code is 
>>>> far from ideal already.
>>>>
>>>> I don't mind removing checks/guards that really don't apply in the 
>>>> try_lock case, but I'm wary of removing all guards without being 
>>>> more sure of that.
>>>
>>> Ok, so how about we just avoid the bad parts, by doing this:
>>>
>>> http://cr.openjdk.java.net/~pliden/8203817/webrev.1
>>
>> That seems a reasonable compromise.
> 
> Good! Thanks for looking at this David!

Erik, are you also happy with this compromise?

/Per

> 
> /Per
> 
>>
>> There may still be possible deadlock issues if we use try-lock to 
>> acquire multiple locks, but we probably don't do that ... and as Erik 
>> noted there are issues with the ranking/deadlock detection anyway.
>>
>> Thanks,
>> David
>>
>>> /Per
>>>
>>>>
>>>> Thanks,
>>>> David
>>>> -----
>>>>
>>>>> Perhaps locking without safepont checks could perform more sanity 
>>>>> checks, but that is a separate issue. I think try_lock should 
>>>>> perform the same checks that locking without safepoint checks does. 
>>>>> The alternatives are then to
>>>>>
>>>>> 1) Remove checking the prelock state like Per suggests for try_lock 
>>>>> (then they do the same checks), or
>>>>> 2) Overhaul the safepoint checking refactoring out the bits that 
>>>>> check the safepointing sanity chrcka from other deadlock checks 
>>>>> (and correct those to check for rank <= special, and not == 
>>>>> special), remove the safepoint checking part from try_lock and 
>>>>> adding the deadlock checking parts to lock without safepoints.
>>>>>
>>>>> Doing #2 seems like a different RFE. In fact I believe that would 
>>>>> be https://bugs.openjdk.java.net/browse/JDK-8184732 
>>>>> <https://bugs.openjdk.java.net/browse/JDK-8184732?jql=text%20~%20%22deadlock%20check%22> that 
>>>>> I filed a while back.
>>>>>
>>>>> In summary, there is a whole bunch of problems in the deadlock 
>>>>> detection system, and #2 makes it hard to not get dragged down in 
>>>>> the rabbit hole. #1 is sufficient to  make try_lock check as much 
>>>>> (or little) as locking without safepoint checking. And I think that 
>>>>> is enough for the scope of this change.
>>>>>
>>>>> Looks good to me.
>>>>>
>>>>> Thanks,
>>>>> /Erik
>>>>>
>>>>> On 25 May 2018, at 08:53, David Holmes <david.holmes at oracle.com 
>>>>> <mailto:david.holmes at oracle.com>> wrote:
>>>>>
>>>>>> Hi Per,
>>>>>>
>>>>>> Exactly what condition(s) does JFR violate? This is throwing away 
>>>>>> all the checks that guard against incorrect monitor use. It's not 
>>>>>> just about whether you'd block trying to acquire the Monitor, it's 
>>>>>> also about whether it is safe to acquire it from that code/thread 
>>>>>> in the first place. (Though I think some of the checks in there 
>>>>>> should also be considering the value of _safepoint_check_required.)
>>>>>>
>>>>>> Thanks,
>>>>>> David
>>>>>>
>>>>>>
>>>>>> On 25/05/2018 4:39 PM, Per Liden wrote:
>>>>>>> In debug builds, Monitor::try_lock() calls check_prelock_state() 
>>>>>>> to check the thread state, etc. The intention is to verify that 
>>>>>>> the call is made from the correct context, a context where we're 
>>>>>>> allowed to block and potentially safepoint. Unlike 
>>>>>>> Monitor::lock(), Monitor::try_lock() will never block, hence the 
>>>>>>> call to check_prelock_state() is overly strict and we should 
>>>>>>> remove it. Removing it would match the behavior of all other 
>>>>>>> non-blocking functions, like 
>>>>>>> Monitor::lock_without_safepoint_check(), which doesn't call 
>>>>>>> check_prelock_state() either (for a good reason).
>>>>>>> The specific problem I've run into with this is related to JFR. 
>>>>>>> Monitor::try_lock() is by JFR to allow non-blocking event 
>>>>>>> generation, so that you can generate JFR events from "any" 
>>>>>>> context without risk blocking/safepointing (the logic is doing 
>>>>>>> something like, if try_lock() fails then put the event on a 
>>>>>>> different queue and let the next owner of the lock handle it). 
>>>>>>> The overly strict checks done by check_prelock_state() in 
>>>>>>> try_lock() breaks this logic, which in turn means that you can't 
>>>>>>> generate JFR event from "any" context as was intended.
>>>>>>> The patch to fix this is a one-liner, just remove the call to 
>>>>>>> check_prelock_state().
>>>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8203817
>>>>>>> Webrev: http://cr.openjdk.java.net/~pliden/8203817/webrev.0
>>>>>>> /Per