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

[Per Liden]

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

/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