RFR (S) 8074355: make MutexLocker smarter about non-JavaThreads

[coleen.phillimore at oracle.com]

On 4/28/19 8:42 PM, David Holmes wrote:
> Hi Coleen,
>
> On 27/04/2019 2:10 am, coleen.phillimore at oracle.com wrote:
>> Summary: Use safepoint_check_always/safepoint_check_never instead of 
>> safepoint_check_sometimes for locks that are taken by JavaThreads and 
>> non-JavaThreads
>
> To clarify: the safepoint_check_[always|never|sometimes] pertains only 
> to the behaviour of JavaThreads that use the lock, independently of 
> whether a lock may be used by both JavaThreads and non-JavaThreads.

Yes.
>
>> This patch moves all but 3 of the locks to not be 
>> safepoint_check_sometimes.  We have plans to fix these three.  Also, 
>
> So have you established that the reasons these were 'sometimes' locks 
> no longer apply and so it is correct to change them? Or are you 
> relying on testing to expose any mistaken assumptions?

Oh, I hope not.   Robbin and I have been looking at them and he thinks 
we can change them for the situations that they had to be sometimes 
locks.  The Heap_lock for example, couldn't be taken with a safepoint 
check on the exit path.

>
>> this patch allows for being explicit about safepoint checking or not 
>> when the thread is a non-java thread, which is something that Kim 
>> objected to in my first patch.
>
> I don't understand what you mean by this. NJTs can currently call 
> either lock() or lock_without_safepoint_check().
>

My first patch added the change for NJTs to just call lock and didn't 
call lock_without_safepoint_check for the safepoint_check_always 
flags.   Now they can call either.   My first patch also made Heap_lock 
an always lock, which it can't be.


>> Tested with mach5 tier1-3.
>>
>> open webrev at 
>> http://cr.openjdk.java.net/~coleenp/2019/8074355.03/webrev
>> bug link https://bugs.openjdk.java.net/browse/JDK-8074355
>
> src/hotspot/share/gc/shared/oopStorage.cpp
>
> How can these mutexes go from currently always needing safepoint 
> checks to now never needing them? Are they in fact never used by 
> JavaThreads?
>
Now, this asserts that they can't be sometimes either.  They asserted 
that they couldn't be "always" locks.  These locks are low level locks 
and should never safepoint check.


> ---
>
> src/hotspot/share/runtime/mutex.hpp
>
>  95   void check_safepoint_state   (Thread* self, bool 
> safepoint_check)   NOT_DEBUG_RETURN;
>
> Please use the same parameter name as in the implementation: 
> do_safepoint_check.

Fixed.

>
> 109   // Java and NonJavaThreads. When the lock is initialized with 
> _safepoint_check_always,
>  110   // that means that whenever the lock is acquired by a 
> JavaThread, it will verify that each
>  111   // acquition from a JavaThread is done with a safepoint check.
>
> That can simplify to just:
>
> 109   // Java and NonJavaThreads. When the lock is initialized with 
> _safepoint_check_always,
> 110   // that means that whenever the lock is acquired by a 
> JavaThread, it will verify that
> 111   // it is done with a safepoint check.

That's better and less redundant.

>
> Should we then continue:
>
> 111   // it is done with a safepoint check. In corollary when the lock
> 112   // is initialized with _safepoint_check_never, that means that
> 113   // whenever the lock is acquired by a JavaThread it will verify
> 114   // that it is done without a safepoint check.
>
> ?

I like it.  Added with some reformatting so the paragraph is same width.
>
> ---
>
> 38   SafepointCheckRequired not_allowed = do_safepoint_check ? 
> _safepoint_check_never : _safepoint_check_always;
> 39   assert(!self->is_Java_thread() || _safepoint_check_required != 
> not_allowed,
>
> I found this code very difficult to understand due to some previous 
> choices. All of the names that start with underscore give the illusion 
> (to me) of being variables (or at least being the same kind of thing) 
> but two are enum values and one is a field. Using 
> this->_safepoint_check_required would at least make it clearer which 
> is the field.

Ew. no. The underscore makes it clear it's a field of the class Monitor.

>
>  43   // Allow NonJavaThreads to lock and wait with a safepoint check 
> for locks that may be shared with JavaThreads.
>  44   assert(self->is_Java_thread() || !do_safepoint_check || 
> _safepoint_check_required != _safepoint_check_never,
>  45          "NonJavaThreads can only check for safepoints if shared 
> with JavaThreads");
>
> This is very confusing: NJTs don't do safepoint checks. I think what 
> you mean here is that you will allow a NJT to call lock() rather than 
> lock_without_safepoint_check() but only if the mutex is "shared with 
> JavaThreads". But always/sometimes/never has nothing to with whether 
> the lock is shared between JTs and NJTs. I understand that a NJT-only 
> mutex should, by convention, be created with _safepoint_check_never - 
> but it really makes no practical difference. Further, a mutex used 
> only by JavaThreads could in theory also be _safepoint_check_never.

It is confusing but this found some wild use of a lock(do safepoint 
check) call for a lock that is now defined as safepoint_check_never.  
The shared lock commentary was because for a shared lock, it can be 
acquired with the safepoint_check parameter from a NonJava thread.

Maybe it should say this instead:

   // NonJavaThreads defined with safepoint_check_never should never ask 
to safepoint check.
   assert(thread->is_Java_thread() || !do_safepoint_check || 
_safepoint_check_required != _safepoint_check_never,
          "NonJavaThread should not check for safepoint");

>
> 47   // Only Threads_lock, Heap_lock and SR_lock may be 
> safepoint_check_sometimes.
> 48   assert(_safepoint_check_required != _safepoint_check_sometimes || 
> this == Threads_lock || this == Heap_lock ||
> 49          this->rank() == suspend_resume,
> 50          "Lock has _safepoint_check_sometimes %s", this->name());
>
> This assert belongs in the constructor, not in every lock operation, 
> as it depends only on the monitor instance not on the thread doing the 
> lock.
>

You're right, that's much better!  Fixed.

Thanks,
Coleen
> ---
>
> Thanks,
> David
>
>
>> Thanks,
>> Coleen