RFR(XS): VM times out in VM_HandshakeAllThreads::doit() with RunThese30M

[Daniel D. Daugherty]

On 8/15/18 4:21 PM, Robbin Ehn wrote:
> Hi Dan, thanks,
>
> On 2018-08-15 22:02, Daniel D. Daugherty wrote:
>> So if HandshakeState::vmthread_can_process_handshake() is only
>> called by the VMThread while at a safepoint, then the target
>> JavaThread cannot be externally resumed while that function is
>> running so this new check:
>
> No we are not at safepoint, this method is never called at safepoint.

Okay I definitely misinterpreted what you were saying in this
email thread... let's try this again.


> All
> other threads should be considered running. Therefore I need to stop 
> resuming targeted suspended thread while VM thread is executing the 
> handshake on behalf of the suspended thread.
>
> Does my comment/assert make more sense in the light of that?

Hmmm.... Here's your comment:

+  // We must block the S/R protocol to resume a thread while the vm thread
+  // is executing the operation, by holding the Threads_lock.
+  // This is problematic, since we now can't remove the Threads_lock in 
handshakes.

Mostly, but I still suggest a rewrite:

   // SafepointSynchronize::safepoint_safe() does not consider an externally
   // suspended thread to be safe. However, this function must be called 
with
   // the Threads_lock held so an externally suspended thread cannot be
   // resumed thus it is safe.

Of course, this comment assumes that the Threads_lock is held in the caller
over the appropriate interval of code...

Dan



>
> /Robbin
>
>>
>>    +  return SafepointSynchronize::safepoint_safe(target, 
>> target->thread_state()) ||
>>    +         target->is_ext_suspended();
>>
>> is okay because that externally suspended thread is also safepoint safe.
>>
>> This assert:
>>
>> +  assert(Threads_lock->owned_by_self(), "Not holding Threads_lock.");
>>
>> doesn't quite match your intent. If 
>> HandshakeState::vmthread_can_process_handshake()
>> should only be called by the VMThread at a safepoint, then a different
>> check would be better:
>>
>>    assert(SafepointSynchronize::is_at_safepoint() && 
>> Thread::current()->is_VM_thread(),
>>           "must be VMThread at a safepoint");
>>
>> Perhaps this for your comment:
>>
>>    // SafepointSynchronize::safepoint_safe() does not consider an 
>> externally
>>    // suspended thread to be safe. However, if we are at a safepoint, 
>> then
>>    // the externally suspended thread cannot be resumed so it is safe.
>>
>> Dan
>>
>>
>>>
>>> Thanks, Robbin
>>>
>>>>
>>>> Thanks,
>>>> David
>>>>
>>
>