RFR: 8231666: ThreadIdTable::grow() invokes invalid thread transition

[David Holmes]

On 2/10/2019 7:15 pm, Robbin Ehn wrote:
> Hi, since holding the Threads_lock while growing can block out 
> safepoints for a
> longer period, I would suggest just skip growing when holding Threads_lock.
> E.g. return before creating the GrowTask.

What if the table is full and must be grown?

That aside, I'd like to know how expensive it is to grow this table. 
What are we talking about here?

David

> /Robbin
> 
> On 2019-10-02 08:46, David Holmes wrote:
>> Hi Daniil,
>>
>> On 2/10/2019 4:13 pm, Daniil Titov wrote:
>>> Please review a change that fixes the issue. The problem here is that 
>>> that the thread is added to the ThreadIdTable  (introduced in [3]) 
>>> while the Threads_lock is held by
>>> JVM_StartThread. When new thread is added  to the thread table the 
>>> table checks if its load factor is greater than required and if so it 
>>> grows itself while polling for safepoints.
>>> After changes [4]  an attempt to block the thread while holding the 
>>> Threads_lock  results in assertion in 
>>> Thread::check_possible_safepoint().
>>>
>>> The fix  proposed by David Holmes ( thank you, David!)  is to skip 
>>> the ThreadBlockInVM inside ThreadIdTable::grow() method if the 
>>> current thread owns the Threads_lock.
>>
>> Sorry but looking at the fix in context now I think it would be better 
>> to do this:
>>
>>      while (gt.do_task(jt)) {
>>        if (Threads_lock->owner() == jt) {
>>          gt.pause(jt);
>>          ThreadBlockInVM tbivm(jt);
>>          gt.cont(jt);
>>        }
>>      }
>>
>> This way we don't waste time with the pause/cont when there's no 
>> safepoint pause going to happen - and the owner() check is quicker 
>> than owned_by_self(). That partially addresses a general concern I 
>> have about how long it may take to grow the table, as we are deferring 
>> safepoints until it is complete in this JVM_StartThread usecase.
>>
>> In the test you don't need all of:
>>
>>    32  * @run clean ThreadStartTest
>>    33  * @run build ThreadStartTest
>>    34  * @run main ThreadStartTest
>>
>> just the last @run suffices to build and run the test.
>>
>> Thanks,
>> David
>> -----
>>
>>> Testing : Mach 5 tier1 and tier2 completed successfully, tier3 is in 
>>> progress.
>>>
>>> [1] Webrev: http://cr.openjdk.java.net/~dtitov/8231666/webrev.01/
>>> [2] Bug: https://bugs.openjdk.java.net/browse/JDK-8231666
>>> [3] https://bugs.openjdk.java.net/browse/JDK-8185005
>>> [4] https://bugs.openjdk.java.net/browse/JDK-8184732
>>>
>>> Best regards,
>>> Danill
>>>
>>>