RFR(s): 8213574: Deadlock in string table expansion when dumping lots of CDS classes

[David Holmes]

Hi Robbin,

On 12/11/2018 11:59 PM, Robbin Ehn wrote:
> Hi all, please review.
> 
> The re-sizing operation is run by the ServiceThread (JavaThread). To be 
> safepoint polite it pauses the operation and do safepoint checks. 
> Operations on
> the hashtable that visit multiple bucket are mutual exclusive. This 
> means you
> can't iterate over the table during a safepoint if there is paused resize.
> 
> The hashtable uses a Mutex, which means if there were other threads 
> using it
> during the safepoint the VM thread sneaking would break it. Since there 
> are no such users it is safe to access it without locks in side the 
> safepoint. That is
> how rehash works today.

Okay - so you are saying that if the existing code is not actually 
broken, then the new code is also not actually broken. I agree. But I'd 
be happier if there was somewhere we could assert this fact. :)

I wonder whether as part of the Mutex rewrite we should add the ability 
to indicate what types of thread can acquire a given mutex, and thus 
ensure non-JavaThreads can't access things they should not ?

> Until we sorted this out better in 8213742,  I'm adding a safepoint 
> scanning
> operation that can handle a paused resize and which skips the lock.
> 
> CR: https://bugs.openjdk.java.net/browse/JDK-8213574
> Webrev: http://cr.openjdk.java.net/~rehn/8213574/webrev/

The Thread argument to do_safepoint_scan seems unused and unnecessary - 
the current thread must be the VMThread (which should be asserted)

Otherwise looks good.

Thanks,
David
-----

> Passes t1-3 and 8213587 new test which triggered the issue.
> 
> Thanks, Robbin