RFR (s): JDK-8013129 Possible deadlock with Metaspace locks due to mixed usage of safepoint aware and non-safepoint aware locking

[Mikael Gerdin]

Hi,

If nobody objects I'd like to push this today, it's causing test 
timeouts in our nightly testing and I'd like to get rid of them as soon 
as possible.

/Mikael

On 2013-04-25 11:10, Mikael Gerdin wrote:
> Hi,
>
> Problem:
> We've seen some hangs in the GC nightly testing recently.
> When I looked at the minidump files from some of those hangs they looked
> like safepoint deadlocks where one thread was parked in
> Mutex::lock_without_safepoint_check on one of the "Metaspace allocation
> locks".
>
> Both of the hangs I investigated also had threads trying to lock the
> same Metaspace lock but without eliding safepoint checks because they
> were originating from Metaspace::deallocate.
>
> I believe that since the change to allocate MethodCounters on demand and
> potentially deallocating them when racing this issue was brought to the
> surface because of more frequent calls to Metaspace::deallocate when not
> at a safepoint.
>
> I was able to reproduce the hang after about an hour by running an
> instrumented build where MethodCounters are allocated and then
> unconditionally deallocated on each entry to Method::build_method_counters.
>
> I can't describe the failure mode in detail since I'm not familiar with
> the Mutex code but I can imagine that the locking state machine is
> broken when we take completely different code paths for the same Mutex.
>
> Suggested fix:
> My suggested fix is to change Metaspace::deallocate to take the lock
> with Mutex::_no_safepoint_check_flag.
>
> With my fix I ran the test that managed to reproduce the failure
> overnight without reproducing the hang.
> I also ran the parallel class loading tests and nashorn's
> test262parallel for good measure.
>
> Webrev: http://cr.openjdk.java.net/~mgerdin/8013129/webrev.0/
> JBS: https://jbs.oracle.com/bugs/browse/JDK-8013129
> bugs.sun.com: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8013129
>
> /Mikael