Correction ...
On 30/06/2014 3:33 PM, David Holmes wrote:
Hi Dan,
I see this has already gone in but I think it is worth looking closer at
this.
On 28/06/2014 2:18 AM, Daniel D. Daugherty wrote:
Greetings,
I have a fix ready for the following bug:
8047720 Xprof hangs on Solaris
https://bugs.openjdk.java.net/browse/JDK-8047720
Here is the webrev URL:
http://cr.openjdk.java.net/~dcubed/8047720-webrev/0-jdk9-hs-rt/
This deadlock occurred between the following threads:
Main thread - Trying to stop the WatcherThread as part of
shutting down the VM; this thread is blocked
on the PeriodicTask_lock which keeps it from
reaching a safepoint.
WatcherThread - Requested a VM_ForceSafepoint to complete
a JavaThread::java_suspend() call as part
of a FlatProfiler record_thread_ticks()
call; this thread owns the PeriodicTask_lock
since it is processing a periodic task.
VMThread - Trying to start a safepoint; this thread is
blocked waiting for the Main thread to reach
a safepoint.
The PeriodicTask_lock is one of the VM internal locks and is
typically managed using Mutex::_no_safepoint_check_flag to
avoid deadlocks. Yes, the irony is dripping on the floor... :-)
What was overlooked here is that the holder of a lock that is acquired
without safepoint checks, must never block at a safepoint whilst holding
that lock. In this case the blocking is indirect, caused by the
synchronous nature of the VM_Operation, rather than a direct result of
"blocking for the safepoint" (which the WatcherThread does not
participate in). I wonder if the WatcherThread should really be using
the async variant of VM_ForceSafepoint here?
The interesting part of this deadlock is that I think that it
is possible for other periodic tasks to hit it. Anything that
causes the WatcherThread to start a safepoint while processing
a periodic task should be susceptible to this race. Think about
the -XX:+DeoptimizeALot option and how it causes VM_Deopt
requests on thread state transitions... Interesting...
I don't think so. You need three threads involved to get the deadlock.
But that isn't the point. As you state this deadlock, at VM shutdown,
could impact any synchronous safepoint operations executed by the
WatcherThread.
That aside ...
In the current case the main thread's locking of the PeriodicTask_lock
without a safepoint check is what causes the problem - that violates the
rules surrounding use of "no safepoint checks". The other methods that a
JavaThread might call that acquire the PeriodicTask_lock do perform the
safepoint checks, so they wouldn't deadlock. Hence it seems to me that
only WatcherThread::stop can lead to this problem. And as
WatcherThread::stop is only called from before_exit, and that can only
be called once, it seems to me that we could/should actually acquire the
lock with a safepoint check.
David
-----
Cheers,
David
Testing:
- I found a way to add delays to the right spots in the
VM to make the deadlock reproduce in just about every
run of the test associated with the bug. The new
os::naked_short_sleep() function is your friend. Thanks
to Fred for adding that! See the bug report for the
debugging diffs.
- 72 hours of running the test in the bug report with
delays enabled for product, fastdebug and jvmg bits
in parallel on my Solaris X86 server.
- JPRT test run
- Aurora Adhoc results are in process; we're having issues
with both a broken testbase build and infra problems
with results not being uploaded.
