RFR(XS) for PeriodicTask_lock cleanup (8072439)

[Markus Gronlund]

Hi Dan,

 

I have taken a look with your suggested patch – I think your suggestion looks very good.

 

I guess the original hang happened because the PeriodicTask_lock was attempted to be acquired by a JavaThread, but the PeriodicTask_lock was still held by someone else. Since the PeriodicTask_lock was taken with “Mutex::_no_safepoint_checks” it meant the JavaThread bypassed the callback for a potentially pending safepoint and instead called parked upon the PeriodicTask_lock straight away...

 

I think this lock should definitely be taken the way you have done in the patch.

 

I also think the placement of OrderAccess::fence() might have been due to some of the constructs being racy, take this for instance:

 

void WatcherThread::start() {

  assert(PeriodicTask_lock->owned_by_self(), "PeriodicTask_lock required");

 

  if (watcher_thread() == NULL && _startable) { _startable is visible since its the same thread

    _should_terminate = false; <<----------------------------- this is set but will not be visible to the WatcherThread being launched (it’s  a 0 in the static initializer however, so it is still “safe”)

    // Create the single instance of WatcherThread

    new WatcherThread();

 

// above the constructor for WatcherThread will start the thread, and the WatcherThread::run() might check _should_terminate before the launching thread releases the PeriodicTask_lock. Not that it will be an issue here, since _should_terminate is set to 0 in its static initializer. But thanks Dan for moving this _should_terminate lower in the loop, at least the WatcherThread will need now need a call to sleep() before reaching it (and sleep needs the PeriodicTask_lock)

 

But for the construct in WatcherThread::stop(), there is no need (any more?) for the OrderAccess::fence(), I think it can be safely removed.

 

Can you also remove the comment in thread.hpp : 704 that says:

 

  volatile static bool _should_terminate; // updated without holding lock

 

As this is not the case any longer.

 

Otherwise it looks good!

 

Thanks for fixing this

 

Cheers

Markus

 

 

 

 

 

 

 

From: Daniel D. Daugherty 
Sent: den 17 februari 2015 23:42
To: Carsten Varming
Cc: Alexander Garthwaite; Rickard Bäckman; David Holmes; Markus Grönlund; Coleen Phillimore; hotspot-runtime-dev at openjdk.java.net; serviceability-dev at openjdk.java.net
Subject: Re: RFR(XS) for PeriodicTask_lock cleanup (8072439)

 

On 2/17/15 3:22 PM, Carsten Varming wrote:



Dear Daniel, 

 

Looks good to me.


Thanks for the fast review.





The line: "OrderAccess::fence();  // ensure WatcherThread sees update in main loop" seems unnecessary as the lock acts as a memory barrier.


Yes, I keep looking at that line from the original work on
JDK-7127792 and wonder why it's there... I'll chase that down
with the original folks...

Dan





 

Carsten

 

On Tue, Feb 17, 2015 at 4:44 PM, Daniel D. Daugherty <HYPERLINK "mailto:daniel.daugherty at oracle.com" \ndaniel.daugherty at oracle.com> wrote:

Greetings,

My fix for the following bug:

    JDK-8047720 Xprof hangs on Solaris

that was pushed to JDK9 last June needs to be cleaned up.

Thanks to Alex Garthwaite (HYPERLINK "mailto:agarthwaite at twitter.com" \nagarthwaite at twitter.com) and Carsten
Varming (HYPERLINK "mailto:varming at gmail.com" \nvarming at gmail.com) for reporting the mess that I made
in WatcherThread::stop() and for suggesting fixes.

This code review is for a general cleanup pass on PeriodicTask_lock
and some of the surrounding code. This is a targeted review in that
I would like to hear from three groups of people:

1) The author and reviewers for:

   JDK-7127792 Add the ability to change an existing PeriodicTask's
               execution interval

   Rickard, David H, and Markus G.

2) The reviewers for:

   JDK-8047720 Xprof hangs on Solaris

   Markus G and Coleen

3) Alex and Carsten


Here's the webrev URL:

HYPERLINK "http://cr.openjdk.java.net/%7Edcubed/8072439-webrev/0-for_jdk9_hs_rt/" \nhttp://cr.openjdk.java.net/~dcubed/8072439-webrev/0-for_jdk9_hs_rt/

I've attached the original RFR for JDK-8047720 that explains
the original deadlock that was being fixed. Similar testing
will be done with this fix.

Dan