RFR(M): 8154736: enhancement of cmpxchg and copy_to_survivor for ppc64

[Hiroshi H Horii]

Hi, Thomas, and David,

Thank you for your comments.

> I think Hiroshi thinks that since the work stealing itself does a CAS
> with barrier after obtaining "new_obj" in the other thread, it should
> be safe (for other threads consuming an object on the task queue).

Thank you. What Thomas thankfully explain is that I wanted to mention why 
relaxed CAS is available for copy_to_survivor.

> I also do not think it is safe as is - for example, at least
> PSPromotionManager::copy_and_push_safe_barrier() reads data from the
> returned new_obj (in another log message :)) regardless of failure.
> 
> That method also reads the forwardee if forwarded, and then again uses
> object information in that same log message. A quick look did not show
> other issues, but don't count this as a review.

Thank you for your comments.

As Carsten suggested, I guess, size may not be necessary for logging when 
CAS is failed (the size will be logged by the other thread that 
successfully operates the CAS). By reducing printing a size of new_obj, 
relaxing CAS for forwarding pointers becomes safe, I believe.

In my understanding, PSPromotionManager::copy_and_push_safe_barrier() 
updates a card table for new_obj. However, this new_obj will not be used 
fro card tables in the same GC as a root of GC because all of entries in 
card tables were registered as tasks before any calls of 
copy_and_push_safe_barrier.

I created a new webrev that reduces print formats when CAS is failed. 
Could you review this and give comments on it?
http://cr.openjdk.java.net/~horii/8154736/webrev.00/

Regards,
Hiroshi
-----------------------
Hiroshi Horii, Ph.D.
IBM Research - Tokyo


Thomas Schatzl <thomas.schatzl at oracle.com> wrote on 09/30/2016 21:02:31:

> From: Thomas Schatzl <thomas.schatzl at oracle.com>
> To: David Holmes <david.holmes at oracle.com>, Hiroshi H 
Horii/Japan/IBM at IBMJP
> Cc: hotspot-compiler-dev <hotspot-compiler-dev at openjdk.java.net>, 
> Tim Ellison <Tim_Ellison at uk.ibm.com>, Michihiro Horie/Japan/
> IBM at IBMJP, "ppc-aix-port-dev at openjdk.java.net" <ppc-aix-port-
> dev at openjdk.java.net>, "hotspot-gc-dev at openjdk.java.net" <hotspot-
> gc-dev at openjdk.java.net>, "hotspot-runtime-dev at openjdk.java.net" 
> <hotspot-runtime-dev at openjdk.java.net>
> Date: 09/30/2016 21:04
> Subject: Re: RFR(M): 8154736: enhancement of cmpxchg and 
> copy_to_survivor for ppc64
> 
> Hi,
> 
> On Fri, 2016-09-30 at 21:12 +1000, David Holmes wrote:
> > On 30/09/2016 8:17 PM, Hiroshi H Horii wrote:
> > > 
> > > Dear David, and Dan,
> > > 
> > > Thank you for your comments.
> > > 
> > > > 
> > > > In
> > > > hotspot/src/share/vm/gc/parallel/psPromotionManager.inline.hpp:
> > > > 266 the log line reads data from the forwardee even when the CAS
> > > > fails. I believe those reads will be unsafe without barriers
> > > > after
> > > > the copy of the content of the object.
> > > > hotspot/src/share/vm/gc/parallel/psPromotionManager.inline.hpp:28
> > > > 8
> > > > same problem as in line 266
> > > Can we use o->size() or new_obj_size instead of new_obj->size()?
> 
> They are not equivalent. Parallel GC and other collectors creatively
> reuse the "length" field of objArrays to indicate progress in the
> scanning them during GC.
> 
> new_obj_size is the result of a call to o->size() (and the compiler may
> redo computations at any point), so has the same issue.
> 
> > > > If you feel that the use of new_obj->size() is potentially unsafe
> > > > then
> > > > the fact we return new_obj means that any use of new_obj by the
> > > > caller
> > > > may also potentially be unsafe.
> > > In my understanding, while copying objects to a survivor space, if
> > > a thread creates a new_obj and sets a pointer with CAS, the other
> > > threads can touch the new_obj after the thread calls
> > > push_contents(new_obj) (Line: 239). In push_contents,
> > > OrderAccess::release_store is called before pushing the object as a
> > > task into a deque of workstealing (taskqueue.inline.hpp). If the
> > > other thread reads the task, all of copy for new_obj is safe.
> > I'm not familiar with the larger picture of the GC protocols here,
> > but just looking at this code fragment in isolation if the CAS fails
> > we read o->forwardee() to set new_obj. That in itself is fine because
> > we're reading the field that we were testing with the CAS. But we
> > could then deference new_obj before the thread that won the CAS calls
> > push_contents; and even if it is after push_contents we have not done
> > an acquire to pair with the release-store in push_contents.
> 
> I think Hiroshi thinks that since the work stealing itself does a CAS
> with barrier after obtaining "new_obj" in the other thread, it should
> be safe (for other threads consuming an object on the task queue).
> 
> > So I'm really not seeing how we can use a barrier-less CAS here.
> 
> I also do not think it is safe as is - for example, at least
> PSPromotionManager::copy_and_push_safe_barrier() reads data from the
> returned new_obj (in another log message :)) regardless of failure.
> 
> That method also reads the forwardee if forwarded, and then again uses
> object information in that same log message. A quick look did not show
> other issues, but don't count this as a review.
> 
> Thanks,
>   Thomas
> 


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