RFR: 7143664: Clean up OrderAccess implementations and usage

David Holmes david.holmes at oracle.com
Tue Feb 24 02:30:54 UTC 2015

On 24/02/2015 4:45 AM, Karen Kinnear wrote:
> On Feb 20, 2015, at 1:24 PM, Erik Österlund wrote:
> And truly many thanks - in my mind you have made this much more conceptually clean and safer.
> I am good with your checking in the parts I have reviewed.

Thanks Karen! So unless someone has something they really want to be 
changed now (and I think everyone has so far said there are no strOng 
objections to anything) then I will push this version:


in a couple of days.

Oh wait - we still haven't heard anything from ppc64 folk - pinging 
Volker (cc'd :) ).


>> Hi Karen,
>> On 20/02/15 17:04, Karen Kinnear wrote:
>>> Erik,
>>> 1. orderAccess.hpp
>>> Were you going to add a comment about MSVC assumptions under C++ Volatile Semantics?
>>> Is it worth documenting in source somewhere what minimal versions of compilers are assumed?
>>> Or which versions of compilers were tested? I think that would help us in future please.
>>> Maybe in the platform-specific files?
>> I did not intend to add this comment to shared. I thought that it would
>> be enough to say that non-volatile to volatile constraints can not be
>> trusted in general. Instead there is a comment explaining this anomaly
>> in orderAccess_windows_x86.inline.hpp where it is exploited, since it is
>> the only documented exception I know of.
>> If we start picking certain examples, it feels like we should rather
>> have either 1) a comprehensive table of what volatile actually means for
>> different compilers with references or question marks where we don't
>> know (and hence need to be conservative assuming the worst), or 2)
>> putting such comments in platform specific files (like MSVC currently).
>> Do you agree? In that case, which one do you prefer?
>> As for documenting compiler versions assumed, would it be enough to
>> stamp the code with the currently used compilers we used now, even
>> though it might actually have worked with earlier versions? The
>> reasoning is that it's unlikely we will go back to using earlier
>> compilers anyway, so finding out when exactly certain compilers started
>> doing what we want them to could possibly be unnecessary, while still
>> having enough information to allow checking if future compilers have
>> provided improved functionality from this version of OrderAccess?
> It would make more sense to document for the individual platforms.
> I'm trying to prevent future "what were they thinking" questions by recording
> for each platform the specific compiler version tested. Not all versions that
> should work, just the ones we tested it with.
>>> 2. orderAccess_windows_x86.inline.hpp
>>> Could you possibly add to the comment about MSVC, that this assumes all of the Scoped templates explicitly cast the addresses to volatiles?
>>> (In case someone changes that some day?)
>> Sure! But is this not clear given the comment in
>> orderAccess_windows_x86.inline.hpp where this is exploited next to the
>> scope template specialization:
>> // Note that in MSVC, volatile memory accesses are explicitly
>> // guaranteed to have acquire release semantics (w.r.t. compiler
>> // reordering) and therefore does not even need a compiler barrier
>> // for normal acquire release accesses.
>> template<> inline void ScopedFence<X_ACQUIRE>::postfix()       { }
>> template<> inline void ScopedFence<RELEASE_X>::prefix()        { }
>> template<> inline void ScopedFence<RELEASE_X_FENCE>::prefix()  { }
>> template<> inline void ScopedFence<RELEASE_X_FENCE>::postfix() {
>> OrderAccess::fence(); }
>> If you don't think this is clear enough, I would happily improve the
>> description.
> Perhaps I am being extra cautious since the very original set of calls all assumed the passed in arguments were
> already volatile, I wanted to save us this problem in 10 years by stating that this assumes that the Scoped templates explicitly
> cast the addresses to volatiles  (which is different than the caller passing in volatiles). So to me there were two ways to get there
> and I thought it would be valuable to specify this is the assumption. Not a big deal.
>>> 3. orderAccess_windows_x86.inline.hpp
>>> So I looked up _ReadWriteBarrier and VS2012 and VS2013 say this is deprecated.
>>> JDK8 builds on VS2010, so ok. JDK9 appears to be planning to move to VS2013, so we may need to change this.
>>> It appears that the recommendation is volatile (with default on x86 of /volatile:ms). Can we use __asm__volatile("" : : : "memory";
>>> compiler _barrier for VS2013 or is there an equivalent you know about?
>> Unfortunately I currently have no access to windows platforms so I can't
>> experiment around. But I did some research when I looked into it and it
>> seems like _ReadWriteBarrier is all we got for now until we adopt C++11
>> std::atomic that Microsoft in the deprecation message on their website
>> suggests we should use instead.
>> It seems to have been deprecated because programmers found it confusing
>> - the name does not suggest it's a compiler-only ordering constraint.
>> But that's fine for our purposes.
>> I just assumed that for now we probably don't want to make such a
>> radical move to C++11, right? Since it was deprecation or C++11 I
>> intentionally used _ReadWriteBarrier anyway even though it is
>> deprecated. Is this a problem?
>> When we need to switch, I believe we will be forced to use C++11 on
>> windows whether we like it or not. It has std::atomic_signal_fence which
>> should work as a drop-in replacement for _ReadWriteBarrier AFAIK. But
>> then again, we might as well just map all calls to std::atomic if we
>> gotta use C++11 anyway...
> Thank you. Looks like an area we will need to follow-up on.
> The C++ documentation says atomic_signal_fence just does atomic reordering.
> The Visual Studio 2012 documentation is not clear on whether it also adds hardware fences.
> So - go ahead and leave this as is and we will need to switch later.
>>> So - do you have plans to do additional changes to the OrderAccess logic? Beyond getting rid of the obsolete entries
>>> when all platforms are ok with it?
>> My biggest concern was correctness and I felt it really had to be done.
>> I use OrderAccess for my own GC code a lot, and needed to tame it a bit
>> to work, and thought I might as well contribute the fixes too. ;)
> Thank you.
>> Having looked into the code a bit I know there is still room
>> improvements in places if you would like me to look into it. Some examples:
>> <possibleImprovements>
>> 1) x86_64 windows - I suspect this can be improved a /lot/. It seems
>> like compilers were broken when the code was written at the very
>> transition point to 64 bit. I believe this can be optimized to use
>> compiler support I expect is in place today.
> Yes.
>> 2) x86 on solaris: solaris studio was also crippled in the past with no
>> inline assembly. I believe these optimizations we see on other platforms
>> could now be ported to solaris as well.
> Yes. yay!
>> 3) _volatile variants of acquire/release/fence/membars: Now we are
>> conservative, taking volatile to non-volatile reordering into account
>> using compiler barriers. If the programmer knows this is not needed
>> since all data published is volatile, maybe it could be good to have
>> variants explicitly stating it won't be necessary.
> Yes.
>> 4) Joining more store(); release(); pairs to store_release() since some
>> platforms like ARM can then use stlr instead of full dmb fence which
>> seems like a good thing in general anyway.
>> All previous suggestions revolve around performance (and I don't know
>> how much we need it). But could also do other things.
>> Testing:
>> 5) Testing code: it is possible using some light template
>> metaprogramming to test if the expected specializations of OrderAccess
>> are indeed used, a bit like @Override in Java - asserting that the
>> generalized behaviour is indeed specialized as expected for all types we
>> expect them to be specialized for.
>> Further refactoring:
>> 6) Forwarding all atomic memory accesses to Atomic:: since it's really a
>> separate concern. Atomic:: should know how to make atomic accesses for
>> types used by OrderAccess, and OrderAccess should in shared code just
>> forward it. Currently Atomic supports all stores but only load of jlong,
>> making assumptions the rest can use volatile only - an assumption I'm
>> not willing to make and spread around in the JVM.
> I share your concerns.
>> </possibleImprovements>
>> The main problem though is that OrderAcces changes are often inherently
>> platform specific, and I have very limited access as an outsider to the
>> platforms.
>> This time, Jesper has been very kind to me, running JPRT for me which
>> made these changes possible, and I'm very happy about that. But maybe I
>> should do less platform specific things when I can't run JPRT. I don't
>> want to bother people with compiling code for me too much, especially
>> not if experimenting with what new compiler features have been added
>> over the years. :)
>>> Looks like we have a couple of follow-up exercises:
>>> 1) check our is_MP usage to ensure we get the compiler_barriers in both paths if we are not using volatile declarations for
>>> all compiler ordering
>>> 2) try applying these templates to any other platforms to see how well they work for us as maintainers
>>> 3) figure out why we use StubRoutines::fence on amd64 Windows but on linux we just use lock; addl 0(sp), MSVC limitation?
>> Yes I could volunteer to do 2 (only open variants of course) and 3 if
>> anyone would be kind to run JPRT for me and/or provide me with machines
>> I can try things out on (which I suspect is impossible?).
> I wasn't asking you to do this work - I was suggesting work we should do. No worries.
>>> I did not review ppc, zero, arm.
>> I should mention Zero changes were reviewed by Severin Gehwolf in the
>> zero-dev list. He did not have any problem with my changes.
> Glad to hear that. Thank you.
>> Thanks for the review Karen! :)
>> /Erik
> thanks,
> Karen
>>> thanks!
>>> Karen
>>> On Jan 22, 2015, at 8:19 PM, Erik Österlund wrote:
>>>> Hi all,
>>>> == Summary of Changes ==
>>>> This changeset fixes issues in OrderAccess on multiple levels from the
>>>> memory model semantics to compiler reorderings, to addressing
>>>> maintainability/portability issues which (almost) had to be fixed in
>>>> order to fix the correctness issues. It is the result of discussions
>>>> found in the previous "OrderAccess Refactoring" thread:
>>>> http://openjdk.5641.n7.nabble.com/OrderAccess-Refactoring-td212050.html
>>>> Bug ID:
>>>> https://bugs.openjdk.java.net/browse/JDK-7143664
>>>> (updated to reflect these related changes)
>>>> Webrev:
>>>> http://cr.openjdk.java.net/~dholmes/7143664/webrev/
>>>> Before I describe more I would like to give special thanks to David
>>>> Holmes for long discussions leading up to the currently proposed
>>>> changes. I would also like to thank Jesper Wilhelmsson for helping me
>>>> run my changes through JPRT.
>>>> == Motivation ==
>>>> This change directly fixes a reported OrderAccess bug due to compiler
>>>> reorderings which is still a vulnerability on almost all TSO platforms:
>>>> https://bugs.openjdk.java.net/browse/JDK-806196
>>>> And directly fixes confusions like release_store() != release() store()
>>>> due to memory model issues previously described in this bug ID.
>>>> At the same time it provides clearer design with separation of concerns
>>>> and generalization/specialization, removing a whole bunch of platform
>>>> specific code which could be generalized. The platform specific files
>>>> now only have a few LoC requirements (~7) to conform to the memory model
>>>> by specifying what the stand alone barriers do. Then optionally
>>>> optimizations to the general approach are possible if platforms support
>>>> it. This also makes it much easier to port to new platforms.
>>>> == Memory Model ==
>>>> The current definitions of acquire/release semantics are a bit fishy
>>>> leading to problems previously described in the bug ID (release_store()
>>>> != release() store()) and some other correctness issues. It has
>>>> therefore been replaced with a new model. I would like to thank David
>>>> Holmes for the long discussions leading up to the newly proposed model.
>>>> The new model is formally defined like this:
>>>> // T1: access_shared_data
>>>> // T1: ]release
>>>> // T1: (...)
>>>> // T1: store(X)
>>>> //
>>>> // T2: load(X)
>>>> // T2: (...)
>>>> // T2: acquire[
>>>> // T2: access_shared_data
>>>> //
>>>> // It is guaranteed that if T2: load(X) synchronizes with (observes the
>>>> // value written by) T1: store(X), then the memory accesses before the
>>>> // T1: ]release happen before the memory accesses after the T2: acquire[.
>>>> The orderAccess.hpp file and bug ID also has a few additional
>>>> explanations making it more intuitive to the user when to use
>>>> acquire/release and the resemblance to TSO abstract machines. Big thanks
>>>> goes to David Holmes for discussing the memory model with me, which
>>>> helped a lot in deriving it.
>>>> Now it holds that release() store() == release_store(), and the other
>>>> correctness issues are fixed as well.
>>>> The new model is also closer to C++11 definitions which could give us
>>>> more relaxed compiler reordering constraints in the future when compiler
>>>> support for C++11 is there and ready.
>>>> == Reliance on C++ Volatile Semantics ==
>>>> The C++ standard section 1.9 "Program Execution" is very vague about
>>>> what the keyword volatile can actually do for us. It gives clear
>>>> constraints in terms of volatile-volatile accesses but says little about
>>>> nonvolatile-volatile accesses. Yet the current implementation heavily
>>>> relies upon volatile to in terms of compiler reordering. But GCC
>>>> explicitly declares that volatiles and non-volatiles may reorder freely
>>>> ( https://gcc.gnu.org/onlinedocs/gcc/Volatiles.html ). The only compiler
>>>> known to explicitly provide the wanted semantics with volatile is MSVC
>>>> 2010 for windows (
>>>> https://msdn.microsoft.com/en-us/library/12a04hfd(v=vs.100).aspx ).
>>>> Compilers not giving explicit guarantees, must be considered unsafe and
>>>> revert to conservative behaviour.
>>>> This was brought to attention after causing bugs, but was only fixed for
>>>> x86 linux. This is a fundamental issue inherent to all TSO platforms
>>>> except windows, and has to be fixed on all of them.
>>>> Several barriers are unsafe to use because they lack compiler reordering
>>>> constraints (e.g. fence and acquire on linux_SPARC). For TSO platforms
>>>> they are typically implemented using dummy loads and stores. This seems
>>>> to be another old volatile reliance that I fixed. These barriers
>>>> sometimes have omitted compiler barriers (which is what we really want).
>>>> This seems to be another example on incorrect reliance on the volatile
>>>> semantics to help us. Therefore dummy loads/stores have been replaced
>>>> with compiler barriers on TSO platforms.
>>>> It is also worth noting that compilers like sun studio did previously
>>>> not support inline asm syntax. Therefore, barriers were implemented in
>>>> .il-files. However, using them does not give explicit compiler
>>>> constraints for reordering AFAIK. Therefore, they have been
>>>> reimplemented using inline asm with explicit compiler reordering
>>>> constraints, as even sun (solaris?) studio now supports this.
>>>> The windows variants have added a windows-style _ReadWriteBarrier()
>>>> compiler barrier similarly.
>>>> == Strange Hardware Reorderings ==
>>>> Fixed a weird inconsistency where acquire, loadstore and loadlaod would
>>>> use isync instead of lwsync for PPC on linux_zero, but not in any other
>>>> PPC platform in the repo. I assumed this is wrong and changed it to
>>>> lwsync instead.
>>>> == Code Redundancy and Refactoring ==
>>>> The OrderAccess code looks like it has been developed over a long period
>>>> of time, with small incremental changes. This seems to have led to a lot
>>>> of code duplication over time. For example, store_fence variants are not
>>>> referenced from anywhere, yet contribute to a lot of the code base and a
>>>> lot of awkwardness (such as being the one only exception not using
>>>> volatiles for some reason). Moreover, store_fence is not used anywhere
>>>> in hotspot because it is not a good fit for either the acquire/release
>>>> semantics or the Java volatile semantics, leaving a big question mark on
>>>> when it should ever be used. I took the liberty of removing it.
>>>> Another redundancy issue is that most of the semantics is exactly the
>>>> same for all platforms, yet all that default boilerplate such as how to
>>>> make atomic accesses, where acquire/release is supposed to be placed
>>>> w.r.t. the memory access, what the different barriers should do etc. is
>>>> copied in redundantly for each os_cpu and each type of memory access for
>>>> each os_cpu. This makes it extremely painful 1) to understand what
>>>> actually defines a certain platform compared to the defaults and 2) to
>>>> correct bugs like those discovered here 3) prevent silly mistakes and
>>>> bugs, by simply having a lot less code defining the behaviour of
>>>> OrderAccess that could go wrong.
>>>> A new architecture/design for OrderAccess is proposed, using a
>>>> generalization/specialization approach.
>>>> A general implementation in /share/ defines how things work and splits
>>>> into different concerns: 1) how to make an atomic memory access, 2)
>>>> where to but barriers w.r.t. the memory access for things like
>>>> load_acquire, release_store and release_store_fence, 3) how these
>>>> barriers are defined.
>>>> This allows a clear split between what is required for following the
>>>> specifications, and optimizations, which become much more readable and
>>>> only optimizations need to be reviewed in depth as the defaults can
>>>> always be trusted given correct standalone barriers.
>>>> The only thing a platform is required to specify, is what an
>>>> implementation of acquire(), release() and fence() should do. If they
>>>> are implemented properly, everything in OrderAccess is guaranteed to
>>>> work according to specification using the generalized code. This makes
>>>> it very easy to support new ports. ALL the other code in the os_cpu
>>>> files is used /only/ for optimization purposes offered for specific
>>>> configurations.
>>>> However, it is highly customizable so that specific platform can perform
>>>> any desired optimizations. For instance this load_acquire on PPC is
>>>> optimized:
>>>> template<> inline jbyte  OrderAccess::specialized_load_acquire<jbyte>
>>>> (volatile jbyte*  p) { register jbyte t = load(p);
>>>> inlasm_acquire_reg(t); return t; }
>>>> This overrides the whole load_acquire implementation to do something
>>>> custom. Platforms like x86 extensively use this for joined fencing
>>>> variants to optimize.
>>>> The default implementation of load_acquire() will make an atomic load()
>>>> followed by acquire() since the semantics is generalized. The
>>>> generalized semantics are defined using inlined postfix/prefix calls
>>>> after/before the atomic access, as defined here:
>>>> template<> inline void ScopedFenceGeneral<X_ACQUIRE>::postfix()       {
>>>> OrderAccess::acquire(); }
>>>> template<> inline void ScopedFenceGeneral<RELEASE_X>::prefix()        {
>>>> OrderAccess::release(); }
>>>> template<> inline void ScopedFenceGeneral<RELEASE_X_FENCE>::prefix()  {
>>>> OrderAccess::release(); }
>>>> template<> inline void ScopedFenceGeneral<RELEASE_X_FENCE>::postfix() {
>>>> OrderAccess::fence();   }
>>>> For platforms that simply wish to override what e.g. acquire means for a
>>>> joined ordered memory access in general, as different to calling stand
>>>> alone acquire(), the semantics can be easily overridden for a platform
>>>> such as windows like on windows:
>>>> template<> inline void ScopedFence<X_ACQUIRE>::postfix()       { }
>>>> template<> inline void ScopedFence<RELEASE_X>::prefix()        { }
>>>> template<> inline void ScopedFence<RELEASE_X_FENCE>::prefix()  { }
>>>> template<> inline void ScopedFence<RELEASE_X_FENCE>::postfix() {
>>>> OrderAccess::fence(); }
>>>> In this example, since Windows (now) has a compiler barrier for acquire,
>>>> but does not need it for joined accesses since volatile has stronger
>>>> guarantees on windows, this is enough to specialize that for joined
>>>> memory accesses, no extra protection is needed.
>>>> == Backward Compatibility and Transitioning ==
>>>> Since the newly proposed code is structured differently to before, a
>>>> #define was added for backward compatibility so that external
>>>> repositories not adhering to this new architecture do not break.
>>>> Furthermore, store_release was declared private and marked as
>>>> deprecated. This allows for a smooth transition into the new style of
>>>> OrderAccess. When the full transition is made in all known repos, the
>>>> #define and store_fence may be safely removed, eventually.
>>>> == Documentation ==
>>>> The documentation seems old and outdated, describing how it works on
>>>> SPARC RMO and IA64, which are nowhere to be found in the repository. It
>>>> also describes things about C++ volatiles which cannot be relied upon.
>>>> The documentation has been cleaned up to match the current state of the
>>>> implementation better, with architectures actually found in the repository.
>>>> == Testing ==
>>>> JPRT. Big thanks to Jesper Wilhelmsson for helping me test these changes.
>>>> Ran some DaCapo benchmarks (I know okay :p) for performance regression
>>>> and there was no perceivable difference.
>>>> Looking forward to feedback on this, and hope to get some reviews. :)
>>>> Thanks,
>>>> Erik

More information about the hotspot-dev mailing list