RFR: 7143664: Clean up OrderAccess implementations and usage

David Holmes david.holmes at oracle.com
Fri Jan 23 07:10:16 UTC 2015

On 23/01/2015 4:36 PM, Dean Long wrote:
>   147 // Ordering a load relative to preceding stores requires a fence,
>   148 // which implies a membar #StoreLoad between the store and load under
>   149 // sparc-TSO.  A fence is required by x86.  On x86, we use explicitly
>   150 // locked add.
>   151 //
> It sounds like the above is saying that fence is the same as StoreLoad ...

That may be a s/store_fence/fence/ typo. The original text was:

// Ordering a load relative to preceding stores requires a store_fence,
// which implies a membar #StoreLoad between the store and load under
// sparc-TSO.  A fence is required by ia64.  On x86, we use locked xchg.

Actually seems like a couple of typos there: ia64 became x86, then we 
have x86 again.

>   152 // 4. store, load  <= is constrained by => store, fence, load
>   153 //
>   154 // Use store, fence to make sure all stores done in an 'interesting'
>   155 // region are made visible prior to both subsequent loads and stores.
> ... and this is like saying to use fence because StoreStore | StoreLoad
> isn't available.

Again this seems an issue with store_fence being changed to "store, 
fence"  which doesn't really make sense.


>> Furthermore, store_release was declared private and marked as
> deprecated.
> I can't find where this was done.
> dl
> On 1/22/2015 5:19 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

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