RFR: JDK-8061964: Insufficient compiler barriers for GCC in OrderAccess functions

Tue Nov 4 13:21:25 UTC 2014

Hi guys,

Small comment on this discussion.

On 04 Nov 2014, at 13:35, Mikael Gerdin <mikael.gerdin at oracle.com> wrote:

> Hi David,
> 
> On 2014-11-04 12:15, David Holmes wrote:
>> Hi Mikael,
>> 
>> Thanks for fixing this.
>> 
>> Given x86 is a TSO system my understanding from the previous discussion
>> in 6973570 is that once you have the compiler barrier the volatile write
>> to the dummy variable in storestore() is no longer needed - it's only
>> purpose was to introduce the compiler barrier (which it failed to do).
> 
> I felt that removing the dummy store would be more risky since I have no good way of verifying that removing the volatile write doesn't break ordering for some other caller of storestore().
> 
>> 
>> Secondly, do we observe the same bug with the release_store operations
>> or are we just being conservative? I would not expect to need the
>> compiler barrier prior to the Atomic::store, but I would assume this is
>> completely harmless and performance neutral, so we should have it for
>> completeness regardless.
> 
> I tried changing the problematic code to do a release_store of _gc_time_stamp and saw the same reordering problem, that's the reason for adding compiler_barrier to the release_store variants.
> 
> Any suggestion on where to put compiler_barrier in order to use it from atomic_linux_x86.inline.hpp?
> 
>> 
>> Finally, you answered Dean regarding the simple acquire() function but
>> the load_acquire variants all rely on volatile semantics - which we've
>> seen not to provide what we expected. So perhaps the load_acquire
>> functions also need a compile_barrier inserted ?
> 
> Perhaps. I haven't observed any problems with the acquire functions so I can't determine if they need the compiler barrier or not.
> 
> It may be a good idea to add the compiler barrier to those functions as well, what are the other reviewers' opinions on this?

The issue is that volatiles in the C++ standard prevent reordering of one volatile access w.r.t. another volatile access, but not other non-volatile accesses.
Both acquire and release semantics however should consider reordering of non-volatile accesses to. Therefore, not issuing a compiler barrier for acquire may not be shown as an issue in code yet, but I'm convinced it's hazardous not to.

Example of a pair of release_store synchronizing with a load_acquire:

release_store in T1:
non-volatile write x_1
release			<-- now enforced properly with compiler barrier
volatile write x_2

load_acquire in T2:
volatile load x_2
acquire 			<--- not currently enforced properly!
non-volatile load x_1

Now by fixing the compiler barrier of release store, the non-volatile write to x_1 is properly kept above the volatile x_2.
However, by not having a compiler barrier for the load_acquire, there is nothing AFAIK that prevents the volatile load x_2 from reordering with the non-volatile load x_1, since volatile accesses are only constrained in order with respect to other volatile accesses.

My 50 cents...

/Erik

> 
> Thanks for the review David.
> /Mikael
> 
> 
>> 
>> Thanks,
>> David
>> 
>> On 3/11/2014 11:01 PM, Mikael Gerdin wrote:
>>> Hi all,
>>> 
>>> Please review this attempt at fixing the OrderAccess functions on Linux
>>> x86 with GCC.
>>> 
>>> While working on another bug I recently discovered that g++ was
>>> reordering stores across a call to OrderAccess::storestore on Linux x86.
>>> 
>>> The G1 code attempts to do an ordered publishing of two values:
>>> _saved_mark_word = _top;
>>> OrderAccess::storestore();
>>> _gc_time_stamp = curr_gc_time_stamp;
>>> 
>>> The types involved are
>>> HeapWord* _top, _saved_mark_word;
>>> volatile unsigned _gc_time_stamp;
>>> 
>>> The incorrect behavior seems to have started when JDK-6973570 was fixed
>>> in JDK 7.
>>> Below, _top is at offset 0x58, _saved_mark_word at 0x18 and
>>> _gc_time_stamp at 0x138, %rbx is "this".
>>> 
>>> /net/jre/onestop/jdk/1.7.0/promoted/all//b108/binaries/linux-x64/jre/lib/amd64/server/libjvm.so:
>>> 
>>> 
>>>   3d9f4d:       39 d0                   cmp    %edx,%eax
>>>   3d9f4f:       73 1c                   jae    3d9f6d
>>> <G1OffsetTableContigSpace::set_saved_mark()+0x3d>
>>>   3d9f51:       48 8b 43 58             mov    0x58(%rbx),%rax
>>>   3d9f55:       48 89 43 18             mov    %rax,0x18(%rbx)
>>>   3d9f59:       48 8b 05 40 f9 70 00    mov    0x70f940(%rip),%rax    #
>>> ae98a0 <_DYNAMIC+0x12f8>
>>>   3d9f60:       48 c7 00 00 00 00 00    movq   $0x0,(%rax)
>>>   3d9f67:       89 93 38 01 00 00       mov    %edx,0x138(%rbx)
>>> 
>>> /net/jre/onestop/jdk/1.7.0/promoted/all//b109/binaries/linux-x64/jre/lib/amd64/server/libjvm.so
>>> 
>>> 
>>>   3da05d:       39 d0                   cmp    %edx,%eax
>>>   3da05f:       73 15                   jae    3da076
>>> <G1OffsetTableContigSpace::set_saved_mark()+0x36>
>>>   3da061:       48 8b 43 58             mov    0x58(%rbx),%rax
>>>   3da065:       c7 45 f4 00 00 00 00    movl   $0x0,-0xc(%rbp)
>>>   3da06c:       89 93 38 01 00 00       mov    %edx,0x138(%rbx)
>>>   3da072:       48 89 43 18             mov    %rax,0x18(%rbx)
>>> 
>>> In b109 the store of %rax to 0x18(%rbx) has been ordered after the store
>>> of %edx to 0x138(%rbx) in the same build as JDK-6973570 was integrated.
>>> 
>>> My suggestion to fix this is to extend all the OrderAccess::release*
>>> variants on x86 with a:
>>> __asm__ volatile ("" : : : "memory");
>>> to attempt to prevent GCC from reordering any memory accesses across
>>> those function calls.
>>> 
>>> I've verified that this solves the issue in the assembly with our
>>> current JDK 9 build platform compilers.
>>> I've also verified that this particular piece of code is compiled
>>> correctly on our other x86 platforms: Solaris, Windows and OS X.
>>> 
>>> Webrev:
>>> http://cr.openjdk.java.net/~mgerdin/8061964/webrev.0/
>>> Bug:
>>> https://bugs.openjdk.java.net/browse/JDK-8061964
>>> Testing:
>>> JPRT, inspecting generated assembly for the function
>>> G1OffsetTableContigSpace::record_top_and_timestamp (as the method is
>>> currently named).
>>> Suggestions of further testing is greatly appreciated.
>>> 
>>> Thanks
>>> Mikael