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

[Mikael Gerdin]

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?

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