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

[Mikael Gerdin]

On 2014-11-05 15:34, Bertrand Delsart wrote:
> Looks good.

Thanks for the review, Bertrand.

/Mikael

>
> Bertrand (not a Reviewer).
>
> On 05/11/2014 14:59, Mikael Gerdin wrote:
>> Hi all,
>>
>> I have an updated webrev with the following changes:
>>
>> * compiler_barrier() calls added after the loads to all
>> load_acquire-variants.
>> * the dummy store in release() is removed.
>>
>> Full webrev at:
>> http://cr.openjdk.java.net/~mgerdin/8061964/webrev.1/
>>
>> Incremental webrev at:
>> http://cr.openjdk.java.net/~mgerdin/8061964/webrev.0_to_1/
>>
>> Thanks for all the feedback so far!
>>
>> /Mikael
>>
>> On 2014-11-03 14:01, 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
>
>