Re: RFR (XS) 8047212: fix race between ObjectMonitor alloc and verification code

[Daniel D. Daugherty]

On 10/20/15, 9:18 PM, David Holmes wrote:

<trimming>

On 21/10/2015 12:44 PM, Daniel D. Daugherty wrote:

On 10/20/15, 8:15 PM, David Holmes wrote:

On 21/10/2015 12:51 AM, Daniel D. Daugherty wrote:

On 10/20/15, 1:53 AM, David Holmes wrote:

src/share/vm/runtime/vmStructs.cpp

Can you not just define volatile_static_field?

Yes, I went that way originally and then I changed my mind to
avoid colliding with the other format. See below.

Why does the ptr aspect need to come into it? Also "static pointer

volatile field" sounds really odd, it is a static, volatile field that

happens to be a pointer-type.

It's meant to be odd because it follows the actual decl:

     static ObjectMonitor * volatile gBlockList;

So "static pointer volatile field" is exactly what I have:

     static ObjectMonitor * volatile gBlockList;

  => (static ObjectMonitor *) volatile gBlockList;

i.e. I have a static ObjectMonitor pointer that is volatile


Compared to these two forms:

     static volatile ObjectMonitor * gBlockList;
     static ObjectMonitor volatile * gBlockList;

  => static (volatile ObjectMonitor) * gBlockList;
  => static (ObjectMonitor volatile) * gBlockList;

i.e. I have a static pointer to a volatile ObjectMonitor.

Hopefully, this makes my reasons a bit more clear...

Not really :) Yes there is a difference between a "volatile pointer to
Foo" and "pointer to a volatile Foo", but for the sake of vmstructs we
don't really seem to need to care about that. The two questions are:
- is the field/variable static
- is the field/variable volatile

I'll have to politely disagree:

Here's the existing volatile non-static macro:

2743 // This macro checks the type of a volatile VMStructEntry by
comparing pointer types
2744 #define CHECK_VOLATILE_NONSTATIC_VM_STRUCT_ENTRY(typeName,
fieldName, type)        \
2745  {typedef type dummyvtype; typeName *dummyObj = NULL; volatile
dummyvtype* dummy = &dummyObj->fieldName; }

And here's the new static pointer volatile macro:

2751 // This macro checks the type of a static pointer volatile
VMStructEntry by comparing pointer types,
2752 // e.g.: "static ObjectMonitor * volatile gBlockList;"
2753 #define CHECK_STATIC_PTR_VOLATILE_VM_STRUCT_ENTRY(typeName,
fieldName, type)       \
2754  {type volatile * dummy = &typeName::fieldName; }

Yes, the variable assignments are different because we have static
versus a non-static situation, but what's more important is where
the "volatile" is positioned.

I see your point. But I think the real problem is that there is a bug in the declaration of CHECK_VOLATILE_NONSTATIC_VM_STRUCT_ENTRY that makes it wrong when used with a pointer type. I think this:

2745 {typedef type dummyvtype; typeName *dummyObj = NULL; volatile dummyvtype* dummy = &dummyObj->fieldName; }

should really be:

2745 {typedef type dummyvtype; typeName *dummyObj = NULL; dummyvtype volatile * dummy = &dummyObj->fieldName; }

Actually, I believe these two are equivalent:

    volatile dummyvtype* dummy =
    dummyvtype volatile * dummy =

based on my reading of the URL that I put in the original webrev...

So it's not a bug, it's one variation of an acceptable style.

and the static version would follow the same form. dummy is a pointer to a volatile field of type dummyvtype. (I'm unclear why the dummyObj variable is introduced though ??).

'dummyObj' is used to access the field: &dummyObj->fieldName

I wonder if Kim wants to wade in on this one :)

Dunno.

Dan

Cheers,
David
-----

In the existing volatile non-static macro, the volatile piece is:

     volatile dummyvtype* dummy = &dummyObj->fieldName;

and in the new static pointer volatile macro, the volatile piece is:

     type volatile * dummy = &typeName::fieldName;

So the CHECK_VOLATILE_NONSTATIC_XXX macro has the "volatile" before
the data type... and the CHECK_STATIC_PTR_VOLATILE_XXX macro
has the "volatile" after the data type...

Dan

Cheers,
David

Dan

Thanks,
David

Testing: Aurora Adhoc RT-SVC nightly batch
          4 inner-complex fastdebug parallel runs for 4+ days and
            600K iterations without seeing this failure; the
experiment

is still running; final results to be reported at the end

            of the review cycle
          JPRT -testset hotspot

This fix:

- makes ObjectMonitor::gBlockList volatile
- uses "OrderAccess::release_store_ptr(&gBlockList, temp)" to
   make sure the new block updates _happen before_ gBlockList is
   changed to refer to the new block
- add SA support for a "static pointer volatile" field like:

     static ObjectMonitor * volatile gBlockList;

See the following link for a nice description of what "volatile"
means in the different positions on a variable/parameter decl line:

http://www.embedded.com/electronics-blogs/beginner-s-corner/4023801/Introduction-to-the-Volatile-Keyword

Thanks, in advance, for any comments, questions or suggestions.

Dan