Request for reviews (M): 7009756: volatile variables could be broken throw reflection API

[Tom Rodriguez]

On Jan 4, 2011, at 3:46 AM, David Holmes wrote:

> Vladimir Kozlov said the following on 01/04/11 14:19:
>> David,
>> What mechanism you are talking about?:
>> "Such a mechanism already exists for regular volatile variable support."
> 
> I mean that we have code in the VM that ensures that load/store of volatile Java variables is atomic. Those Unsafe macros should have utilized similar code, not plain load/store.
> 
> > supports_cx8 and lock are not needed here if you have atomic long move
>> instruction.
> 
> _if_ you have them. That's what supports_cx8 is telling you. (Yes I know it's an abuse but that's the way it is).
> 
>> supports_cx8 is NOT indicating "whether atomic 64-bit operations are supported",
>> it only indicates that HW has 8 bytes atomic CMPXCHG instruction which is not used here.
> 
> Please read the comment at 259-260 that you indicated you would "clean up"
> 
> 259 // Volatile long versions must use locks if !VM_Version::supports_cx8().
> 260 // support_cx8 is a surrogate for 'supports atomic long memory ops'.

That's the point of the fix, supports_cx8 is a bad proxy for whether you have atomic long operations.  32 bit x86 doesn't have an atomic long move but it does have cx8.  If other architectures need to use the ObjectLocker for atomic longs then we need some new feature test to drive that logic.  How are the compiler intrinsics for those routines implemented for those architectures?

tom

> 
>> I think this code was duplicated without thinking from Unsafe_CompareAndSwapLong()
>> when it was fixed long time ago to support HW without cx8.
> 
> I think it stems from there being platforms without the ability to do 64-bit atomic load/stores.
> 
> David
> 
>> Thanks,
>> Vladimir
>> On 1/3/11 6:48 PM, David Holmes wrote:
>>> Vladimir Kozlov said the following on 01/04/11 12:28:
>>>> http://cr.openjdk.java.net/~kvn/7009756/webrev
>>>> 
>>>> Fixed 7009756: volatile variables could be broken throw reflection API
>>>> 
>>>> Unsafe_SetLongVolatile() and Unsafe_GetLongVolatile() are not atomic
>>>> on 32 bit x86 (C++ generates two 32 bit loads/stores).
>>>> 
>>>> Use Atomic::load() and Atomic::store() to access a volatile long.
>>>> 
>>>> Verified with bug's test on all 32 bit platforms. Passed JPRT.
>>> 
>>> Hold up there! The Unsafe code uses supports_cx8 to determine whether atomic 64-bit operations are supported, and if not
>>> it falls back to using explicit locking. This change totally removes that for these methods and so breaks all platforms
>>> where cx8 is not supported.
>>> 
>>> If anything is wrong it is these macros:
>>> 
>>> 155 #define GET_FIELD_VOLATILE(obj, offset, type_name, v) \
>>> 156 oop p = JNIHandles::resolve(obj); \
>>> 157 volatile type_name v = *(volatile type_name*)index_oop_from_field_offset_long(p, offset)
>>> 158
>>> 159 #define SET_FIELD_VOLATILE(obj, offset, type_name, x) \
>>> 160 oop p = JNIHandles::resolve(obj); \
>>> 161 *(volatile type_name*)index_oop_from_field_offset_long(p, offset) = x; \
>>> 162 OrderAccess::fence();
>>> 163
>>> 
>>> because we should be using a load/store mechanism that is aware of the volatile aspect and so enforces the 64-bit
>>> atomicity requirement at a lower level. Such a mechanism already exists for regular volatile variable support.
>>> 
>>> David