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

[David Holmes]

Further, note that the specification for Unsafe does not make it clear 
that these methods are supposed to implement the atomicity properties of 
Java volatiles, as opposed to simply adhering to the memory model 
properties.

David

David Holmes said the following on 01/04/11 12:48:
> 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