GCC built-in atomic operations and memory barriers

[Toby Douglass]

Gilles Chanteperdrix wrote:
> Russell King - ARM Linux wrote:
>> The kernel API is the syscall interface and associated data structures.
>> It does not include everything you can find in kernel headers.
> 
> Are the functions found in the vectors page part of the kernel API?
> Because if that is so, then they provide a way to implement cmpxchg and
> barriers in user-space.

GCC (I had overlooked this) has a function, __sync_synchronize(), which 
provides a full memory barrier.  I do not know how it is implemented on 
ARM, but the CAS I wrote certainly would not work (and did not work) 
without a memory barrier and with this function inserted, does work.

The GCC atomic CAS for ARM appears to call the OS's compare-exchange 
function, which certainly is broken, by lacking memory barriers, on the 
Linux I am developing on.

Either way, I now have my own platform independent ARM __asm__ for LL/CS 
CAS, with memory barriers, so I'm happy.  Also, I got my OpenGL wrapper 
DLL working last night :-)