atomic operations
int atomic_xchg ( volatile __global int *p, int val)
Swaps the old value stored at location p with new value given by val. Returns old value.
from: https://www.khronos.org/registry/OpenCL/sdk/1.1/docs/man/xhtml/atomic_xchg.html
原文：https://www.cnblogs.com/aspirs/p/15024086.html