memory barrier and atomic_t on linux

Question

Recently, I am reading some Linux kernel space codes, I see this

uint64_t used;
uint64_t blocked;

used = atomic64_read(&g_variable->used);       //#1         


        

Answer 1

x86 CPUs don’t do read-after-read reordering, so it is sufficient to prevent the compiler from doing any reordering. On other platforms such as PowerPC, things will look a lot different.