How is Code in do_sys_settimeofday() safe in case of SMP and Nest Kernel Path?

Hi, all,

I got a question, that is, I am confused by the following code in
do_sys_settimeofday().

if (tz) {
   /* SMP safe, global irq locking makes it work. */
       sys_tz = *tz;
       if (firsttime) {
           firsttime = 0;
           if (!tv)
               warp_clock();
   }
}


For my understanding, an assignment between structs should be a
bit-wise copy. Such operation is not atomic, so it can not be supposed
SMP-safe. And the subsequent test-and-assign operation on firsttime is
not atomic, either.

If the comments mean the subsequent code is SMP-safe and can prevent
nest-kernel-path, how does it achieves that?

Thank you in advance.
Feng,Dong
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