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

From: Dong Feng
Date: Fri Sep 29 2006 - 10:34:17 EST

Hi, all,

I got a question, that is, I am confused by the following code in

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

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.
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