Race Condition over sys_tz

["Dong Feng" <middle.fengdong@gmail.com>]

The operations on sys_tz, so far known to me in sys_settimeofday and
sys_gettimeofday, is neither atomic nor protected by any lock. I
suspect it probably causes unpredictable behavior when multiple
processes try to set the system time zone simultaneously.

Following is the code fragment extracted from do_sys_settimeofday().
The function is invoked by sys_settimeofday() without locking. At
least two non-atomic operations:

1. struct copy between *tz and sys_tz.
2. The test-and-operate over firsttime.

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