xiaoxiang781216 commented on a change in pull request #4270:
URL: https://github.com/apache/incubator-nuttx/pull/4270#discussion_r682611007
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File path: libs/libc/time/lib_localtime.c
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@@ -2528,9 +2537,20 @@ void tzset(void)
{
FAR const char *name;
- tz_semtake(&g_lcl_sem);
-
name = getenv("TZ");
+ if (g_lcl_isset > 0 && name && strcmp(g_lcl_tzname, name) == 0)
+ {
+ return;
+ }
Review comment:
> Thanks for providing this example scenario. I am still having a hard
time to understand how the first check copes with this concurrency between
threads.
> Let's assume threads A and B.
> Thread A fails the check (which means timezone info was still not
initialized). But, the OS interrupts thread A before it locks the semaphore.
> Thread B then calls `tzset` and locks the semaphore, completes the
initialization of timezone info and unlock the semaphore.
> Thread A later resumes execution, finally locks the semaphore and, as
expected, finds that timezone is already initialized.
> The same outcome would happen if the first check did not exist, wouldn't
it?
>
> Sorry if there is something too obvious I am not realizing, but I'd like
to be convinced that the first check is not useless.
Yes, but the problem is that you can't take the lock and then do the check
in the interrupt.
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