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SubjectRe: [PATCH] time/fs - file's time race with vgettimeofday
On 07/02, Oleg Nesterov wrote:
>
> But, get_seconds() is also used by sys_time(), and we should have the
> same problem with another trivial test-case:
>
> #include <stdio.h>
> #include <sys/time.h>
> #include <time.h>
>
> int main(void)
> {
> struct timeval tv;
> int sec;
>
> do {
> gettimeofday(&tv, NULL);
> sec = time(NULL);
> } while (tv.tv_sec <= sec);
>
> printf("gtod: %ld.%06ld\n", tv.tv_sec, tv.tv_usec);
> printf("time: %d.000000\n", sec);
> return 0;
> }
>
> However, this test-case can't trigger the problem. Confused.

Aha. libc's time() doesn't use sys_time(), it uses __vsyscall(1)
vtime()->do_vgettimeofday().

This one quickly triggers the "time goes backward" case.

#include <stdio.h>
#include <sys/time.h>
#include <time.h>
#include <unistd.h>
#include <sys/syscall.h>

int main(void)
{
struct timeval tv;
int sec;
do {
gettimeofday(&tv, NULL);
sec = syscall(__NR_time, NULL);
} while (tv.tv_sec <= sec);
printf("gtod: %ld.%06ld\n", tv.tv_sec, tv.tv_usec);
printf("time: %d.000000\n", sec);
return 0;
}
Not that I think this "problem" should be fixed, just curious.

Oleg.



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