x86: Fix vtime/file timestamp inconsistencies

Due to vtime calling vgettimeofday(), its possible that an application
could call  time();create("stuff",O_RDRW);  only to see the file's
creation timestamp to be before the value returned by time.

A similar way to reproduce the issue is to compare the vsyscall time()
with the syscall time(), and observe ordering issues.

The modified test case from Oleg Nesterov below can illustrate this:

int main(void)
{
	time_t sec1,sec2;
	do {
		sec1 = time(&sec2);
		sec2 = syscall(__NR_time, NULL);
	} while (sec1 <= sec2);

	printf("vtime: %d.000000\n", sec1);
	printf("time: %d.000000\n", sec2);
	return 0;
}

The proper fix is to make vtime use the same time value as
current_kernel_time() (which is exported via update_vsyscall) instead of
vgettime().

Thanks to Jiri Olsa for bringing up the issue and catching bugs in
earlier verisons of this fix.

Signed-off-by: John Stultz <johnstul@us.ibm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
LKML-Reference: <1279068988-21864-2-git-send-email-johnstul@us.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/vsyscall_64.c

@@ -169,13 +169,18 @@ int __vsyscall(0) vgettimeofday(struct timeval * tv, struct timezone * tz)
  * unlikely */
 time_t __vsyscall(1) vtime(time_t *t)
 {
-	struct timeval tv;
+	unsigned seq;
 	time_t result;
 	if (unlikely(!__vsyscall_gtod_data.sysctl_enabled))
 		return time_syscall(t);
 
-	vgettimeofday(&tv, NULL);
-	result = tv.tv_sec;
+	do {
+		seq = read_seqbegin(&__vsyscall_gtod_data.lock);
+
+		result = __vsyscall_gtod_data.wall_time_sec;
+
+	} while (read_seqretry(&__vsyscall_gtod_data.lock, seq));
+
 	if (t)
 		*t = result;
 	return result;