x86-64: Optimize vDSO time()

This function just reads a 64-bit variable that's updated
atomically, so we don't need any locks.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Borislav Petkov <bp@amd64.org>
Link: http://lkml.kernel.org/r/%3C40e2700f8cda4d511e5910be1e633025d28b36c2.1306156808.git.luto%40mit.edu%3E
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/vdso/vclock_gettime.c

@@ -180,12 +180,8 @@ notrace time_t __vdso_time(time_t *t)
 	if (unlikely(!VVAR(vsyscall_gtod_data).sysctl_enabled))
 		return time_syscall(t);
 
-	do {
-		seq = read_seqbegin(&VVAR(vsyscall_gtod_data).lock);
-
-		result = VVAR(vsyscall_gtod_data).wall_time_sec;
-
-	} while (read_seqretry(&VVAR(vsyscall_gtod_data).lock, seq));
+	/* This is atomic on x86_64 so we don't need any locks. */
+	result = ACCESS_ONCE(VVAR(vsyscall_gtod_data).wall_time_sec);
 
 	if (t)
 		*t = result;