powerpc/vdso: Remove redundant locking in update_vsyscall_tz()

The locking in update_vsyscall_tz() is not only unnecessary because the vdso
code copies the data unproteced in __kernel_gettimeofday() but also
introduces a hard to reproduce race condition between update_vsyscall()
and update_vsyscall_tz(), which causes user space process to loop
forever in vdso code.

The following patch removes the locking from update_vsyscall_tz().

Locking is not only unnecessary because the vdso code copies the data
unprotected in __kernel_gettimeofday() but also erroneous because updating
the tb_update_count is not atomic and introduces a hard to reproduce race
condition between update_vsyscall() and update_vsyscall_tz(), which further
causes user space process to loop forever in vdso code.

The below scenario describes the race condition,
x==0	Boot CPU			other CPU
	proc_P: x==0
	    timer interrupt
		update_vsyscall
x==1		    x++;sync		settimeofday
					    update_vsyscall_tz
x==2						x++;sync
x==3		    sync;x++
						sync;x++
	proc_P: x==3 (loops until x becomes even)

Because the ++ operator would be implemented as three instructions and not
atomic on powerpc.

A similar change was made for x86 in commit 6c260d58
("x86: vdso: Remove bogus locking in update_vsyscall_tz")
Signed-off-by: Shan Hai <shan.hai@windriver.com>
CC: <stable@vger.kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/time.c

@@ -770,13 +770,8 @@ void update_vsyscall_old(struct timespec *wall_time, struct timespec *wtm,
 
 void update_vsyscall_tz(void)
 {
-	/* Make userspace gettimeofday spin until we're done. */
-	++vdso_data->tb_update_count;
-	smp_mb();
 	vdso_data->tz_minuteswest = sys_tz.tz_minuteswest;
 	vdso_data->tz_dsttime = sys_tz.tz_dsttime;
-	smp_mb();
-	++vdso_data->tb_update_count;
 }
 
 static void __init clocksource_init(void)