core: Replace __get_cpu_var with __this_cpu_read if not used for an address.

__get_cpu_var() can be replaced with this_cpu_read and will then use a
single read instruction with implied address calculation to access the
correct per cpu instance.

However, the address of a per cpu variable passed to __this_cpu_read()
cannot be determined (since it's an implied address conversion through
segment prefixes).  Therefore apply this only to uses of __get_cpu_var
where the address of the variable is not used.

Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Hugh Dickins <hughd@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/exit.c

@@ -69,7 +69,7 @@ static void __unhash_process(struct task_struct *p, bool group_dead)
 
 		list_del_rcu(&p->tasks);
 		list_del_init(&p->sibling);
-		__get_cpu_var(process_counts)--;
+		__this_cpu_dec(process_counts);
 	}
 	list_del_rcu(&p->thread_group);
 }