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>

include/asm-generic/irq_regs.h

@@ -22,15 +22,15 @@ DECLARE_PER_CPU(struct pt_regs *, __irq_regs);
 
 static inline struct pt_regs *get_irq_regs(void)
 {
-	return __get_cpu_var(__irq_regs);
+	return __this_cpu_read(__irq_regs);
 }
 
 static inline struct pt_regs *set_irq_regs(struct pt_regs *new_regs)
 {
-	struct pt_regs *old_regs, **pp_regs = &__get_cpu_var(__irq_regs);
+	struct pt_regs *old_regs;
 
-	old_regs = *pp_regs;
-	*pp_regs = new_regs;
+	old_regs = __this_cpu_read(__irq_regs);
+	__this_cpu_write(__irq_regs, new_regs);
 	return old_regs;
 }