irq_work: Use per cpu atomics instead of regular atomics

The irq work queue is a per cpu object and it is sufficient for
synchronization if per cpu atomics are used. Doing so simplifies
the code and reduces the overhead of the code.

Before:

christoph@linux-2.6$ size kernel/irq_work.o
   text	   data	    bss	    dec	    hex	filename
    451	      8	      1	    460	    1cc	kernel/irq_work.o

After:

christoph@linux-2.6$ size kernel/irq_work.o 
   text	   data	    bss	    dec	    hex	filename
    438	      8	      1	    447	    1bf	kernel/irq_work.o

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Christoph Lameter <cl@linux.com>

kernel/irq_work.c

@@ -77,21 +77,21 @@ void __weak arch_irq_work_raise(void)
  */
 static void __irq_work_queue(struct irq_work *entry)
 {
-	struct irq_work **head, *next;
+	struct irq_work *next;
 
-	head = &get_cpu_var(irq_work_list);
+	preempt_disable();
 
 	do {
-		next = *head;
+		next = __this_cpu_read(irq_work_list);
 		/* Can assign non-atomic because we keep the flags set. */
 		entry->next = next_flags(next, IRQ_WORK_FLAGS);
-	} while (cmpxchg(head, next, entry) != next);
+	} while (this_cpu_cmpxchg(irq_work_list, next, entry) != next);
 
 	/* The list was empty, raise self-interrupt to start processing. */
 	if (!irq_work_next(entry))
 		arch_irq_work_raise();
 
-	put_cpu_var(irq_work_list);
+	preempt_enable();
 }
 
 /*
@@ -120,16 +120,16 @@ EXPORT_SYMBOL_GPL(irq_work_queue);
  */
 void irq_work_run(void)
 {
-	struct irq_work *list, **head;
+	struct irq_work *list;
 
-	head = &__get_cpu_var(irq_work_list);
-	if (*head == NULL)
+	if (this_cpu_read(irq_work_list) == NULL)
 		return;
 
 	BUG_ON(!in_irq());
 	BUG_ON(!irqs_disabled());
 
-	list = xchg(head, NULL);
+	list = this_cpu_xchg(irq_work_list, NULL);
+
 	while (list != NULL) {
 		struct irq_work *entry = list;