ACCESS_ONCE usage inside llist_add_batch function

[john@jjdev.com (John de la Garza)]

On Sat, Feb 28, 2015 at 10:12:23PM +0200, Cihangir Akturk wrote:
> Reading the lib/llist.c file in the kernel sources, I came across
> the llist_add_bach function defined like this;
> 
> bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
> 		     struct llist_head *head)
> {
> 	struct llist_node *first;
> 
> 	do {
> 		new_last->next = first = ACCESS_ONCE(head->first);
> 	} while (cmpxchg(&head->first, first, new_first) != first);
> 
> 	return !first;
> }
> 
> One thing bugging my mind is the ACCESS_ONCE macro. Is it really
> needed here ? I mean I would write this function with ACCES_ONCE
> moved outside the loop like as follows;
> 
Do you know what ACCESS_ONCE does?

I found this comment:

"The compiler
 * is also forbidden from reordering successive instances of ACCESS_ONCE()"

from: include/linux/compiler.h


This might be something that does need to be called (to tell the compiler
something) on every loop iteration.