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.
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