Re: [RFC PATCH 03/21] list: Annotate lockless list primitives with data_race()

[Jann Horn]

On Tue, Mar 24, 2020 at 4:37 PM Will Deacon <will@xxxxxxxxxx> wrote:
> Some list predicates can be used locklessly even with the non-RCU list
> implementations, since they effectively boil down to a test against
> NULL. For example, checking whether or not a list is empty is safe even
> in the presence of a concurrent, tearing write to the list head pointer.
> Similarly, checking whether or not an hlist node has been hashed is safe
> as well.
>
> Annotate these lockless list predicates with data_race() and READ_ONCE()
> so that KCSAN and the compiler are aware of what's going on. The writer
> side can then avoid having to use WRITE_ONCE() in the non-RCU
> implementation.
[...]
>  static inline int list_empty(const struct list_head *head)
>  {
> -       return READ_ONCE(head->next) == head;
> +       return data_race(READ_ONCE(head->next) == head);
>  }
[...]
>  static inline int hlist_unhashed(const struct hlist_node *h)
>  {
> -       return !READ_ONCE(h->pprev);
> +       return data_race(!READ_ONCE(h->pprev));
>  }

This is probably valid in practice for hlist_unhashed(), which
compares with NULL, as long as the most significant byte of all kernel
pointers is non-zero; but I think list_empty() could realistically
return false positives in the presence of a concurrent tearing store?
This could break the following code pattern:

/* optimistic lockless check */
if (!list_empty(&some_list)) {
  /* slowpath */
  mutex_lock(&some_mutex);
  list_for_each(tmp, &some_list) {
    ...
  }
  mutex_unlock(&some_mutex);
}

(I'm not sure whether patterns like this appear commonly though.)