On Thu, Jun 13, 2024 at 10:26:11AM +0530, Nilay Shroff wrote:
On 6/12/24 21:21, Keith Busch wrote:
+static inline void list_cut(struct list_head *list,
struct list_head *head, struct list_head *entry)+{
- list->next = entry;
- list->prev = head->prev;
- head->prev = entry->prev;
- entry->prev->next = head;
- entry->prev = list;
- list->prev->next = list;
+}
I am wondering whether we really need the _rcu version of list_cut here? I think that @head could point to an _rcu protected list and that's true for this patch. So there might be concurrent readers accessing @head using _rcu list-traversal primitives, such as list_for_each_entry_rcu().
An _rcu version of list_cut():
static inline void list_cut_rcu(struct list_head *list, struct list_head *head, struct list_head *entry) { list->next = entry; list->prev = head->prev; head->prev = entry->prev; rcu_assign_pointer(list_next_rcu(entry->prev), head); entry->prev = list; list->prev->next = list; }
I was initially thinking similiar, but this is really just doing a "list_del", and the rcu version calls the same generic __list_del() helper. To make this more clear, we could change
head->prev = entry->prev; entry->prev->next = head;
To just this:
__list_del(entry->prev, head);
And that also gets the "WRITE_ONCE" usage right.
But that's not the problem for the rcu case. It's the last line that's the problem:
list->prev->next = list;
We can't change forward pointers for any element being detached from @head because a reader iterating the list may see that new pointer value and end up in the wrong list, breaking iteration. A synchronize rcu needs to happen before forward pointers can be mucked with, so it still needs to be done in two steps. Oh bother...