Hi all,
My dependency checker is flagging yet another broken dependency. For
context, see [1].
Thankfully, it is fairly straight-forward to explain this time.
> stable_node = page_stable_node(page);
Line 2032 in mm/ksm.c::cmp_and_merge_page() sees the return value of a
call to "page_stable_node()", which can depend on a "READ_ONCE()", being
assigned to "stable_node".
> if (stable_node) {
> if (stable_node->head != &migrate_nodes &&
> get_kpfn_nid(READ_ONCE(stable_node->kpfn)) !=
> NUMA(stable_node->nid)) {
> stable_node_dup_del(stable_node); ‣dup: stable_node
> stable_node->head = &migrate_nodes;
> list_add(&stable_node->list, stable_node->head);
The dependency chain then runs into the two following if's, through an
assignment of "migrate_nodes" to "stable_node->head" (line 2038) and
finally reaches a call to "list_add()" (line 2039) where
"stable_node->head" gets passed as the second function argument.
> }
> }
>
> static inline void list_add(struct list_head *new, struct list_head *head)
> {
> __list_add(new, head, head->next);
> }
>
> static inline void __list_add(struct list_head *new,
> struct list_head *prev,
> struct list_head *next)
> {
> if (!__list_add_valid(new, prev, next))
> return;
>
> next->prev = new;
> new->next = next;
> new->prev = prev;
> WRITE_ONCE(prev->next, new);
> }
By being passed into "list_add()" via "stable_node->head", the
dependency chain eventually reaches a "WRITE_ONCE()" in "__list_add()"
whose destination address, "stable_node->head->next", is part of the
dependency chain and therefore carries an address dependency.
However, as a result of the assignment in line 2038, Clang knows that
"stable_node->head" is "migrate_nodes" and replaces it, thereby breaking
the dependency chain.
What do you think?
Many thanks,
Paul
--
[1]: https://lore.kernel.org/all/Yk7%2FT8BJITwz+Og1@Pauls-MacBook-Pro.local/
