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SubjectRe: Litmus test for question from Al Viro
On Thu, Oct 01, 2020 at 02:39:25PM -0400, Alan Stern wrote:

> The problem with a plain write is that it isn't guaranteed to be atomic
> in any sense. In principle, the compiler could generate code for CPU1
> which would write 0 to V->A more than once.
>
> Although I strongly doubt that any real compiler would actually do this,
> the memory model does allow for it, out of an overabundance of caution.

Point... OK, not a problem - actually there will be WRITE_ONCE() for other
reasons; the real-life (pseudo-)code is
spin_lock(&file->f_lock);
to_free = NULL;
head = file->f_ep;
if (head->first == &epitem->fllink && epitem->fllink.next == NULL) {
/* the set will go empty */
file->f_ep = NULL;
if (!is_file_epoll(file)) {
/*
* not embedded into struct eventpoll; we want it
* freed unless it's on the check list, in which
* case we leave it for reverse path check to free.
*/
v = container_of(head, struct ep_head, epitems);
if (!smp_load_acquire(&v->next))
to_free = v;
}
}
hlist_del_rcu(&epitem->fllink);
spin_unlock(file->f_lock);
kfree(to_free);
and hlist_del_rcu() will use WRITE_ONCE() to store the updated forward links.

That goes into ep_remove() and CPU1 side of that thing is the final (set-emptying)
call. CPU2 side is the list traversal step in reverse_path_check() and
in clear_tfile_check_list():
// under rcu_read_lock()
to_free = head;
epitem = rcu_dereference(hlist_first_rcu(&head->epitems));
if (epitem) {
spin_lock(&epitem->file->f_lock);
if (!hlist_empty(&head->epitems))
to_free = NULL;
head->next = NULL;
spin_unlock(&epitem->file->f_lock);
}
kfree(to_free);

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