Re: [PATCH 2/8] uprobes: revamp uprobe refcounting and lifetime management

Andrii Nakryiko
2024-08-05T17:29:20Z

On Mon, Aug 5, 2024 at 6:44 AM Oleg Nesterov <oleg@redhat.com> wrote:
>
> On 07/31, Andrii Nakryiko wrote:
> >
> > @@ -732,11 +776,13 @@ static struct uprobe *alloc_uprobe(struct inode *inode, loff_t offset,
> > uprobe->ref_ctr_offset = ref_ctr_offset;
> > init_rwsem(&uprobe->register_rwsem);
> > init_rwsem(&uprobe->consumer_rwsem);
> > + RB_CLEAR_NODE(&uprobe->rb_node);
>
> I guess RB_CLEAR_NODE() is not necessary?

I definitely needed that with my batch API changes, but it might be
that I don't need it anymore. But I'm a bit hesitant to remove it,
because if we ever get put_uprobe() on an uprobe that hasn't been
inserted into RB-tree yet, this will cause a hard to understand crash.
RB_CLEAR_NODE() in __insert_uprobe() is critical to have, this one is
kind of optional (but still feels right to initialize the field
properly).

Let me know if you feel strongly about this, though.

>
> > @@ -1286,15 +1296,19 @@ static void build_probe_list(struct inode *inode,
> > u = rb_entry(t, struct uprobe, rb_node);
> > if (u->inode != inode || u->offset < min)
> > break;
> > + u = try_get_uprobe(u);
> > + if (!u) /* uprobe already went away, safe to ignore */
> > + continue;
> > list_add(&u->pending_list, head);
>
> cosmetic nit, feel to ignore, but to me
>
> if (try_get_uprobe(u))
> list_add(&u->pending_list, head);
>
> looks more readable.

It's not my code base to enforce my preferences, but I'll at least
explain why I disagree. To me, something like `if (some condition)
<break/continue>;` is a very clear indication that this item (or even
the rest of items in case of break) won't be processed anymore.

While

if (some inverted condition)
<do some something useful>
<might be some more code>

... is a pattern that requires double-checking that we really are not
going to use that item later on.

So I'll invert this just to not be PITA, but I disagree :)

>
> Other than the lack of kfree() in put_uprobe() and WARN() in _unregister()
> the patch looks good to me.

yep, fixed that locally already. Thanks for the review!

>
> Oleg.
>