Re: [PATCH v3 04/13] uprobes: travers uprobe's consumer list locklessly under SRCU protection

Andrii Nakryiko
2024-08-22T16:59:41Z

On Thu, Aug 22, 2024 at 7:22 AM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Mon, Aug 12, 2024 at 09:29:08PM -0700, Andrii Nakryiko wrote:
>
> SNIP
>
> > @@ -1125,18 +1103,31 @@ void uprobe_unregister(struct uprobe *uprobe, struct uprobe_consumer *uc)
> > int err;
> >
> > down_write(&uprobe->register_rwsem);
> > - if (WARN_ON(!consumer_del(uprobe, uc))) {
> > - err = -ENOENT;
> > - } else {
> > - err = register_for_each_vma(uprobe, NULL);
> > - /* TODO : cant unregister? schedule a worker thread */
> > - if (unlikely(err))
> > - uprobe_warn(current, "unregister, leaking uprobe");
> > - }
> > +
> > + list_del_rcu(&uc->cons_node);
>
> hi,
> I'm using this patchset as base for my changes and stumbled on this today,
> I'm probably missing something, but should we keep the 'uprobe->consumer_rwsem'
> lock around the list_del_rcu?
>

Note that original code also didn't take consumer_rwsem, but rather
kept register_rwsem (which we still use).

There is a bit of mix of using register_rwsem and consumer_rwsem for
working with consumer list. Code hints at this as being undesirable
and "temporary", but you know, it's not broken :)

Anyways, my point is that we didn't change the behavior, this should
be fine. That _rcu() in list_del_rcu() is not about lockless
modification of the list, but rather modification in such a way as to
keep lockless RCU-protected *readers* correct. It just does some more
memory barrier/release operations more carefully.

> jirka
>
>
> > + err = register_for_each_vma(uprobe, NULL);
> > +
> > up_write(&uprobe->register_rwsem);
> >
> > - if (!err)
> > - put_uprobe(uprobe);
> > + /* TODO : cant unregister? schedule a worker thread */
> > + if (unlikely(err)) {
> > + uprobe_warn(current, "unregister, leaking uprobe");
> > + goto out_sync;
> > + }
> > +
> > + put_uprobe(uprobe);
> > +