Re: [PATCH 1/3] uprobes: allow put_uprobe() from non-sleepable softirq context

Andrii Nakryiko
2024-09-17T08:20:05Z

On Sun, Sep 15, 2024 at 4:49 PM Oleg Nesterov <oleg@redhat.com> wrote:
>
> On 09/09, Andrii Nakryiko wrote:
> >
> > Currently put_uprobe() might trigger mutex_lock()/mutex_unlock(), which
> > makes it unsuitable to be called from more restricted context like softirq.
> >
> > Let's make put_uprobe() agnostic to the context in which it is called,
> > and use work queue to defer the mutex-protected clean up steps.
>
> ...
>
> > +static void uprobe_free_deferred(struct work_struct *work)
> > +{
> > + struct uprobe *uprobe = container_of(work, struct uprobe, work);
> > +
> > + /*
> > + * If application munmap(exec_vma) before uprobe_unregister()
> > + * gets called, we don't get a chance to remove uprobe from
> > + * delayed_uprobe_list from remove_breakpoint(). Do it here.
> > + */
> > + mutex_lock(&delayed_uprobe_lock);
> > + delayed_uprobe_remove(uprobe, NULL);
> > + mutex_unlock(&delayed_uprobe_lock);
> > +
> > + kfree(uprobe);
> > +}
> > +
> > static void uprobe_free_rcu(struct rcu_head *rcu)
> > {
> > struct uprobe *uprobe = container_of(rcu, struct uprobe, rcu);
> >
> > - kfree(uprobe);
> > + INIT_WORK(&uprobe->work, uprobe_free_deferred);
> > + schedule_work(&uprobe->work);
> > }
>
> This is still wrong afaics...
>
> If put_uprobe() can be called from softirq (after the next patch), then
> put_uprobe() and all other users of uprobes_treelock should use
> write_lock_bh/read_lock_bh to avoid the deadlock.

Ok, I see the problem, that's unfortunate.

I see three ways to handle that:

1) keep put_uprobe() as is, and instead do schedule_work() from the
timer thread to postpone put_uprobe(). (but I'm not a big fan of this)
2) move uprobes_treelock part of put_uprobe() into rcu callback, I
think it has no bearing on correctness, uprobe_is_active() is there
already to handle races between putting uprobe and removing it from
uprobes_tree (I prefer this one over #1 )
3) you might like this the most ;) I think I can simplify
hprobes_expire() from patch #2 to not need put_uprobe() at all, if I
protect uprobe lifetime with non-sleepable
rcu_read_lock()/rcu_read_unlock() and perform try_get_uprobe() as the
very last step after cmpxchg() succeeded.

I'm leaning towards #3, but #2 seems fine to me as well.

>
> To be honest... I simply can't force myself to even try to read 2/3 ;) I'll
> try to do this later, but I am sure I will never like it, sorry.

This might sound rude, but the goal here is not to make you like it :)
The goal is to improve performance with minimal complexity. And I'm
very open to any alternative proposals as to how to make uretprobes
RCU-protected to avoid refcounting in the hot path.

I think #3 proposal above will make it a bit more palatable (but there
is still locklessness, cmpxchg, etc, I see no way around that,
unfortunately).

>
> Oleg.
>