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

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Message-ID: <CAEf4BzYvkAYL4pPcA7ayiR_VT=g4Y1SMZy4MNX3QEV3H=PjYvw@mail.gmail.com>
Date: Mon, 5 Aug 2024 10:29:07 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Andrii Nakryiko <andrii@...nel.org>, linux-trace-kernel@...r.kernel.org, 
	peterz@...radead.org, rostedt@...dmis.org, mhiramat@...nel.org, 
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org, jolsa@...nel.org, 
	paulmck@...nel.org
Subject: Re: [PATCH 2/8] uprobes: revamp uprobe refcounting and lifetime management

On Mon, Aug 5, 2024 at 6:44 AM Oleg Nesterov <oleg@...hat.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.
>