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

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Message-ID: <20240806104500.GA20881@redhat.com>
Date: Tue, 6 Aug 2024 12:45:00 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.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 08/05, Andrii Nakryiko wrote:
>
> 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,

OK, lets keep it, it doesn't hurt. Just it wasn't clear to me why did
you add this initialization in this patch.

> > > @@ -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>

OK, I won't insist. To me the most confusing part is

	u = try_get_uprobe(u);
	if (!u)
	...

If you read this code for the 1st time (or you are trying to recall it
after 10 years ;) it looks as if try_get_uprobe() can return another uprobe.

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

If you disagree, then don't change it ;)

Oleg.