linux-kernel - Re: [PATCHv2 bpf-next 1/9] uprobe: Add support for session consumer

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Date: Tue, 2 Jul 2024 13:52:38 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Oleg Nesterov <oleg@...hat.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org, Martin KaFai Lau <kafai@...com>, 
	Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>, 
	John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...omium.org>, 
	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCHv2 bpf-next 1/9] uprobe: Add support for session consumer

On Tue, Jul 2, 2024 at 9:11 AM Jiri Olsa <olsajiri@...il.com> wrote:
>
> On Tue, Jul 02, 2024 at 03:04:08PM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 01, 2024 at 06:41:07PM +0200, Jiri Olsa wrote:
> >
> > > +static void
> > > +uprobe_consumer_account(struct uprobe *uprobe, struct uprobe_consumer *uc)
> > > +{
> > > +   static unsigned int session_id;
> > > +
> > > +   if (uc->session) {
> > > +           uprobe->sessions_cnt++;
> > > +           uc->session_id = ++session_id ?: ++session_id;
> > > +   }
> > > +}
> >
> > The way I understand this code, you create a consumer every time you do
> > uprobe_register() and unregister makes it go away.
> >
> > Now, register one, then 4g-1 times register+unregister, then register
> > again.
> >
> > The above seems to then result in two consumers with the same
> > session_id, which leads to trouble.
> >
> > Hmm?
>
> ugh true.. will make it u64 :)
>
> I think we could store uprobe_consumer pointer+ref in session_consumer,
> and that would make the unregister path more interesting.. will check

More interesting how? It's actually a great idea, uprobe_consumer
pointer itself is a unique ID and 64-bit. We can still use lowest bit
for RC (see my other reply).

>
> thanks,
> jirka