linux-kernel - Re: [PATCH v4 4/8] uprobes: travers uprobe's consumer list locklessly under SRCU protection

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Message-ID: <20240903182523.GH17936@redhat.com>
Date: Tue, 3 Sep 2024 20:25:23 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Jiri Olsa <olsajiri@...il.com>, 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, paulmck@...nel.org,
	willy@...radead.org, surenb@...gle.com, akpm@...ux-foundation.org,
	linux-mm@...ck.org
Subject: Re: [PATCH v4 4/8] uprobes: travers uprobe's consumer list
 locklessly under SRCU protection

On 09/03, Andrii Nakryiko wrote:
>
> On Sat, Aug 31, 2024 at 9:19 AM Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > I was thinking about another seq counter incremented in register(), so
> > that handler_chain() can detect the race with uprobe_register() and skip
> > unapply_uprobe() in this case. This is what Peter did in one of his series.
> > Still changes the current behaviour, but not too much.
>
> We could do that, but then worst case, when we do detect registration
> race, what do we do?

Do nothing and skip unapply_uprobe().

> But as you said, this all can/should be addressed as a follow up
> discussion.

Yes, yes,

> You mentioned some clean ups you wanted to do, let's
> discuss all that as part of that?

Yes, sure.

And please note that in reply to myself I also mentioned that I am stupid
and these cleanups can't help to change/improve this behaviour ;)

> > The only in-kernel user of UPROBE_HANDLER_REMOVE is perf, and it is fine.
> >
>
> Well, BPF program can accidentally trigger this as well, but that's a
> bug, we should fix it ASAP in the bpf tree.

not sure, but...

> > And in general, this change makes the API less "flexible".
>
> it maybe makes a weird and too-flexible case a bit more work to
> implement. Because if consumer want to be that flexible, they can
> still define filter that will be coordinated between filter() and
> handler() implementation.

perhaps, but lets discuss this later, on top of your series.

> > But once again, I agree that it would be better to apply your series first,
> > then add the fixes in (unlikely) case it breaks something.
>
> Yep, agreed, thanks! Will send a new version ASAP, so we have a common
> base to work on top of.

Thanks. Hopefully Peter will queue your V5 soon.

Oleg.