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

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Message-ID: <20240831161914.GA9683@redhat.com>
Date: Sat, 31 Aug 2024 18:19:15 +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 08/30, Andrii Nakryiko wrote:
>
> On Fri, Aug 30, 2024 at 1:21 PM Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > I'll probably write another email (too late for me today), but I agree
> > that "avoid register_rwsem in handler_chain" is obviously a good goal,
> > lets discuss the possible cleanups or even fixlets later, when this
> > series is already applied.
> >
>
> Sounds good. It seems like I'll need another revision due to missing
> include, so if there is any reasonably straightforward clean up we
> should do, I can just incorporate that into my series.

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.

But see below,

> I still think it's fine, tbh.

and perhaps you are right,

> Which uprobe user violates this contract
> in the kernel?

The only in-kernel user of UPROBE_HANDLER_REMOVE is perf, and it is fine.

But there are out-of-tree users, say systemtap, I have no idea if this
change can affect them.

And in general, this change makes the API less "flexible".

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.

But. Since you are going to send another version, may I ask you to add a
note into the changelog to explain that this patch assumes (and enforces)
the rule about handler/filter consistency?

Oleg.