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

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Message-ID: <ZtWCAN_F1DkFAFNp@krava>
Date: Mon, 2 Sep 2024 12:14:40 +0300
From: Jiri Olsa <olsajiri@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>,
	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 Sat, Aug 31, 2024 at 06:19:15PM +0200, Oleg Nesterov wrote:
> 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.

FWIW I (strongly) agree with merging this change and fixing the rest as follow up

thanks,
jirka

> 
> 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.
>