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

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Message-ID: <CAEf4BzYE7+YgM7HMb-JceoC33f=irjHkj=5x46WaXdCcgTk4xg@mail.gmail.com>
Date: Tue, 3 Sep 2024 10:27:49 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Oleg Nesterov <oleg@...hat.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 Sat, Aug 31, 2024 at 9:19 AM Oleg Nesterov <oleg@...hat.com> 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.

We could do that, but then worst case, when we do detect registration
race, what do we do? We still have to do the same. So instead of
polluting the logic with seq counter it's best to just codify the
protocol and take advantage of that.

But as you said, this all can/should be addressed as a follow up
discussion. You mentioned some clean ups you wanted to do, let's
discuss all that as part of that?

>
> 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.
>

Well, BPF program can accidentally trigger this as well, but that's a
bug, we should fix it ASAP in the bpf tree.


> 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".

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.

>
> 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.

>
> 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?

Yep, will do. I will also leave a comment next to the filter callback
definition in uprobe_consumer about this.

>
> Oleg.
>