linux-kernel - Re: [PATCH v3 04/13] uprobes: travers uprobe's consumer list locklessly under SRCU protection

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Message-ID: <CAEf4Bzb-1na=S9+XVpEpmtDE4mJLQRywZJ6wB8JyN++2Si6Pgw@mail.gmail.com>
Date: Thu, 22 Aug 2024 09:59:29 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Andrii Nakryiko <andrii@...nel.org>, linux-trace-kernel@...r.kernel.org, 
	peterz@...radead.org, oleg@...hat.com, 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 v3 04/13] uprobes: travers uprobe's consumer list
 locklessly under SRCU protection

On Thu, Aug 22, 2024 at 7:22 AM Jiri Olsa <olsajiri@...il.com> wrote:
>
> On Mon, Aug 12, 2024 at 09:29:08PM -0700, Andrii Nakryiko wrote:
>
> SNIP
>
> > @@ -1125,18 +1103,31 @@ void uprobe_unregister(struct uprobe *uprobe, struct uprobe_consumer *uc)
> >       int err;
> >
> >       down_write(&uprobe->register_rwsem);
> > -     if (WARN_ON(!consumer_del(uprobe, uc))) {
> > -             err = -ENOENT;
> > -     } else {
> > -             err = register_for_each_vma(uprobe, NULL);
> > -             /* TODO : cant unregister? schedule a worker thread */
> > -             if (unlikely(err))
> > -                     uprobe_warn(current, "unregister, leaking uprobe");
> > -     }
> > +
> > +     list_del_rcu(&uc->cons_node);
>
> hi,
> I'm using this patchset as base for my changes and stumbled on this today,
> I'm probably missing something, but should we keep the 'uprobe->consumer_rwsem'
> lock around the list_del_rcu?
>

Note that original code also didn't take consumer_rwsem, but rather
kept register_rwsem (which we still use).

There is a bit of mix of using register_rwsem and consumer_rwsem for
working with consumer list. Code hints at this as being undesirable
and "temporary", but you know, it's not broken :)

Anyways, my point is that we didn't change the behavior, this should
be fine. That _rcu() in list_del_rcu() is not about lockless
modification of the list, but rather modification in such a way as to
keep lockless RCU-protected *readers* correct. It just does some more
memory barrier/release operations more carefully.

> jirka
>
>
> > +     err = register_for_each_vma(uprobe, NULL);
> > +
> >       up_write(&uprobe->register_rwsem);
> >
> > -     if (!err)
> > -             put_uprobe(uprobe);
> > +     /* TODO : cant unregister? schedule a worker thread */
> > +     if (unlikely(err)) {
> > +             uprobe_warn(current, "unregister, leaking uprobe");
> > +             goto out_sync;
> > +     }
> > +
> > +     put_uprobe(uprobe);
> > +