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

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Message-ID: <Zsd28aEHBLkRpUQs@krava>
Date: Thu, 22 Aug 2024 19:35:45 +0200
From: Jiri Olsa <olsajiri@...il.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,
	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 09:59:29AM -0700, Andrii Nakryiko wrote:
> 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).

humm, consumer_del took consumer_rwsem, right?

jirka

> 
> 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);
> > > +