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

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Message-ID: <CAEf4Bza43g4O-OP2ZTvri2+rnUVzKcV6LCRugzt7AcFqmkSP4g@mail.gmail.com>
Date: Thu, 22 Aug 2024 10:51:54 -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 10:35 AM Jiri Olsa <olsajiri@...il.com> wrote:
>
> 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?
>

Ah, it was inside consume_del(), sorry, my bad. I can add nested
consumer_rwsem back, but what I mentioned earlier, regiser_rwsem is
sort of interchangeable and sufficient enough for working with
consumer list, it seems. There are a bunch of places where we iterated
this list without holding consumer_rwsem lock and that doesn't break
anything.

Also, consumer_add() and consumer_del() are always called with
register_rwsem, so that consumer_rwsem isn't necessary.

We also have prepare_uprobe() holding consumer_rwsem and there is a
comment about abuse of that rwsem and suggestion to move it to
registration, I never completely understood that. But prepare_uprobe()
doesn't seem to modify consumers list at all.

And the one remaining use of consumer_rwsem is filter_chain(), which
for handler_chain() will be also called under register_rwsem, if
purely lockless traversal is not enough.

There are two other calls to filter_chain() that are not protected by
register_rwsem, so just because of those two maybe we should keep
consumer_rwsem, but so far all the stress testing never caught any
problem.


> 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);
> > > > +