Re: [PATCH v2 04/12] uprobes: revamp uprobe refcounting and lifetime management

Jiri Olsa
2024-07-06T17:05:31Z

On Sat, Jul 06, 2024 at 07:00:34PM +0200, Jiri Olsa wrote:
> On Fri, Jul 05, 2024 at 05:37:05PM +0200, Oleg Nesterov wrote:
> > Tried to read this patch, but I fail to understand it. It looks
> > obvioulsy wrong to me, see below.
> >
> > I tend to agree with the comments from Peter, but lets ignore them
> > for the moment.
> >
> > On 07/01, Andrii Nakryiko wrote:
> > >
> > > static void put_uprobe(struct uprobe *uprobe)
> > > {
> > > - if (refcount_dec_and_test(&uprobe->ref)) {
> > > + s64 v;
> > > +
> > > + /*
> > > + * here uprobe instance is guaranteed to be alive, so we use Tasks
> > > + * Trace RCU to guarantee that uprobe won't be freed from under us, if
> > > + * we end up being a losing "destructor" inside uprobe_treelock'ed
> > > + * section double-checking uprobe->ref value below.
> > > + * Note call_rcu_tasks_trace() + uprobe_free_rcu below.
> > > + */
> > > + rcu_read_lock_trace();
> > > +
> > > + v = atomic64_add_return(UPROBE_REFCNT_PUT, &uprobe->ref);
> > > +
> > > + if (unlikely((u32)v == 0)) {
> >
> > I must have missed something, but how can this ever happen?
> >
> > Suppose uprobe_register(inode) is called the 1st time. To simplify, suppose
> > that this binary is not used, so _register() doesn't install breakpoints/etc.
> >
> > IIUC, with this change (u32)uprobe->ref == 1 when uprobe_register() succeeds.
> >
> > Now suppose that uprobe_unregister() is called right after that. It does
> >
> > uprobe = find_uprobe(inode, offset);
> >
> > this increments the counter, (u32)uprobe->ref == 2
> >
> > __uprobe_unregister(...);
> >
> > this wont't change the counter,
>
> __uprobe_unregister calls delete_uprobe that calls put_uprobe ?

ugh, wrong sources.. ok, don't know ;-)

jirka

>
> jirka
>
> >
> > put_uprobe(uprobe);
> >
> > this drops the reference added by find_uprobe(), (u32)uprobe->ref == 1.
> >
> > Where should the "final" put_uprobe() come from?
> >
> > IIUC, this patch lacks another put_uprobe() after consumer_del(), no?
> >
> > Oleg.
> >