On Tue, Sep 02, 2025 at 05:50:32PM +0800, menglong.d...@linux.dev wrote:
> On 2025/9/2 17:17 Herbert Xu <herb...@gondor.apana.org.au> write:
> > Menglong Dong <dong...@chinatelecom.cn> wrote:
> > >
> > > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> > > index fb127fa95f21..fece0f849c1c 100644
> > > --- a/kernel/trace/fprobe.c
> > > +++ b/kernel/trace/fprobe.c
> > > @@ -269,7 +269,9 @@ static int fprobe_entry(struct ftrace_graph_ent
> > > *trace, struct fgraph_ops *gops,
> > > if (WARN_ON_ONCE(!fregs))
> > > return 0;
> > >
> > > + rcu_read_lock();
> > > head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params);
> > > + rcu_read_unlock();
> > > reserved_words = 0;
> > > rhl_for_each_entry_rcu(node, pos, head, hlist) {
> > > if (node->addr != func)
> >
> > Actually this isn't quite right. I know that it is a false-positive
> > so that it's actually safe, but if you're going to mark it with
> > rcu_read_lock, it should cover both the lookup as well as the
> > dereference which happens in the loop rhl_for_each_entry_rcu.
>
> Yeah, I understand. The rcu_read_lock() here is totally used to
> suppress the suspicious rcu usage warning, not for the protection.
> So I used it just for the rhltable_lookup() to reduce the impact.
> Maybe I should add some comment for it.
My point is that after a lookup you will be doing some sort of a
dereference on the RCU pointer. That would cause exactly the same
splat that rhltable_lookup itself generated.
For example, rhl_for_each_entry_rcu should have created the same
warning, but it doesn't because for some reason it is using
rcu_dereference_raw. I'll need to dig up the history of this
to see if there is a good reason for it to not warn.
Cheers,
--
Email: Herbert Xu <herb...@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
