Re: [PATCH] lockdep: Speed up lockdep_unregister_key() with expedited RCU synchronization

[Boqun Feng]

On Mon, Mar 24, 2025 at 01:23:50PM +0100, Eric Dumazet wrote:
[...]
> > > ---
> > >  kernel/locking/lockdep.c | 6 ++++--
> > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > > index 4470680f02269..a79030ac36dd4 100644
> > > --- a/kernel/locking/lockdep.c
> > > +++ b/kernel/locking/lockdep.c
> > > @@ -6595,8 +6595,10 @@ void lockdep_unregister_key(struct lock_class_key *key)
> > >       if (need_callback)
> > >               call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
> > >
> > > -     /* Wait until is_dynamic_key() has finished accessing k->hash_entry. */
> > > -     synchronize_rcu();

I feel a bit confusing even for the old comment, normally I would expect
the caller of lockdep_unregister_key() should guarantee the key has been
unpublished, in other words, there is no way a lockdep_unregister_key()
could race with a register_lock_class()/lockdep_init_map_type(). The
synchronize_rcu() is not needed then.

Let's say someone breaks my assumption above, then when doing a
register_lock_class() with a key about to be unregister, I cannot see
anything stops the following:

	CPU 0				CPU 1
	=====				=====
	register_lock_class():
	  ...
	  } else if (... && !is_dynamic_key(lock->key)) {
	  	// ->key is not unregistered yet, so this branch is not
		// taken.
	  	return NULL;
	  }
	  				lockdep_unregister_key(..);
					// key unregister, can be free
					// any time.
	  key = lock->key->subkeys + subclass; // BOOM! UAF.

So either we don't need the synchronize_rcu() here or the
synchronize_rcu() doesn't help at all. Am I missing something subtle
here?

Regards,
Boqun

> > > +     /* Wait until is_dynamic_key() has finished accessing k->hash_entry.
> > > +      * This needs to be quick, since it is called in critical sections
> > > +      */
> > > +     synchronize_rcu_expedited();
> > >  }
> > >  EXPORT_SYMBOL_GPL(lockdep_unregister_key);
> >
> > So I fundamentally despise synchronize_rcu_expedited(), also your
> > comment style is broken.
> >
> > Why can't qdisc call this outside of the lock?
> 
> Good luck with that, and anyway the time to call it 256 times would
> still hurt Breno use case.
> 
> My suggestion was to change lockdep_unregister_key() contract, and use
> kfree_rcu() there
> 
> > I think we should redesign lockdep_unregister_key() to work on a separately
> > allocated piece of memory,
> > then use kfree_rcu() in it.
> >
> > Ie not embed a "struct lock_class_key" in the struct Qdisc, but a pointer to
> >
> > struct ... {
> >      struct lock_class_key key;
> >      struct rcu_head  rcu;
> > }
> 
> More work because it requires changing all lockdep_unregister_key() users.