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

[Boqun Feng]

On Mon, Mar 24, 2025 at 12:21:07PM -0700, Boqun Feng wrote:
> 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 unpublished, in other words, there is no way a lockdep_unregister_key()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?
> 

Oh! Maybe I was missing register_lock_class() must be called with irq
disabled, which is also an RCU read-side critical section.

Regards,
Boqun

> 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.