Re: [PATCH v8] KEYS: Add a list for unreferenced keys

[Jarkko Sakkinen <jarkko@kernel.org>]

On Fri, Apr 11, 2025 at 04:59:11PM +0100, David Howells wrote:
> Jarkko Sakkinen <jarkko@kernel.org> wrote:
> 
> > +	spin_lock_irqsave(&key_graveyard_lock, flags);
> > +	list_splice_init(&key_graveyard, &graveyard);
> > +	spin_unlock_irqrestore(&key_graveyard_lock, flags);
> 
> I would wrap this bit in a check to see if key_graveyard is empty so that we
> can avoid disabling irqs and taking the lock if the graveyard is empty.

Can do, and does make sense.

> 
> > +		if (!refcount_inc_not_zero(&key->usage)) {
> 
> Sorry, but eww.  You're going to wangle the refcount twice on every key on the
> system every time the gc does a pass.  Further, in some cases inc_not_zero is
> not the fastest op in the world.

One could alternatively "test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) &&
!refcount_inc_not_zero(&key->usage))" without mb() on either side and
set_bit() could be at the beginning of key_put().

Race at worst would be an extra refcount_inc_not_zero() but not often.

> 
> > +			spin_lock_irqsave(&key_graveyard_lock, flags);
> > +			list_add_tail(&key->graveyard_link, &key_graveyard);
> > +			spin_unlock_irqrestore(&key_graveyard_lock, flags);
> >  			schedule_work(&key_gc_work);
> 
> This is going to enable and disable interrupts twice and that can be
> expensive, depending on the arch.  I wonder if it would be better to do:
> 
> 			local_irq_save(flags);
> 			spin_lock(&key_graveyard_lock);
> 			list_add_tail(&key->graveyard_link, &key_graveyard);
> 			spin_unlock(&key_graveyard_lock);
> 			schedule_work(&key_gc_work);
> 			local_irq_restore(flags);

I like this but shouldn't this also comprehend the quota update before
(just asking for completeness sake)?

> 
> David
> 
> 

BR, Jarkko