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

[Jarkko Sakkinen]

On Fri, Apr 11, 2025 at 11:37:25PM +0300, Jarkko Sakkinen wrote:
> On Fri, Apr 11, 2025 at 04:59:11PM +0100, David Howells wrote:
> > Jarkko Sakkinen <jar...@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

Refactoring the changes to key_put() would be (draft):

void key_put(struct key *key)
{
        unsigned long flags;

        if (!key)
                return;

        key_check(key);

        if (!refcount_dec_and_test(&key->usage))
                return;

        local_irq_save(flags);

        set_bit(KEY_FLAG_FINAL_PUT, &key->flags);

        /* Remove user's key tracking and quota: */
        if (test_bit(KEY_FLAG_IN_QUOTA, &key->flags)) {
                spin_lock(&key->user->lock);
                key->user->qnkeys--;
                key->user->qnbytes -= key->quotalen;
                spin_unlock(&key->user->lock);
        }

        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);
}

And:

static bool key_get_gc(struct key *key)
{
        return !test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) &&   /* fast path */
               refcount_inc_not_zero(&key->usage)               /* slow path */
}

In the gc-loop:

        if (!key_get_gc(&key)) {
                key = NULL;
                gc_state |= KEY_GC_REAP_AGAIN;
                goto skip_dead_key;
        }

[none yet compiled]

BR, Jarkko