Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache_free callback

[Vlastimil Babka <vbabka@suse.cz>]

On 6/17/24 6:33 PM, Jason A. Donenfeld wrote:
> On Mon, Jun 17, 2024 at 6:30 PM Uladzislau Rezki <urezki@gmail.com> wrote:
>> Here if an "err" is less then "0" means there are still objects
>> whereas "is_destroyed" is set to "true" which is not correlated
>> with a comment:
>>
>> "Destruction happens when no objects"
> 
> The comment is just poorly written. But the logic of the code is right.
> 
>>
>> >  out_unlock:
>> >       mutex_unlock(&slab_mutex);
>> >       cpus_read_unlock();
>> > diff --git a/mm/slub.c b/mm/slub.c
>> > index 1373ac365a46..7db8fe90a323 100644
>> > --- a/mm/slub.c
>> > +++ b/mm/slub.c
>> > @@ -4510,6 +4510,8 @@ void kmem_cache_free(struct kmem_cache *s, void *x)
>> >               return;
>> >       trace_kmem_cache_free(_RET_IP_, x, s);
>> >       slab_free(s, virt_to_slab(x), x, _RET_IP_);
>> > +     if (s->is_destroyed)
>> > +             kmem_cache_destroy(s);
>> >  }
>> >  EXPORT_SYMBOL(kmem_cache_free);
>> >
>> > @@ -5342,9 +5344,6 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
>> >               if (!slab->inuse) {
>> >                       remove_partial(n, slab);
>> >                       list_add(&slab->slab_list, &discard);
>> > -             } else {
>> > -                     list_slab_objects(s, slab,
>> > -                       "Objects remaining in %s on __kmem_cache_shutdown()");
>> >               }
>> >       }
>> >       spin_unlock_irq(&n->list_lock);
>> >
>> Anyway it looks like it was not welcome to do it in the kmem_cache_free()
>> function due to performance reason.
> 
> "was not welcome" - Vlastimil mentioned *potential* performance
> concerns before I posted this. I suspect he might have a different
> view now, maybe?
> 
> Vlastimil, this is just checking a boolean (which could be
> unlikely()'d), which should have pretty minimal overhead. Is that
> alright with you?

Well I doubt we can just set and check it without any barriers? The
completion of the last pending kfree_rcu() might race with
kmem_cache_destroy() in a way that will leave the cache there forever, no?
And once we add barriers it becomes a perf issue?

> Jason