Re: [PATCH rcu 5/8] slab: Explain why SLAB_DESTROY_BY_RCU reference before locking

[Vlastimil Babka <vbabka@suse.cz>]

On 10/20/22 00:46, Paul E. McKenney wrote:
> It is not obvious to the casual user why it is absolutely necessary to
> acquire a reference to a SLAB_DESTROY_BY_RCU structure before acquiring
> a lock in that structure.  Therefore, add a comment explaining this point.

s/SLAB_DESTROY_BY_RCU/SLAB_TYPESAFE_BY_RCU/ in subject, commit log and the
added comment? :)

> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Roman Gushchin <roman.gushchin@linux.dev>
> Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> Cc: <linux-mm@kvack.org>
> ---
>  include/linux/slab.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 90877fcde70bd..446303e385265 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -76,6 +76,12 @@
>   * rcu_read_lock before reading the address, then rcu_read_unlock after
>   * taking the spinlock within the structure expected at that address.
>   *
> + * Note that it is not possible to acquire a lock within a structure
> + * allocated with SLAB_DESTROY_BY_RCU without first acquiring a reference
> + * as described above.  The reason is that SLAB_DESTROY_BY_RCU pages are
> + * not zeroed before being given to the slab, which means that any locks
> + * must be initialized after each and every kmem_struct_alloc().
> + *

Wonder if slab caches with a constructor should be OK here as AFAIK it
should mean the object has to be in the initialized state both when
allocated and freed?

>   * Note that SLAB_TYPESAFE_BY_RCU was originally named SLAB_DESTROY_BY_RCU.
>   */
>  /* Defer freeing slabs to RCU */