Subject: Re: [PATCH 06/17] batman-adv: replace call_rcu by kfree_rcu for simple kmem_cache_free callback

From: Sven Eckelmann <[email-masked]>
Date: Mon, 14 Oct 2024 09:03:30 +0200

On Sunday, 13 October 2024 22:16:53 CEST Julia Lawall wrote:
> Since SLOB was removed and since
> commit 6c6c47b063b5 ("mm, slab: call kvfree_rcu_barrier() from kmem_cache_destroy()"),
> it is not necessary to use call_rcu when the callback only performs
> kmem_cache_free. Use kfree_rcu() directly.
> 
> The changes were made using Coccinelle.
> 
> Signed-off-by: Julia Lawall <[email-masked]>
> 
> ---
>  net/batman-adv/translation-table.c |   47 ++-----------------------------------
>  1 file changed, 3 insertions(+), 44 deletions(-)

This was tried and we noticed that it is not safe [1]. So, I would get 
confirmation that commit 6c6c47b063b5 ("mm, slab: call kvfree_rcu_barrier() 
from kmem_cache_destroy()") is fixing the problem which we had at that time. 
The commit message sounds like it but I just want to be sure.

Kind regards,
	Sven

[1] https://lore.kernel.org/r/[email-masked]

> 
> diff --git a/net/batman-adv/translation-table.c b/net/batman-adv/translation-table.c
> index 2243cec18ecc..b21ff3c36b07 100644
> --- a/net/batman-adv/translation-table.c
> +++ b/net/batman-adv/translation-table.c
> @@ -208,20 +208,6 @@ batadv_tt_global_hash_find(struct batadv_priv *bat_priv, const u8 *addr,
>  	return tt_global_entry;
>  }
>  
> -/**
> - * batadv_tt_local_entry_free_rcu() - free the tt_local_entry
> - * @rcu: rcu pointer of the tt_local_entry
> - */
> -static void batadv_tt_local_entry_free_rcu(struct rcu_head *rcu)
> -{
> -	struct batadv_tt_local_entry *tt_local_entry;
> -
> -	tt_local_entry = container_of(rcu, struct batadv_tt_local_entry,
> -				      common.rcu);
> -
> -	kmem_cache_free(batadv_tl_cache, tt_local_entry);
> -}
> -
>  /**
>   * batadv_tt_local_entry_release() - release tt_local_entry from lists and queue
>   *  for free after rcu grace period
> @@ -236,7 +222,7 @@ static void batadv_tt_local_entry_release(struct kref *ref)
>  
>  	batadv_softif_vlan_put(tt_local_entry->vlan);
>  
> -	call_rcu(&tt_local_entry->common.rcu, batadv_tt_local_entry_free_rcu);
> +	kfree_rcu(tt_local_entry, common.rcu);
>  }
>  
>  /**
> @@ -254,20 +240,6 @@ batadv_tt_local_entry_put(struct batadv_tt_local_entry *tt_local_entry)
>  		 batadv_tt_local_entry_release);
>  }
>  
> -/**
> - * batadv_tt_global_entry_free_rcu() - free the tt_global_entry
> - * @rcu: rcu pointer of the tt_global_entry
> - */
> -static void batadv_tt_global_entry_free_rcu(struct rcu_head *rcu)
> -{
> -	struct batadv_tt_global_entry *tt_global_entry;
> -
> -	tt_global_entry = container_of(rcu, struct batadv_tt_global_entry,
> -				       common.rcu);
> -
> -	kmem_cache_free(batadv_tg_cache, tt_global_entry);
> -}
> -
>  /**
>   * batadv_tt_global_entry_release() - release tt_global_entry from lists and
>   *  queue for free after rcu grace period
> @@ -282,7 +254,7 @@ void batadv_tt_global_entry_release(struct kref *ref)
>  
>  	batadv_tt_global_del_orig_list(tt_global_entry);
>  
> -	call_rcu(&tt_global_entry->common.rcu, batadv_tt_global_entry_free_rcu);
> +	kfree_rcu(tt_global_entry, common.rcu);
>  }
>  
>  /**
> @@ -407,19 +379,6 @@ static void batadv_tt_global_size_dec(struct batadv_orig_node *orig_node,
>  	batadv_tt_global_size_mod(orig_node, vid, -1);
>  }
>  
> -/**
> - * batadv_tt_orig_list_entry_free_rcu() - free the orig_entry
> - * @rcu: rcu pointer of the orig_entry
> - */
> -static void batadv_tt_orig_list_entry_free_rcu(struct rcu_head *rcu)
> -{
> -	struct batadv_tt_orig_list_entry *orig_entry;
> -
> -	orig_entry = container_of(rcu, struct batadv_tt_orig_list_entry, rcu);
> -
> -	kmem_cache_free(batadv_tt_orig_cache, orig_entry);
> -}
> -
>  /**
>   * batadv_tt_orig_list_entry_release() - release tt orig entry from lists and
>   *  queue for free after rcu grace period
> @@ -433,7 +392,7 @@ static void batadv_tt_orig_list_entry_release(struct kref *ref)
>  				  refcount);
>  
>  	batadv_orig_node_put(orig_entry->orig_node);
> -	call_rcu(&orig_entry->rcu, batadv_tt_orig_list_entry_free_rcu);
> +	kfree_rcu(orig_entry, rcu);
>  }
>  
>  /**
> 
>