Subject: Re: [PATCH net-next] rhashtable: Fix remove logic to avoid cross references between buckets

From: Ying Xue <[email-masked]>
Date: Mon, 9 Feb 2015 10:44:50 +0800

On 02/07/2015 12:08 AM, Thomas Graf wrote:
> The remove logic properly searched the remaining chain for a matching
> entry with an identical hash but it did this while searching from both
> the old and new table. Instead in order to not leave stale references
> behind we need to:
> 
>  1. When growing and searching from the new table:
>     Search remaining chain for entry with same hash to avoid having
>     the new table directly point to a entry with a different hash.
> 
>  2. When shrinking and searching from the old table:
>     Check if the element after the removed would create a cross
>     reference and avoid it if so.
> 
> These bugs were present from the beginning in nft_hash.
> 
> Also, both insert functions calculated the hash based on the mask of
> the new table. This worked while growing. Wwhile shrinking, the mask
> of the inew table is smaller than the mask of the old table. This lead
> to a bit not being taken into account when selecting the bucket lock
> and thus caused the wrong bucket to be locked eventually.
> 

Good Job!
My previously reported issue has been gone, thanks for your hard work!

Regards,
Ying

> Fixes: 7e1e77636e36 ("lib: Resizable, Scalable, Concurrent Hash Table")
> Fixes: 97defe1ecf86 ("rhashtable: Per bucket locks & deferred expansion/shrinking")
> Reported-by: Ying Xue <[email-masked]>
> Signed-off-by: Thomas Graf <[email-masked]>
> ---
>  lib/rhashtable.c | 28 +++++++++++++++++-----------
>  1 file changed, 17 insertions(+), 11 deletions(-)
> 
> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> index 5919d63..e96fc00 100644
> --- a/lib/rhashtable.c
> +++ b/lib/rhashtable.c
> @@ -552,8 +552,10 @@ static void rhashtable_wakeup_worker(struct rhashtable *ht)
>  static void __rhashtable_insert(struct rhashtable *ht, struct rhash_head *obj,
>  				struct bucket_table *tbl, u32 hash)
>  {
> -	struct rhash_head *head = rht_dereference_bucket(tbl->buckets[hash],
> -							 tbl, hash);
> +	struct rhash_head *head;
> +
> +	hash = rht_bucket_index(tbl, hash);
> +	head = rht_dereference_bucket(tbl->buckets[hash], tbl, hash);
>  
>  	ASSERT_BUCKET_LOCK(ht, tbl, hash);
>  
> @@ -593,7 +595,7 @@ void rhashtable_insert(struct rhashtable *ht, struct rhash_head *obj)
>  
>  	tbl = rht_dereference_rcu(ht->future_tbl, ht);
>  	old_tbl = rht_dereference_rcu(ht->tbl, ht);
> -	hash = head_hashfn(ht, tbl, obj);
> +	hash = obj_raw_hashfn(ht, rht_obj(ht, obj));
>  
>  	lock_buckets(tbl, old_tbl, hash);
>  	__rhashtable_insert(ht, obj, tbl, hash);
> @@ -627,8 +629,8 @@ bool rhashtable_remove(struct rhashtable *ht, struct rhash_head *obj)
>  	bool ret = false;
>  
>  	rcu_read_lock();
> -	tbl = old_tbl = rht_dereference_rcu(ht->tbl, ht);
> -	new_tbl = rht_dereference_rcu(ht->future_tbl, ht);
> +	old_tbl = rht_dereference_rcu(ht->tbl, ht);
> +	tbl = new_tbl = rht_dereference_rcu(ht->future_tbl, ht);
>  	new_hash = obj_raw_hashfn(ht, rht_obj(ht, obj));
>  
>  	lock_buckets(new_tbl, old_tbl, new_hash);
> @@ -643,15 +645,19 @@ restart:
>  
>  		ASSERT_BUCKET_LOCK(ht, tbl, hash);
>  
> -		if (unlikely(new_tbl != tbl)) {
> -			rht_for_each_continue(he2, he->next, tbl, hash) {
> +		if (old_tbl->size > new_tbl->size && tbl == old_tbl &&
> +		    !rht_is_a_nulls(obj->next) &&
> +		    head_hashfn(ht, tbl, obj->next) != hash) {
> +			rcu_assign_pointer(*pprev, (struct rhash_head *) rht_marker(ht, hash));
> +		} else if (unlikely(old_tbl->size < new_tbl->size && tbl == new_tbl)) {
> +			rht_for_each_continue(he2, obj->next, tbl, hash) {
>  				if (head_hashfn(ht, tbl, he2) == hash) {
>  					rcu_assign_pointer(*pprev, he2);
>  					goto found;
>  				}
>  			}
>  
> -			INIT_RHT_NULLS_HEAD(*pprev, ht, hash);
> +			rcu_assign_pointer(*pprev, (struct rhash_head *) rht_marker(ht, hash));
>  		} else {
>  			rcu_assign_pointer(*pprev, obj->next);
>  		}
> @@ -666,8 +672,8 @@ found:
>  	 * resizing. Thus traversing both is fine and the added cost is
>  	 * very rare.
>  	 */
> -	if (tbl != new_tbl) {
> -		tbl = new_tbl;
> +	if (tbl != old_tbl) {
> +		tbl = old_tbl;
>  		goto restart;
>  	}
>  
> @@ -835,7 +841,7 @@ bool rhashtable_lookup_compare_insert(struct rhashtable *ht,
>  	rcu_read_lock();
>  	old_tbl = rht_dereference_rcu(ht->tbl, ht);
>  	new_tbl = rht_dereference_rcu(ht->future_tbl, ht);
> -	new_hash = head_hashfn(ht, new_tbl, obj);
> +	new_hash = obj_raw_hashfn(ht, rht_obj(ht, obj));
>  
>  	lock_buckets(new_tbl, old_tbl, new_hash);
>  
>