Re: [PATCH 1/6] rhashtable: improve documentation for rhashtable_walk_peek()

[Herbert Xu]

On Tue, Mar 27, 2018 at 10:33:04AM +1100, NeilBrown wrote:
> The documentation for rhashtable_walk_peek() wrong.  It claims to
> return the *next* entry, whereas it in fact returns the *previous*
> entry.
> However if no entries have yet been returned - or if the iterator
> was reset due to a resize event, then rhashtable_walk_peek()
> *does* return the next entry, but also advances the iterator.
> 
> I suspect that this interface should be discarded and the one user
> should be changed to not require it.  Possibly this patch should be
> seen as a first step in that conversation.
> 
> This patch mostly corrects the documentation, but does make a
> small code change so that the documentation can be correct without
> listing too many special cases.  I don't think the one user will
> be affected by the code change.
> 
> Signed-off-by: NeilBrown <neilb@xxxxxxxx>

We should cc Tom Herbert too since he wrote this code.

> ---
>  lib/rhashtable.c |   17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> index 3825c30aaa36..24a57ca494cb 100644
> --- a/lib/rhashtable.c
> +++ b/lib/rhashtable.c
> @@ -853,13 +853,17 @@ void *rhashtable_walk_next(struct rhashtable_iter *iter)
>  EXPORT_SYMBOL_GPL(rhashtable_walk_next);
>  
>  /**
> - * rhashtable_walk_peek - Return the next object but don't advance the iterator
> + * rhashtable_walk_peek - Return the previously returned object without advancing the iterator
>   * @iter:	Hash table iterator
>   *
> - * Returns the next object or NULL when the end of the table is reached.
> + * Returns the last object returned, or NULL if no object has yet been returned.
> + * If the previously returned object has since been removed, then some other arbitrary
> + * object maybe returned, or possibly NULL will be returned.  In that case, the
> + * iterator might be advanced.
>   *
>   * Returns -EAGAIN if resize event occurred.  Note that the iterator
> - * will rewind back to the beginning and you may continue to use it.
> + * will rewind back to the beginning and rhashtable_walk_next() should be
> + * used to get the next object.
>   */
>  void *rhashtable_walk_peek(struct rhashtable_iter *iter)
>  {
> @@ -880,7 +884,12 @@ void *rhashtable_walk_peek(struct rhashtable_iter *iter)
>  		 * the table hasn't changed.
>  		 */
>  		iter->skip--;
> -	}
> +	} else
> +		/* ->skip is only zero after rhashtable_walk_start()
> +		 * or when the iterator is reset.  In this case there
> +		 * is no previous object to return.
> +		 */
> +		return NULL;
>  
>  	return __rhashtable_walk_find_next(iter);
>  }
> 
> 

-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
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