Re: [PATCH RFC v2] rhashtable: implement rhashtable_walk_peek() using rhashtable_walk_last_seen()

Tom Herbert
2018-06-14T17:41:05Z

On Mon, Jun 11, 2018 at 7:48 PM, NeilBrown <neilb@suse.com> wrote:
>
> rhashtable_walk_last_seen() does most of the work that
> rhashtable_walk_peek() needs done, so use it and put
> it in a "static inline".
> Also update the documentation for rhashtable_walk_peek() to clarify
> the expected use case.
>
> Signed-off-by: NeilBrown <neilb@suse.com>

Acked-by: Tom Herbert <tom@quantonium.net>

> ---
>
> v2 as static-inline - suggested by Tom.
>
> Thanks,
> NeilBrown
>
> include/linux/rhashtable.h | 29 ++++++++++++++++++++++++++++-
> lib/rhashtable.c | 34 ----------------------------------
> 2 files changed, 28 insertions(+), 35 deletions(-)
>
> diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
> index d63b472e9d50..96ebc2690027 100644
> --- a/include/linux/rhashtable.h
> +++ b/include/linux/rhashtable.h
> @@ -247,10 +247,37 @@ static inline void rhashtable_walk_start(struct rhashtable_iter *iter)
> }
>
> void *rhashtable_walk_next(struct rhashtable_iter *iter);
> -void *rhashtable_walk_peek(struct rhashtable_iter *iter);
> void *rhashtable_walk_last_seen(struct rhashtable_iter *iter);
> void rhashtable_walk_stop(struct rhashtable_iter *iter) __releases(RCU);
>
> +/**
> + * rhashtable_walk_peek - Return the next object to use in an interrupted walk
> + * @iter: Hash table iterator
> + *
> + * Returns the "current" object or NULL when the end of the table is reached.
> + * When an rhashtable_walk is interrupted with rhashtable_walk_stop(),
> + * it is often because an object was found that could not be processed
> + * immediately, possible because there is no more space to encode details
> + * of the object (e.g. when producing a seq_file from the table).
> + * When the walk is restarted, the same object needs to be processed again,
> + * if possible. The object might have been removed from the table while
> + * the walk was paused, so it might not be available. In that case, the
> + * normal "next" object should be treated as "current".
> + *
> + * To support this common case, rhashtable_walk_peek() returns the
> + * appropriate object to process after an interrupted walk, either the
> + * one that was most recently returned, or if that doesn't exist - the
> + * next one.
> + *
> + * Returns -EAGAIN if resize event occurred. In that case the iterator
> + * will rewind back to the beginning and you may continue to use it.
> + */
> +static inline void *rhashtable_walk_peek(struct rhashtable_iter *iter)
> +{
> + return rhashtable_walk_last_seen(iter) ?:
> + rhashtable_walk_next(iter);
> +}
> +
> void rhashtable_free_and_destroy(struct rhashtable *ht,
> void (*free_fn)(void *ptr, void *arg),
> void *arg);
> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> index 45f2554399a5..354275037df3 100644
> --- a/lib/rhashtable.c
> +++ b/lib/rhashtable.c
> @@ -915,40 +915,6 @@ 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
> - * @iter: Hash table iterator
> - *
> - * Returns the next object or NULL when the end of the table is reached.
> - *
> - * Returns -EAGAIN if resize event occurred. Note that the iterator
> - * will rewind back to the beginning and you may continue to use it.
> - */
> -void *rhashtable_walk_peek(struct rhashtable_iter *iter)
> -{
> - struct rhlist_head *list = iter->list;
> - struct rhashtable *ht = iter->ht;
> - struct rhash_head *p = iter->p;
> -
> - if (p)
> - return rht_obj(ht, ht->rhlist ? &list->rhead : p);
> -
> - /* No object found in current iter, find next one in the table. */
> -
> - if (iter->skip) {
> - /* A nonzero skip value points to the next entry in the table
> - * beyond that last one that was found. Decrement skip so
> - * we find the current value. __rhashtable_walk_find_next
> - * will restore the original value of skip assuming that
> - * the table hasn't changed.
> - */
> - iter->skip--;
> - }
> -
> - return __rhashtable_walk_find_next(iter);
> -}
> -EXPORT_SYMBOL_GPL(rhashtable_walk_peek);
> -
> /**
> * rhashtable_walk_last_seen - Return the previously returned object, if available
> * @iter: Hash table iterator
> --
> 2.14.0.rc0.dirty
>