RE: [RFC] lib/st_ring: add single thread ring

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> -----Original Message-----
> From: Mattias Rönnblom <hofors@lysator.liu.se>
> Sent: Monday, August 21, 2023 4:14 PM
> To: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>;
> jackmin@nvidia.com; konstantin.v.ananyev@yandex.ru
> Cc: dev@dpdk.org; Ruifeng Wang <Ruifeng.Wang@arm.com>; Aditya
> Ambadipudi <Aditya.Ambadipudi@arm.com>; Wathsala Wathawana
> Vithanage <wathsala.vithanage@arm.com>; nd <nd@arm.com>
> Subject: Re: [RFC] lib/st_ring: add single thread ring
> 
> On 2023-08-21 08:04, Honnappa Nagarahalli wrote:
> > Add a single thread safe and multi-thread unsafe ring data structure.
> 
> One must have set the bar very low, if one needs to specify that an API is
> single-thread safe.
> 
> > This library provides an simple and efficient alternative to
> > multi-thread safe ring when multi-thread safety is not required.
> >
> > Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> > ---
> > v1:
> > 1) The code is very prelimnary and is not even compiled
> > 2) This is intended to show the APIs and some thoughts on
> > implementation
> 
> If you haven't done it already, maybe it might be worth looking around in the
> code base for already-existing, more-or-less open-coded fifo/circular buffer
> type data structures. Just to make sure those can be eliminated if this makes
> it into DPDK.
> 
> There's one in rte_event_eth_rx_adapter.c, and I think one in the SW
> eventdev as well. Seems to be one in cmdline_cirbuf.h as well. I'm sure there
> are many more.
I knew there are some, but have not looked at them yet. I will look at them.

> 
> You could pick some other name for it, instead of the slightly awkward
> "st_ring" (e.g., "fifo", "cbuf", "cbuffer", "circ_buffer"). That would also leave
> you with more freedom to stray from the MT safe ring API without surprising
> the user, if needed (and I think it is needed).
The thought was to make it clear that this is for single-thread use (i.e.not even producer and consumer on different threads), may be I do not need to try hard.
"fifo" might not be good option given that dequeue/enqueue at both ends of the ring are required/allowed.
Wikipedia [1] and others [2], [3] indicates that this data structure should be called 'deque' (pronounced as deck). I would prefer to go with this (assuming this will be outside of 'rte_ring')

[1] https://en.wikipedia.org/wiki/Double-ended_queue
[2] https://www.geeksforgeeks.org/deque-set-1-introduction-applications/
[3] https://stackoverflow.com/questions/3880254/why-do-we-need-deque-data-structures-in-the-real-world#:~:text=A%20Deque%20is%20a%20double,thing%20on%20front%20of%20queue.

> 
> Hopefully you can reduce API complexity compared to the MT-safe version.
> Having a name for these kinds of data structures doesn't make a lot of sense,
> for example. Skip the dump function. Relax from always_inline to just regular
> inline.
Yes, plan is to reduce complexity (compared to rte_ring) and some APIs can be skipped until there is a need.

> 
> I'm not sure you need bulk/burst type operations. Without any memory
> fences, an optimizing compiler should do a pretty good job of unrolling
> multiple-element access type operations, assuming you leave the ST ring
> code in the header files (otherwise LTO is needed).
IMO, bulk/burst APIs are about the functionality rather than loop unrolling. APIs to work with single objects can be skipped (use bulk APIs with n=1).

> 
> I think you will want a peek-type operation on the reader side. That more for
> convenience, rather than that I think the copies will actually be there in the
> object code (such should be eliminated by the compiler, given that the
> barriers are gone).
> 
> > 3) More APIs and the rest of the implementation will come in subsequent
> >     versions
> >
> >   lib/st_ring/rte_st_ring.h | 567
> ++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 567 insertions(+)
> >   create mode 100644 lib/st_ring/rte_st_ring.h
> >
> > diff --git a/lib/st_ring/rte_st_ring.h b/lib/st_ring/rte_st_ring.h new
> > file mode 100644 index 0000000000..8cb8832591
> > --- /dev/null
> > +++ b/lib/st_ring/rte_st_ring.h
> > @@ -0,0 +1,567 @@
> > +/* SPDX-License-Identifier: BSD-3-Clause
> > + * Copyright(c) 2023 Arm Limited
> > + */
> > +
> > +#ifndef _RTE_ST_RING_H_
> > +#define _RTE_ST_RING_H_
> > +
> > +/**
> > + * @file
> > + * RTE Signle Thread Ring (ST Ring)
> > + *
> > + * The ST Ring is a fixed-size queue intended to be accessed
> > + * by one thread at a time. It does not provide concurrent access to
> > + * multiple threads. If there are multiple threads accessing the ST
> > +ring,
> > + * then the threads have to use locks to protect the ring from
> > + * getting corrupted.
> 
> You are basically saying the same thing three times here.
> 
> > + *
> > + * - FIFO (First In First Out)
> > + * - Maximum size is fixed; the pointers are stored in a table.
> > + * - Consumer and producer part of same thread.
> > + * - Multi-thread producers and consumers need locking.
> 
> ...two more times here. One might get the impression you really don't trust
> the reader.
> 
> > + * - Single/Bulk/burst dequeue at Tail or Head
> > + * - Single/Bulk/burst enqueue at Head or Tail
> 
> Does this not sound more like a deque, than a FIFO/circular buffer? Are there
> any examples where this functionality (the double-endedness) is needed in
> the DPDK code base?
I see, you are calling it 'deque' as well. Basically, this patch originated due to a requirement in MLX PMD [1]

[1] https://github.com/DPDK/dpdk/blob/main/drivers/net/mlx5/mlx5_hws_cnt.h#L381

> 
> > + *
> > + */
> > +
> > +#ifdef __cplusplus
> > +extern "C" {
> > +#endif
> > +
> > +#include <rte_st_ring_core.h>
> > +#include <rte_st_ring_elem.h>
> 
> Is the intention to provide a ring with compile-time variable element size? In
> other words, where the elements of a particular ring instance has the same
> element size, but different rings may have different element sizes.
> 
> Seems like a good idea to me, in that case. Although often you will have
> pointers, it would be useful to store larger things like small structs, and
> maybe smaller elements as well.
Yes, the idea is to make the element size flexible and also compile-time constant.

> 
> > +
> > +/**
> > + * Calculate the memory size needed for a ST ring
> > + *
> > + * This function returns the number of bytes needed for a ST ring,
> > +given
> > + * the number of elements in it. This value is the sum of the size of
> > + * the structure rte_st_ring and the size of the memory needed by the
> > + * elements. The value is aligned to a cache line size.
> > + *
> > + * @param count
> > + *   The number of elements in the ring (must be a power of 2).
> > + * @return
> > + *   - The memory size needed for the ST ring on success.
> > + *   - -EINVAL if count is not a power of 2.
> > + */
> > +ssize_t rte_st_ring_get_memsize(unsigned int count);
> > +
> > +/**
> > + * Initialize a ST ring structure.
> > + *
> > + * Initialize a ST ring structure in memory pointed by "r". The size
> > +of the
> > + * memory area must be large enough to store the ring structure and
> > +the
> > + * object table. It is advised to use rte_st_ring_get_memsize() to
> > +get the
> > + * appropriate size.
> > + *
> > + * The ST ring size is set to *count*, which must be a power of two.
> > + * The real usable ring size is *count-1* instead of *count* to
> > + * differentiate a full ring from an empty ring.
> > + *
> > + * The ring is not added in RTE_TAILQ_ST_RING global list. Indeed,
> > +the
> > + * memory given by the caller may not be shareable among dpdk
> > + * processes.
> > + *
> > + * @param r
> > + *   The pointer to the ring structure followed by the elements table.
> > + * @param name
> > + *   The name of the ring.
> > + * @param count
> > + *   The number of elements in the ring (must be a power of 2,
> > + *   unless RTE_ST_RING_F_EXACT_SZ is set in flags).
> > + * @param flags
> > + *   An OR of the following:
> > + *   - RTE_ST_RING_F_EXACT_SZ: If this flag is set, the ring will hold
> > + *     exactly the requested number of entries, and the requested size
> > + *     will be rounded up to the next power of two, but the usable space
> > + *     will be exactly that requested. Worst case, if a power-of-2 size is
> > + *     requested, half the ring space will be wasted.
> > + *     Without this flag set, the ring size requested must be a power of 2,
> > + *     and the usable space will be that size - 1.
> > + * @return
> > + *   0 on success, or a negative value on error.
> > + */
> > +int rte_st_ring_init(struct rte_st_ring *r, const char *name,
> > +	unsigned int count, unsigned int flags);
> > +
> > +/**
> > + * Create a new ST ring named *name* in memory.
> > + *
> > + * This function uses ``memzone_reserve()`` to allocate memory. Then
> > +it
> > + * calls rte_st_ring_init() to initialize an empty ring.
> > + *
> > + * The new ring size is set to *count*, which must be a power of two.
> > + * The real usable ring size is *count-1* instead of *count* to
> > + * differentiate a full ring from an empty ring.
> > + *
> > + * The ring is added in RTE_TAILQ_ST_RING list.
> > + *
> > + * @param name
> > + *   The name of the ring.
> > + * @param count
> > + *   The size of the ring (must be a power of 2,
> > + *   unless RTE_ST_RING_F_EXACT_SZ is set in flags).
> > + * @param socket_id
> > + *   The *socket_id* argument is the socket identifier in case of
> > + *   NUMA. The value can be *SOCKET_ID_ANY* if there is no NUMA
> > + *   constraint for the reserved zone.
> > + * @param flags
> > + *   - RTE_ST_RING_F_EXACT_SZ: If this flag is set, the ring will hold exactly
> the
> > + *     requested number of entries, and the requested size will be rounded
> up
> > + *     to the next power of two, but the usable space will be exactly that
> > + *     requested. Worst case, if a power-of-2 size is requested, half the
> > + *     ring space will be wasted.
> > + *     Without this flag set, the ring size requested must be a power of 2,
> > + *     and the usable space will be that size - 1.
> > + * @return
> > + *   On success, the pointer to the new allocated ring. NULL on error with
> > + *    rte_errno set appropriately. Possible errno values include:
> > + *    - E_RTE_NO_CONFIG - function could not get pointer to rte_config
> structure
> > + *    - EINVAL - count provided is not a power of 2
> > + *    - ENOSPC - the maximum number of memzones has already been
> allocated
> > + *    - EEXIST - a memzone with the same name already exists
> > + *    - ENOMEM - no appropriate memory area found in which to create
> memzone
> > + */
> > +struct rte_st_ring *rte_st_ring_create(const char *name, unsigned int
> count,
> > +				 int socket_id, unsigned int flags);
> > +
> > +/**
> > + * De-allocate all memory used by the ring.
> > + *
> > + * @param r
> > + *   Ring to free.
> > + *   If NULL then, the function does nothing.
> > + */
> > +void rte_st_ring_free(struct rte_st_ring *r);
> > +
> > +/**
> > + * Dump the status of the ring to a file.
> > + *
> > + * @param f
> > + *   A pointer to a file for output
> > + * @param r
> > + *   A pointer to the ring structure.
> > + */
> > +void rte_st_ring_dump(FILE *f, const struct rte_st_ring *r);
> > +
> > +/**
> > + * Enqueue fixed number of objects on a ST ring.
> > + *
> > + * This function copies the objects at the head of the ring and
> > + * moves the head index.
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @param obj_table
> > + *   A pointer to a table of void * pointers (objects).
> > + * @param n
> > + *   The number of objects to add in the ring from the obj_table.
> > + * @param free_space
> > + *   if non-NULL, returns the amount of space in the ring after the
> > + *   enqueue operation has finished.
> > + * @return
> > + *   The number of objects enqueued, either 0 or n
> > + */
> > +static __rte_always_inline unsigned int
> > +rte_st_ring_enqueue_bulk(struct rte_st_ring *r, void * const *obj_table,
> > +		      unsigned int n, unsigned int *free_space) {
> > +	return rte_st_ring_enqueue_bulk_elem(r, obj_table, sizeof(void *),
> > +			n, free_space);
> > +}
> > +
> > +/**
> > + * Enqueue upto a maximum number of objects on a ST ring.
> > + *
> > + * This function copies the objects at the head of the ring and
> > + * moves the head index.
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @param obj_table
> > + *   A pointer to a table of void * pointers (objects).
> > + * @param n
> > + *   The number of objects to add in the ring from the obj_table.
> > + * @param free_space
> > + *   if non-NULL, returns the amount of space in the ring after the
> > + *   enqueue operation has finished.
> > + * @return
> > + *   - n: Actual number of objects enqueued.
> > + */
> > +static __rte_always_inline unsigned int
> > +rte_st_ring_enqueue_burst(struct rte_st_ring *r, void * const *obj_table,
> > +		      unsigned int n, unsigned int *free_space) {
> > +	return rte_st_ring_enqueue_burst_elem(r, obj_table, sizeof(void *),
> > +			n, free_space);
> > +}
> > +
> > +/**
> > + * Enqueue one object on a ST ring.
> > + *
> > + * This function copies one object at the head of the ring and
> > + * moves the head index.
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @param obj
> > + *   A pointer to the object to be added.
> > + * @return
> > + *   - 0: Success; objects enqueued.
> > + *   - -ENOBUFS: Not enough room in the ring to enqueue; no object is
> enqueued.
> > + */
> > +static __rte_always_inline int
> > +rte_st_ring_enqueue(struct rte_st_ring *r, void *obj) {
> > +	return rte_st_ring_enqueue_elem(r, &obj, sizeof(void *)); }
> > +
> > +/**
> > + * Enqueue fixed number of objects on a ST ring at the tail.
> > + *
> > + * This function copies the objects at the tail of the ring and
> > + * moves the tail index (backwards).
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @param obj_table
> > + *   A pointer to a table of void * pointers (objects).
> > + * @param n
> > + *   The number of objects to add in the ring from the obj_table.
> > + * @param free_space
> > + *   if non-NULL, returns the amount of space in the ring after the
> > + *   enqueue operation has finished.
> > + * @return
> > + *   The number of objects enqueued, either 0 or n
> > + */
> > +static __rte_always_inline unsigned int
> > +rte_st_ring_enqueue_at_tail_bulk(struct rte_st_ring *r,
> > +				 void * const *obj_table, unsigned int n,
> > +				 unsigned int *free_space)
> > +{
> > +	return rte_st_ring_enqueue_at_tail_bulk_elem(r, obj_table,
> > +			sizeof(void *), n, free_space);
> > +}
> > +
> > +/**
> > + * Enqueue upto a maximum number of objects on a ST ring at the tail.
> > + *
> > + * This function copies the objects at the tail of the ring and
> > + * moves the tail index (backwards).
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @param obj_table
> > + *   A pointer to a table of void * pointers (objects).
> > + * @param n
> > + *   The number of objects to add in the ring from the obj_table.
> > + * @param free_space
> > + *   if non-NULL, returns the amount of space in the ring after the
> > + *   enqueue operation has finished.
> > + * @return
> > + *   - n: Actual number of objects enqueued.
> > + */
> > +static __rte_always_inline unsigned int
> > +rte_st_ring_enqueue_at_tail_burst(struct rte_st_ring *r,
> > +				  void * const *obj_table, unsigned int n,
> > +				  unsigned int *free_space)
> > +{
> > +	return rte_st_ring_enqueue_at_tail_burst_elem(r, obj_table,
> > +			sizeof(void *), n, free_space);
> > +}
> > +
> > +/**
> > + * Enqueue one object on a ST ring at tail.
> > + *
> > + * This function copies one object at the tail of the ring and
> > + * moves the tail index (backwards).
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @param obj
> > + *   A pointer to the object to be added.
> > + * @return
> > + *   - 0: Success; objects enqueued.
> > + *   - -ENOBUFS: Not enough room in the ring to enqueue; no object is
> enqueued.
> > + */
> > +static __rte_always_inline int
> > +rte_st_ring_enqueue_at_tail(struct rte_st_ring *r, void *obj) {
> > +	return rte_st_ring_enqueue_at_tail_elem(r, &obj, sizeof(void *)); }
> > +
> > +/**
> > + * Dequeue a fixed number of objects from a ST ring.
> > + *
> > + * This function copies the objects from the tail of the ring and
> > + * moves the tail index.
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @param obj_table
> > + *   A pointer to a table of void * pointers (objects) that will be filled.
> > + * @param n
> > + *   The number of objects to dequeue from the ring to the obj_table.
> > + * @param available
> > + *   If non-NULL, returns the number of remaining ring entries after the
> > + *   dequeue has finished.
> > + * @return
> > + *   The number of objects dequeued, either 0 or n
> > + */
> > +static __rte_always_inline unsigned int
> > +rte_st_ring_dequeue_bulk(struct rte_st_ring *r, void **obj_table,
> unsigned int n,
> > +		unsigned int *available)
> > +{
> > +	return rte_st_ring_dequeue_bulk_elem(r, obj_table, sizeof(void *),
> > +			n, available);
> > +}
> > +
> > +/**
> > + * Dequeue upto a maximum number of objects from a ST ring.
> > + *
> > + * This function copies the objects from the tail of the ring and
> > + * moves the tail index.
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @param obj_table
> > + *   A pointer to a table of void * pointers (objects) that will be filled.
> > + * @param n
> > + *   The number of objects to dequeue from the ring to the obj_table.
> > + * @param available
> > + *   If non-NULL, returns the number of remaining ring entries after the
> > + *   dequeue has finished.
> > + * @return
> > + *   - Number of objects dequeued
> > + */
> > +static __rte_always_inline unsigned int
> > +rte_st_ring_dequeue_burst(struct rte_st_ring *r, void **obj_table,
> > +		unsigned int n, unsigned int *available) {
> > +	return rte_st_ring_dequeue_burst_elem(r, obj_table, sizeof(void *),
> > +			n, available);
> > +}
> > +
> > +/**
> > + * Dequeue one object from a ST ring.
> > + *
> > + * This function copies one object from the tail of the ring and
> > + * moves the tail index.
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @param obj_p
> > + *   A pointer to a void * pointer (object) that will be filled.
> > + * @return
> > + *   - 0: Success, objects dequeued.
> > + *   - -ENOENT: Not enough entries in the ring to dequeue, no object is
> > + *     dequeued.
> > + */
> > +static __rte_always_inline int
> > +rte_st_ring_dequeue(struct rte_st_ring *r, void **obj_p) {
> > +	return rte_st_ring_dequeue_elem(r, obj_p, sizeof(void *)); }
> > +
> > +/**
> > + * Dequeue a fixed number of objects from a ST ring from the head.
> > + *
> > + * This function copies the objects from the head of the ring and
> > + * moves the head index (backwards).
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @param obj_table
> > + *   A pointer to a table of void * pointers (objects) that will be filled.
> > + * @param n
> > + *   The number of objects to dequeue from the ring to the obj_table.
> > + * @param available
> > + *   If non-NULL, returns the number of remaining ring entries after the
> > + *   dequeue has finished.
> > + * @return
> > + *   The number of objects dequeued, either 0 or n
> > + */
> > +static __rte_always_inline unsigned int
> > +rte_st_ring_dequeue_at_head_bulk(struct rte_st_ring *r, void
> **obj_table, unsigned int n,
> > +		unsigned int *available)
> > +{
> > +	return rte_st_ring_dequeue_bulk_elem(r, obj_table, sizeof(void *),
> > +			n, available);
> > +}
> > +
> > +/**
> > + * Dequeue upto a maximum number of objects from a ST ring from the
> head.
> > + *
> > + * This function copies the objects from the head of the ring and
> > + * moves the head index (backwards).
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @param obj_table
> > + *   A pointer to a table of void * pointers (objects) that will be filled.
> > + * @param n
> > + *   The number of objects to dequeue from the ring to the obj_table.
> > + * @param available
> > + *   If non-NULL, returns the number of remaining ring entries after the
> > + *   dequeue has finished.
> > + * @return
> > + *   - Number of objects dequeued
> > + */
> > +static __rte_always_inline unsigned int
> > +rte_st_ring_dequeue_at_head_burst(struct rte_st_ring *r, void
> **obj_table,
> > +		unsigned int n, unsigned int *available) {
> > +	return rte_st_ring_dequeue_burst_elem(r, obj_table, sizeof(void *),
> > +			n, available);
> > +}
> > +
> > +/**
> > + * Dequeue one object from a ST ring from the head.
> > + *
> > + * This function copies the objects from the head of the ring and
> > + * moves the head index (backwards).
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @param obj_p
> > + *   A pointer to a void * pointer (object) that will be filled.
> > + * @return
> > + *   - 0: Success, objects dequeued.
> > + *   - -ENOENT: Not enough entries in the ring to dequeue, no object is
> > + *     dequeued.
> > + */
> > +static __rte_always_inline int
> > +rte_st_ring_at_head_dequeue(struct rte_st_ring *r, void **obj_p) {
> > +	return rte_st_ring_dequeue_elem(r, obj_p, sizeof(void *)); }
> > +
> > +/**
> > + * Flush a ST ring.
> > + *
> > + * This function flush all the elements in a ST ring
> > + *
> > + * @warning
> > + * Make sure the ring is not in use while calling this function.
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + */
> > +void
> > +rte_st_ring_reset(struct rte_st_ring *r);
> > +
> > +/**
> > + * Return the number of entries in a ST ring.
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @return
> > + *   The number of entries in the ring.
> > + */
> > +static inline unsigned int
> > +rte_st_ring_count(const struct rte_st_ring *r) {
> > +	uint32_t count = (r->head - r->tail) & r->mask;
> > +	return count;
> > +}
> > +
> > +/**
> > + * Return the number of free entries in a ST ring.
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @return
> > + *   The number of free entries in the ring.
> > + */
> > +static inline unsigned int
> > +rte_st_ring_free_count(const struct rte_st_ring *r) {
> > +	return r->capacity - rte_st_ring_count(r); }
> > +
> > +/**
> > + * Test if a ST ring is full.
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @return
> > + *   - 1: The ring is full.
> > + *   - 0: The ring is not full.
> > + */
> > +static inline int
> > +rte_st_ring_full(const struct rte_st_ring *r) {
> > +	return rte_st_ring_free_count(r) == 0; }
> > +
> > +/**
> > + * Test if a ST ring is empty.
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @return
> > + *   - 1: The ring is empty.
> > + *   - 0: The ring is not empty.
> > + */
> > +static inline int
> > +rte_st_ring_empty(const struct rte_st_ring *r) {
> > +	return r->tail == r->head;
> > +}
> > +
> > +/**
> > + * Return the size of the ring.
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @return
> > + *   The size of the data store used by the ring.
> > + *   NOTE: this is not the same as the usable space in the ring. To query that
> > + *   use ``rte_st_ring_get_capacity()``.
> > + */
> > +static inline unsigned int
> > +rte_st_ring_get_size(const struct rte_st_ring *r) {
> > +	return r->size;
> > +}
> > +
> > +/**
> > + * Return the number of elements which can be stored in the ring.
> > + *
> > + * @param r
> > + *   A pointer to the ring structure.
> > + * @return
> > + *   The usable size of the ring.
> > + */
> > +static inline unsigned int
> > +rte_st_ring_get_capacity(const struct rte_st_ring *r) {
> > +	return r->capacity;
> > +}
> > +
> > +/**
> > + * Dump the status of all rings on the console
> > + *
> > + * @param f
> > + *   A pointer to a file for output
> > + */
> > +void rte_st_ring_list_dump(FILE *f);
> > +
> > +/**
> > + * Search a ST ring from its name
> > + *
> > + * @param name
> > + *   The name of the ring.
> > + * @return
> > + *   The pointer to the ring matching the name, or NULL if not found,
> > + *   with rte_errno set appropriately. Possible rte_errno values include:
> > + *    - ENOENT - required entry not available to return.
> > + */
> > +struct rte_st_ring *rte_st_ring_lookup(const char *name);
> > +
> > +#ifdef __cplusplus
> > +}
> > +#endif
> > +
> > +#endif /* _RTE_ST_RING_H_ */