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![Magnus Karlsson](/assets/img/avatar_default.png)
The ring buffer code of XDP sockets is missing a memory barrier on the consumer side between the load of the data and the write that signals that it is ok for the producer to put new data into the buffer. On architectures that does not guarantee that stores are not reordered with older loads, the producer might put data into the ring before the consumer had the chance to read it. As IA does guarantee this ordering, it would only need a compiler barrier here, but there are no primitives in Linux for this specific case (hinder writes to be ordered before older reads) so I had to add a smp_mb() here which will translate into a run-time synch operation on IA. Added a longish comment in the code explaining what each barrier in the ring implementation accomplishes and what would happen if we removed one of them. Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
312 lines
7.9 KiB
C
312 lines
7.9 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/* XDP user-space ring structure
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* Copyright(c) 2018 Intel Corporation.
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*/
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#ifndef _LINUX_XSK_QUEUE_H
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#define _LINUX_XSK_QUEUE_H
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#include <linux/types.h>
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#include <linux/if_xdp.h>
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#include <net/xdp_sock.h>
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#define RX_BATCH_SIZE 16
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#define LAZY_UPDATE_THRESHOLD 128
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struct xdp_ring {
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u32 producer ____cacheline_aligned_in_smp;
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u32 consumer ____cacheline_aligned_in_smp;
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};
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/* Used for the RX and TX queues for packets */
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struct xdp_rxtx_ring {
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struct xdp_ring ptrs;
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struct xdp_desc desc[0] ____cacheline_aligned_in_smp;
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};
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/* Used for the fill and completion queues for buffers */
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struct xdp_umem_ring {
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struct xdp_ring ptrs;
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u64 desc[0] ____cacheline_aligned_in_smp;
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};
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struct xsk_queue {
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u64 chunk_mask;
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u64 size;
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u32 ring_mask;
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u32 nentries;
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u32 prod_head;
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u32 prod_tail;
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u32 cons_head;
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u32 cons_tail;
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struct xdp_ring *ring;
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u64 invalid_descs;
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};
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/* The structure of the shared state of the rings are the same as the
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* ring buffer in kernel/events/ring_buffer.c. For the Rx and completion
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* ring, the kernel is the producer and user space is the consumer. For
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* the Tx and fill rings, the kernel is the consumer and user space is
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* the producer.
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*
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* producer consumer
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*
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* if (LOAD ->consumer) { LOAD ->producer
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* (A) smp_rmb() (C)
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* STORE $data LOAD $data
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* smp_wmb() (B) smp_mb() (D)
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* STORE ->producer STORE ->consumer
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* }
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*
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* (A) pairs with (D), and (B) pairs with (C).
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*
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* Starting with (B), it protects the data from being written after
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* the producer pointer. If this barrier was missing, the consumer
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* could observe the producer pointer being set and thus load the data
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* before the producer has written the new data. The consumer would in
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* this case load the old data.
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*
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* (C) protects the consumer from speculatively loading the data before
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* the producer pointer actually has been read. If we do not have this
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* barrier, some architectures could load old data as speculative loads
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* are not discarded as the CPU does not know there is a dependency
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* between ->producer and data.
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*
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* (A) is a control dependency that separates the load of ->consumer
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* from the stores of $data. In case ->consumer indicates there is no
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* room in the buffer to store $data we do not. So no barrier is needed.
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*
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* (D) protects the load of the data to be observed to happen after the
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* store of the consumer pointer. If we did not have this memory
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* barrier, the producer could observe the consumer pointer being set
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* and overwrite the data with a new value before the consumer got the
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* chance to read the old value. The consumer would thus miss reading
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* the old entry and very likely read the new entry twice, once right
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* now and again after circling through the ring.
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*/
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/* Common functions operating for both RXTX and umem queues */
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static inline u64 xskq_nb_invalid_descs(struct xsk_queue *q)
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{
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return q ? q->invalid_descs : 0;
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}
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static inline u32 xskq_nb_avail(struct xsk_queue *q, u32 dcnt)
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{
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u32 entries = q->prod_tail - q->cons_tail;
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if (entries == 0) {
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/* Refresh the local pointer */
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q->prod_tail = READ_ONCE(q->ring->producer);
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entries = q->prod_tail - q->cons_tail;
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}
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return (entries > dcnt) ? dcnt : entries;
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}
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static inline u32 xskq_nb_free(struct xsk_queue *q, u32 producer, u32 dcnt)
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{
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u32 free_entries = q->nentries - (producer - q->cons_tail);
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if (free_entries >= dcnt)
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return free_entries;
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/* Refresh the local tail pointer */
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q->cons_tail = READ_ONCE(q->ring->consumer);
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return q->nentries - (producer - q->cons_tail);
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}
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/* UMEM queue */
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static inline bool xskq_is_valid_addr(struct xsk_queue *q, u64 addr)
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{
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if (addr >= q->size) {
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q->invalid_descs++;
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return false;
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}
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return true;
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}
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static inline u64 *xskq_validate_addr(struct xsk_queue *q, u64 *addr)
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{
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while (q->cons_tail != q->cons_head) {
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struct xdp_umem_ring *ring = (struct xdp_umem_ring *)q->ring;
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unsigned int idx = q->cons_tail & q->ring_mask;
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*addr = READ_ONCE(ring->desc[idx]) & q->chunk_mask;
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if (xskq_is_valid_addr(q, *addr))
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return addr;
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q->cons_tail++;
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}
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return NULL;
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}
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static inline u64 *xskq_peek_addr(struct xsk_queue *q, u64 *addr)
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{
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if (q->cons_tail == q->cons_head) {
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smp_mb(); /* D, matches A */
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WRITE_ONCE(q->ring->consumer, q->cons_tail);
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q->cons_head = q->cons_tail + xskq_nb_avail(q, RX_BATCH_SIZE);
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/* Order consumer and data */
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smp_rmb();
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}
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return xskq_validate_addr(q, addr);
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}
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static inline void xskq_discard_addr(struct xsk_queue *q)
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{
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q->cons_tail++;
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}
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static inline int xskq_produce_addr(struct xsk_queue *q, u64 addr)
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{
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struct xdp_umem_ring *ring = (struct xdp_umem_ring *)q->ring;
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if (xskq_nb_free(q, q->prod_tail, 1) == 0)
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return -ENOSPC;
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/* A, matches D */
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ring->desc[q->prod_tail++ & q->ring_mask] = addr;
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/* Order producer and data */
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smp_wmb(); /* B, matches C */
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WRITE_ONCE(q->ring->producer, q->prod_tail);
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return 0;
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}
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static inline int xskq_produce_addr_lazy(struct xsk_queue *q, u64 addr)
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{
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struct xdp_umem_ring *ring = (struct xdp_umem_ring *)q->ring;
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if (xskq_nb_free(q, q->prod_head, LAZY_UPDATE_THRESHOLD) == 0)
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return -ENOSPC;
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/* A, matches D */
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ring->desc[q->prod_head++ & q->ring_mask] = addr;
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return 0;
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}
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static inline void xskq_produce_flush_addr_n(struct xsk_queue *q,
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u32 nb_entries)
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{
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/* Order producer and data */
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smp_wmb(); /* B, matches C */
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q->prod_tail += nb_entries;
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WRITE_ONCE(q->ring->producer, q->prod_tail);
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}
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static inline int xskq_reserve_addr(struct xsk_queue *q)
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{
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if (xskq_nb_free(q, q->prod_head, 1) == 0)
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return -ENOSPC;
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/* A, matches D */
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q->prod_head++;
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return 0;
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}
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/* Rx/Tx queue */
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static inline bool xskq_is_valid_desc(struct xsk_queue *q, struct xdp_desc *d)
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{
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if (!xskq_is_valid_addr(q, d->addr))
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return false;
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if (((d->addr + d->len) & q->chunk_mask) != (d->addr & q->chunk_mask) ||
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d->options) {
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q->invalid_descs++;
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return false;
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}
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return true;
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}
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static inline struct xdp_desc *xskq_validate_desc(struct xsk_queue *q,
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struct xdp_desc *desc)
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{
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while (q->cons_tail != q->cons_head) {
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struct xdp_rxtx_ring *ring = (struct xdp_rxtx_ring *)q->ring;
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unsigned int idx = q->cons_tail & q->ring_mask;
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*desc = READ_ONCE(ring->desc[idx]);
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if (xskq_is_valid_desc(q, desc))
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return desc;
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q->cons_tail++;
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}
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return NULL;
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}
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static inline struct xdp_desc *xskq_peek_desc(struct xsk_queue *q,
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struct xdp_desc *desc)
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{
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if (q->cons_tail == q->cons_head) {
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smp_mb(); /* D, matches A */
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WRITE_ONCE(q->ring->consumer, q->cons_tail);
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q->cons_head = q->cons_tail + xskq_nb_avail(q, RX_BATCH_SIZE);
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/* Order consumer and data */
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smp_rmb(); /* C, matches B */
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}
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return xskq_validate_desc(q, desc);
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}
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static inline void xskq_discard_desc(struct xsk_queue *q)
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{
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q->cons_tail++;
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}
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static inline int xskq_produce_batch_desc(struct xsk_queue *q,
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u64 addr, u32 len)
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{
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struct xdp_rxtx_ring *ring = (struct xdp_rxtx_ring *)q->ring;
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unsigned int idx;
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if (xskq_nb_free(q, q->prod_head, 1) == 0)
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return -ENOSPC;
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/* A, matches D */
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idx = (q->prod_head++) & q->ring_mask;
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ring->desc[idx].addr = addr;
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ring->desc[idx].len = len;
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return 0;
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}
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static inline void xskq_produce_flush_desc(struct xsk_queue *q)
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{
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/* Order producer and data */
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smp_wmb(); /* B, matches C */
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q->prod_tail = q->prod_head,
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WRITE_ONCE(q->ring->producer, q->prod_tail);
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}
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static inline bool xskq_full_desc(struct xsk_queue *q)
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{
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return xskq_nb_avail(q, q->nentries) == q->nentries;
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}
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static inline bool xskq_empty_desc(struct xsk_queue *q)
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{
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return xskq_nb_free(q, q->prod_tail, q->nentries) == q->nentries;
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}
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void xskq_set_umem(struct xsk_queue *q, u64 size, u64 chunk_mask);
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struct xsk_queue *xskq_create(u32 nentries, bool umem_queue);
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void xskq_destroy(struct xsk_queue *q_ops);
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/* Executed by the core when the entire UMEM gets freed */
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void xsk_reuseq_destroy(struct xdp_umem *umem);
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#endif /* _LINUX_XSK_QUEUE_H */
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