Merge branch 'load-acquire/store-release barriers for'

Björn Töpel says:

====================

This two-patch series introduces load-acquire/store-release barriers
for the AF_XDP rings.

For most contemporary architectures, this is more effective than a
SPSC ring based on smp_{r,w,}mb() barriers. More importantly,
load-acquire/store-release semantics make the ring code easier to
follow.

This is effectively the change done in commit 6c43c091bdc5
("documentation: Update circular buffer for
load-acquire/store-release"), but for the AF_XDP rings.

Both libbpf and the kernel-side are updated.

Full details are outlined in the commits!

Thanks to the LKMM-folks (Paul/Alan/Will) for helping me out in this
complicated matter!

Changelog

v1[1]->v2:
* Expanded the commit message for patch 1, and included the LKMM
  litmus tests. Hopefully this clear things up. (Daniel)

* Clarified why the smp_mb()/smp_load_acquire() is not needed in (A);
  control dependency with load to store. (Toke)

[1] https://lore.kernel.org/bpf/20210301104318.263262-1-bjorn.topel@gmail.com/

Thanks,
Björn
====================

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

1 files changed, 13 insertions, 17 deletions

diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
index 2823b7c3302d..2ac3802c2cd7 100644
--- a/net/xdp/xsk_queue.h
+++ b/net/xdp/xsk_queue.h
@@ -47,19 +47,18 @@ struct xsk_queue {
 	u64 queue_empty_descs;
 };
 
-/* The structure of the shared state of the rings are the same as the
- * ring buffer in kernel/events/ring_buffer.c. For the Rx and completion
- * ring, the kernel is the producer and user space is the consumer. For
- * the Tx and fill rings, the kernel is the consumer and user space is
- * the producer.
+/* The structure of the shared state of the rings are a simple
+ * circular buffer, as outlined in
+ * Documentation/core-api/circular-buffers.rst. For the Rx and
+ * completion ring, the kernel is the producer and user space is the
+ * consumer. For the Tx and fill rings, the kernel is the consumer and
+ * user space is the producer.
  *
  * producer                         consumer
  *
- * if (LOAD ->consumer) {           LOAD ->producer
- *                    (A)           smp_rmb()       (C)
+ * if (LOAD ->consumer) {  (A)      LOAD.acq ->producer  (C)
  *    STORE $data                   LOAD $data
- *    smp_wmb()       (B)           smp_mb()        (D)
- *    STORE ->producer              STORE ->consumer
+ *    STORE.rel ->producer (B)      STORE.rel ->consumer (D)
  * }
  *
  * (A) pairs with (D), and (B) pairs with (C).
@@ -78,7 +77,8 @@ struct xsk_queue {
  *
  * (A) is a control dependency that separates the load of ->consumer
  * from the stores of $data. In case ->consumer indicates there is no
- * room in the buffer to store $data we do not. So no barrier is needed.
+ * room in the buffer to store $data we do not. The dependency will
+ * order both of the stores after the loads. So no barrier is needed.
  *
  * (D) protects the load of the data to be observed to happen after the
  * store of the consumer pointer. If we did not have this memory
@@ -227,15 +227,13 @@ static inline u32 xskq_cons_read_desc_batch(struct xsk_queue *q,
 
 static inline void __xskq_cons_release(struct xsk_queue *q)
 {
-	smp_mb(); /* D, matches A */
-	WRITE_ONCE(q->ring->consumer, q->cached_cons);
+	smp_store_release(&q->ring->consumer, q->cached_cons); /* D, matchees A */
 }
 
 static inline void __xskq_cons_peek(struct xsk_queue *q)
 {
 	/* Refresh the local pointer */
-	q->cached_prod = READ_ONCE(q->ring->producer);
-	smp_rmb(); /* C, matches B */
+	q->cached_prod = smp_load_acquire(&q->ring->producer);  /* C, matches B */
 }
 
 static inline void xskq_cons_get_entries(struct xsk_queue *q)
@@ -397,9 +395,7 @@ static inline int xskq_prod_reserve_desc(struct xsk_queue *q,
 
 static inline void __xskq_prod_submit(struct xsk_queue *q, u32 idx)
 {
-	smp_wmb(); /* B, matches C */
-
-	WRITE_ONCE(q->ring->producer, idx);
+	smp_store_release(&q->ring->producer, idx); /* B, matches C */
 }
 
 static inline void xskq_prod_submit(struct xsk_queue *q)