From 44e1cfca417c5ef0db908f3836ec3ba704ef1de2 Mon Sep 17 00:00:00 2001 From: Andrew Turner Date: Mon, 19 Aug 2024 10:06:52 +0100 Subject: [PATCH] buf_ring: Use atomic operations with br_cons_tail Use an atomic operation with a memory barrier loading br_cons_tail from the producer thread and storing to it in the consumer thread. On dequeue we need to read the pointer value from the buf_ring before moving the consumer tail as that indicates the entry is available to be used. The store release atomic operation guarantees this. In the enqueueing thread we then need to use a load acquire atomic operation to ensure writing to this entry can only happen after the tail has been read and checked. Reported by: Ali Saidi Co-developed by: Ali Saidi Reviewed by: markj Sponsored by: Arm Ltd Differential Revision: https://reviews.freebsd.org/D46152 --- sys/sys/buf_ring.h | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/sys/sys/buf_ring.h b/sys/sys/buf_ring.h index dec0f971ae4..9aeb5aa19fa 100644 --- a/sys/sys/buf_ring.h +++ b/sys/sys/buf_ring.h @@ -91,7 +91,7 @@ buf_ring_enqueue(struct buf_ring *br, void *buf) do { prod_head = br->br_prod_head; prod_next = prod_head + 1; - cons_tail = br->br_cons_tail; + cons_tail = atomic_load_acq_32(&br->br_cons_tail); if ((int32_t)(cons_tail + br->br_prod_size - prod_next) < 1) { rmb(); @@ -229,7 +229,7 @@ buf_ring_dequeue_sc(struct buf_ring *br) panic("inconsistent list cons_tail=%d cons_head=%d", br->br_cons_tail, cons_head); #endif - br->br_cons_tail = cons_next; + atomic_store_rel_32(&br->br_cons_tail, cons_next); return (buf); } @@ -257,7 +257,7 @@ buf_ring_advance_sc(struct buf_ring *br) #ifdef DEBUG_BUFRING br->br_ring[cons_head & mask] = NULL; #endif - br->br_cons_tail = cons_next; + atomic_store_rel_32(&br->br_cons_tail, cons_next); } /*