git: 44e1cfca417c - main - buf_ring: Use atomic operations with br_cons_tail

The branch main has been updated by andrew:

URL: https://cgit.FreeBSD.org/src/commit/?id=44e1cfca417c5ef0db908f3836ec3ba704ef1de2

commit 44e1cfca417c5ef0db908f3836ec3ba704ef1de2
Author:     Andrew Turner <andrew@FreeBSD.org>
AuthorDate: 2024-08-19 09:06:52 +0000
Commit:     Andrew Turner <andrew@FreeBSD.org>
CommitDate: 2024-08-19 10:53:11 +0000

    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 <alisaidi@amazon.com>
    Co-developed by: Ali Saidi <alisaidi@amazon.com>
    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 dec0f971ae44..9aeb5aa19fa8 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);
 }
 
 /*