[PATCH bpf v2 7/8] libbpf: ringbuf: Use compiler atomics

[Tamir Duberstein]

Consumer-side ring buffer code uses architecture-specific smp_* helpers
for shared memory accesses.

Use compiler atomics instead. They provide equivalent acquire and
release ordering through a portable userspace interface and allow the
next commit to use compiler fences in the wakeup protocol without mixing
atomic interfaces.

Reviewed-by: Emil Tsalapatis <emil@xxxxxxxxxxxxxxx>
Assisted-by: Codex:gpt-5.5
Signed-off-by: Tamir Duberstein <tamird@xxxxxxxxxx>
---
 tools/lib/bpf/ringbuf.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
index 1c5bce2b5e12..141f2cbe56eb 100644
--- a/tools/lib/bpf/ringbuf.c
+++ b/tools/lib/bpf/ringbuf.c
@@ -270,14 +270,14 @@ static int64_t ringbuf_process_ring(struct ring *r, size_t n)
 	if (n == 0)
 		return 0;
 
-	cons_pos = smp_load_acquire(r->consumer_pos);
+	cons_pos = __atomic_load_n(r->consumer_pos, __ATOMIC_ACQUIRE);
 	do {
 		got_new_data = false;
-		prod_pos = smp_load_acquire(r->producer_pos);
+		prod_pos = __atomic_load_n(r->producer_pos, __ATOMIC_ACQUIRE);
 		/* Positions wrap; the consumer cannot logically pass the producer. */
 		while (cons_pos != prod_pos) {
 			len_ptr = r->data + (cons_pos & r->mask);
-			len = smp_load_acquire(len_ptr);
+			len = __atomic_load_n(len_ptr, __ATOMIC_ACQUIRE);
 
 			/* sample not committed yet, bail out for now */
 			if (len & BPF_RINGBUF_BUSY_BIT)
@@ -291,14 +291,16 @@ static int64_t ringbuf_process_ring(struct ring *r, size_t n)
 				err = r->sample_cb(r->ctx, sample, len);
 				if (err < 0) {
 					/* update consumer pos and bail out */
-					smp_store_release(r->consumer_pos,
-							  cons_pos);
+					__atomic_store_n(r->consumer_pos,
+							 cons_pos,
+							 __ATOMIC_RELEASE);
 					return err;
 				}
 				cnt++;
 			}
 
-			smp_store_release(r->consumer_pos, cons_pos);
+			__atomic_store_n(r->consumer_pos, cons_pos,
+					 __ATOMIC_RELEASE);
 
 			if (cnt >= n)
 				goto done;
@@ -413,8 +415,8 @@ struct ring *ring_buffer__ring(struct ring_buffer *rb, unsigned int idx)
 
 unsigned long ring__consumer_pos(const struct ring *r)
 {
-	/* Synchronizes with smp_store_release() in ringbuf_process_ring(). */
-	return smp_load_acquire(r->consumer_pos);
+	/* Synchronizes with the release store in ringbuf_process_ring(). */
+	return __atomic_load_n(r->consumer_pos, __ATOMIC_ACQUIRE);
 }
 
 unsigned long ring__producer_pos(const struct ring *r)
@@ -422,7 +424,7 @@ unsigned long ring__producer_pos(const struct ring *r)
 	/* Synchronizes with smp_store_release() in __bpf_ringbuf_reserve() in
 	 * the kernel.
 	 */
-	return smp_load_acquire(r->producer_pos);
+	return __atomic_load_n(r->producer_pos, __ATOMIC_ACQUIRE);
 }
 
 size_t ring__avail_data_size(const struct ring *r)

-- 
2.55.0.rc0.159.gbe5d7338c2