toys

commit 027d9b04bb1fae62fbb1c842395ad1dacacc121f
parent 23ce869de42558066c42f905411e89354040cf23
Author: Mikołaj Lenczewski <mikolaj@lenczewski.org>
Date:   Tue,  9 Dec 2025 12:18:36 +0000

Warn about usefulness of spsc_queue_length()

Diffstat:
M
queue.h
 | 
20
++++++++++++++++++--

1 file changed, 18 insertions(+), 2 deletions(-)
diff --git a/queue.h b/queue.h
@@ -220,11 +220,27 @@ spsc_queue_free(struct spsc_queue *queue)
 	munmap(queue->ptr, queue->cap * 2);
 }
 
+// NOTE: this (spsc_queue_length()) and its cousin (spsc_queue_capacity()) are
+//       not a great interface. For one, they are slow as molasses, and to be
+//       honest I'm neither sure of it being correct or it being useful (since
+//       in high-contention scenarios the length of the queue at the moment of
+//       calling the function will become stale pretty quickly). It might be
+//       far better to simply attempt a read or write, and detect a full queue
+//       by the return value (a return value of NULL means it was not possible
+//       to reserve a buffer of the given size).
+//
+//       Included solely for "feature parity" with the unsynchronised queue
+
 inline size_t
 spsc_queue_length(struct spsc_queue *queue)
 {
-	size_t head = atomic_load_explicit(&queue->head, memory_order_acquire);
-	size_t tail = atomic_load_explicit(&queue->tail, memory_order_acquire);
+	size_t head, tail;
+
+	do {
+		head = atomic_load_explicit(&queue->head, memory_order_acquire);
+		tail = atomic_load_explicit(&queue->tail, memory_order_acquire);
+	} while (atomic_load_explicit(&queue->head, memory_order_acquire) != head);
+
 	return head - tail;
 }