Add option for getting a completion without waiting

This now exposes two helpers:

io_uring_get_completion()
	Return a completion, if we have one (or more) available in
	the ring

io_uring_wait_completion()
	Return a completion, waiting for it if necessary

Signed-off-by: Jens Axboe <axboe@kernel.dk>

src/io_uring.c

@@ -9,38 +9,54 @@
 #include "liburing.h"
 #include "barrier.h"
 
-/*
+static int __io_uring_get_completion(int fd, struct io_uring_cq *cq,
- * Return an IO completion, waiting for it it necessary.
+				     struct io_uring_event **ev_ptr, int wait)
- */
-int io_uring_get_completion(int fd, struct io_uring_cq *cq,
-			    struct io_uring_event **ev_ptr)
 {
 	const unsigned mask = *cq->kring_mask;
-	struct io_uring_event *ev = NULL;
 	unsigned head;
 	int ret;
 
+	*ev_ptr = NULL;
 	head = *cq->khead;
 	do {
 		read_barrier();
 		if (head != *cq->ktail) {
-			ev = &cq->events[head & mask];
+			*ev_ptr = &cq->events[head & mask];
 			break;
 		}
+		if (!wait)
+			break;
 		ret = io_uring_enter(fd, 0, 1, IORING_ENTER_GETEVENTS);
 		if (ret < 0)
 			return -errno;
 	} while (1);
 
-	if (ev) {
+	if (*ev_ptr) {
 		*cq->khead = head + 1;
 		write_barrier();
 	}
 
-	*ev_ptr = ev;
 	return 0;
 }
 
+/*
+ * Return an IO completion, if one is readily available
+ */
+int io_uring_get_completion(int fd, struct io_uring_cq *cq,
+			    struct io_uring_event **ev_ptr)
+{
+	return __io_uring_get_completion(fd, cq, ev_ptr, 0);
+}
+
+/*
+ * Return an IO completion, waiting for it if necessary
+ */
+int io_uring_wait_completion(int fd, struct io_uring_cq *cq,
+			     struct io_uring_event **ev_ptr)
+{
+	return __io_uring_get_completion(fd, cq, ev_ptr, 1);
+}
+
 /*
  * Submit iocbs acquired from io_uring_get_iocb() to the kernel.
  *

src/liburing.h

@@ -51,6 +51,8 @@ extern void io_uring_queue_exit(int fd, struct io_uring_sq *sq,
 	struct io_uring_cq *cq);
 extern int io_uring_get_completion(int fd, struct io_uring_cq *cq,
 	struct io_uring_event **ev_ptr);
+extern int io_uring_wait_completion(int fd, struct io_uring_cq *cq,
+	struct io_uring_event **ev_ptr);
 extern int io_uring_submit(int fd, struct io_uring_sq *sq);
 extern struct io_uring_iocb *io_uring_get_iocb(struct io_uring_sq *sq);
 

src/liburing.map

@@ -3,6 +3,7 @@ LIBURING_0.1 {
 		io_uring_queue_init;
 		io_uring_queue_exit;
 		io_uring_get_completion;
+		io_uring_wait_completion;
 		io_uring_submit;
 		io_uring_get_iocb;
 	local: