diff --git a/ext/io/event/selector/uring.c b/ext/io/event/selector/uring.c
index 65ca8f5d..d9be022f 100644
--- a/ext/io/event/selector/uring.c
+++ b/ext/io/event/selector/uring.c
@@ -34,6 +34,7 @@
 
 enum {
 	DEBUG = 0,
+	DEBUG_COMPLETION = 0,
 };
 
 static VALUE IO_Event_Selector_URing = Qnil;
@@ -165,6 +166,8 @@ struct IO_Event_Selector_URing_Completion * IO_Event_Selector_URing_Completion_a
 		IO_Event_List_clear(&completion->list);
 	}
 	
+	if (DEBUG_COMPLETION) fprintf(stderr, "IO_Event_Selector_URing_Completion_acquire(%p, limit=%ld)\n", (void*)completion, selector->completions.limit);
+	
 	waiting->completion = completion;
 	completion->waiting = waiting;
 	
@@ -174,6 +177,8 @@ struct IO_Event_Selector_URing_Completion * IO_Event_Selector_URing_Completion_a
 inline static
 void IO_Event_Selector_URing_Completion_cancel(struct IO_Event_Selector_URing_Completion *completion)
 {
+	if (DEBUG_COMPLETION) fprintf(stderr, "IO_Event_Selector_URing_Completion_cancel(%p)\n", (void*)completion);
+	
 	if (completion->waiting) {
 		completion->waiting->completion = NULL;
 		completion->waiting = NULL;
@@ -183,13 +188,17 @@ void IO_Event_Selector_URing_Completion_cancel(struct IO_Event_Selector_URing_Co
 inline static
 void IO_Event_Selector_URing_Completion_release(struct IO_Event_Selector_URing *selector, struct IO_Event_Selector_URing_Completion *completion)
 {
+	if (DEBUG_COMPLETION) fprintf(stderr, "IO_Event_Selector_URing_Completion_release(%p)\n", (void*)completion);
+	
 	IO_Event_Selector_URing_Completion_cancel(completion);
 	IO_Event_List_prepend(&selector->free_list, &completion->list);
 }
 
 inline static
-void IO_Event_Selector_URing_Waiting_cancel(struct IO_Event_Selector_URing *selector, struct IO_Event_Selector_URing_Waiting *waiting)
+void IO_Event_Selector_URing_Waiting_cancel(struct IO_Event_Selector_URing_Waiting *waiting)
 {
+	if (DEBUG_COMPLETION) fprintf(stderr, "IO_Event_Selector_URing_Waiting_cancel(%p, %p)\n", (void*)waiting, (void*)waiting->completion);
+	
 	if (waiting->completion) {
 		waiting->completion->waiting = NULL;
 		waiting->completion = NULL;
@@ -198,10 +207,13 @@ void IO_Event_Selector_URing_Waiting_cancel(struct IO_Event_Selector_URing *sele
 	waiting->fiber = 0;
 }
 
+struct IO_Event_List_Type IO_Event_Selector_URing_Completion_Type = {};
+
 void IO_Event_Selector_URing_Completion_initialize(void *element)
 {
 	struct IO_Event_Selector_URing_Completion *completion = element;
 	IO_Event_List_initialize(&completion->list);
+	completion->list.type = &IO_Event_Selector_URing_Completion_Type;
 }
 
 void IO_Event_Selector_URing_Completion_free(void *element)
@@ -357,6 +369,12 @@ int io_uring_submit_now(struct IO_Event_Selector_URing *selector) {
 	}
 }
 
+static
+void IO_Event_Selector_URing_submit_sqe(struct io_uring_sqe *sqe)
+{
+	if (DEBUG) fprintf(stderr, "IO_Event_Selector_URing_submit_sqe(%p): user_data=%p opcode=%d\n", sqe, (void*)sqe->user_data, sqe->opcode);
+}
+
 // Submit a pending operation. This does not submit the operation immediately, but instead defers it to the next call to `io_uring_submit_flush` or `io_uring_submit_now`. This is useful for operations that are not urgent, but should be used with care as it can lead to a deadlock if the submission queue is not flushed.
 static
 void io_uring_submit_pending(struct IO_Event_Selector_URing *selector) {
@@ -407,7 +425,7 @@ VALUE process_wait_ensure(VALUE _arguments) {
 	
 	close(arguments->descriptor);
 	
-	IO_Event_Selector_URing_Waiting_cancel(arguments->selector, arguments->waiting);
+	IO_Event_Selector_URing_Waiting_cancel(arguments->waiting);
 	
 	return Qnil;
 }
@@ -442,6 +460,7 @@ VALUE IO_Event_Selector_URing_process_wait(VALUE self, VALUE fiber, VALUE _pid,
 	if (DEBUG) fprintf(stderr, "IO_Event_Selector_URing_process_wait:io_uring_prep_poll_add(%p)\n", (void*)fiber);
 	io_uring_prep_poll_add(sqe, descriptor, POLLIN|POLLHUP|POLLERR);
 	io_uring_sqe_set_data(sqe, completion);
+	IO_Event_Selector_URing_submit_sqe(sqe);
 	io_uring_submit_pending(selector);
 	
 	return rb_ensure(process_wait_transfer, (VALUE)&process_wait_arguments, process_wait_ensure, (VALUE)&process_wait_arguments);
@@ -485,12 +504,18 @@ static
 VALUE io_wait_ensure(VALUE _arguments) {
 	struct io_wait_arguments *arguments = (struct io_wait_arguments *)_arguments;
 	
-	// We may want to consider cancellation. Be aware that the order of operations is important here:
-	// io_uring_prep_cancel(sqe, (void*)arguments->waiting, 0);
-	// io_uring_sqe_set_data(sqe, NULL);
-	// io_uring_submit_now(selector);
+	if (DEBUG) fprintf(stderr, "io_wait_ensure:io_uring_prep_cancel(waiting=%p, completion=%p)\n", (void*)arguments->waiting, (void*)arguments->waiting->completion);
 	
-	IO_Event_Selector_URing_Waiting_cancel(arguments->selector, arguments->waiting);
+	// If the operation is still in progress, cancel it:
+	if (arguments->waiting->completion) {
+		struct io_uring_sqe *sqe = io_get_sqe(arguments->selector);
+		io_uring_prep_cancel(sqe, (void*)arguments->waiting->completion, 0);
+		io_uring_sqe_set_data(sqe, NULL);
+		IO_Event_Selector_URing_submit_sqe(sqe);
+		io_uring_submit_now(arguments->selector);
+	}
+	
+	IO_Event_Selector_URing_Waiting_cancel(arguments->waiting);
 	
 	return Qnil;
 };
@@ -502,6 +527,8 @@ VALUE io_wait_transfer(VALUE _arguments) {
 	
 	IO_Event_Selector_fiber_transfer(selector->backend.loop, 0, NULL);
 	
+	if (DEBUG) fprintf(stderr, "io_wait_transfer:waiting=%p, result=%d\n", (void*)arguments->waiting, arguments->waiting->result);
+	
 	if (arguments->waiting->result) {
 		// We explicitly filter the resulting events based on the requested events.
 		// In some cases, poll will report events we didn't ask for.
@@ -531,6 +558,7 @@ VALUE IO_Event_Selector_URing_io_wait(VALUE self, VALUE fiber, VALUE io, VALUE e
 	struct IO_Event_Selector_URing_Completion *completion = IO_Event_Selector_URing_Completion_acquire(selector, &waiting);
 	
 	io_uring_sqe_set_data(sqe, completion);
+	IO_Event_Selector_URing_submit_sqe(sqe);
 	
 	// If we are going to wait, we assume that we are waiting for a while:
 	io_uring_submit_pending(selector);
@@ -581,10 +609,11 @@ io_read_submit(VALUE _arguments)
 	struct IO_Event_Selector_URing *selector = arguments->selector;
 	struct io_uring_sqe *sqe = io_get_sqe(selector);
 	
-	if (DEBUG) fprintf(stderr, "io_read_submit:io_uring_prep_read(fiber=%p, descriptor=%d, buffer=%p, length=%ld)\n", (void*)arguments->waiting, arguments->descriptor, arguments->buffer, arguments->length);
+	if (DEBUG) fprintf(stderr, "io_read_submit:io_uring_prep_read(waiting=%p, completion=%p, descriptor=%d, buffer=%p, length=%ld)\n", (void*)arguments->waiting, (void*)arguments->waiting->completion, arguments->descriptor, arguments->buffer, arguments->length);
 	
 	io_uring_prep_read(sqe, arguments->descriptor, arguments->buffer, arguments->length, io_seekable(arguments->descriptor));
 	io_uring_sqe_set_data(sqe, arguments->waiting->completion);
+	IO_Event_Selector_URing_submit_sqe(sqe);
 	io_uring_submit_now(selector);
 	
 	IO_Event_Selector_fiber_transfer(selector->backend.loop, 0, NULL);
@@ -600,16 +629,17 @@ io_read_ensure(VALUE _arguments)
 	
 	struct io_uring_sqe *sqe = io_get_sqe(selector);
 	
-	if (DEBUG) fprintf(stderr, "io_read_cancel:io_uring_prep_cancel(fiber=%p)\n", (void*)arguments->waiting);
+	if (DEBUG) fprintf(stderr, "io_read_ensure:io_uring_prep_cancel(waiting=%p, completion=%p)\n", (void*)arguments->waiting, (void*)arguments->waiting->completion);
 	
-	// If the operation has already completed, we don't need to cancel it:
-	if (!arguments->waiting->result) {
-		io_uring_prep_cancel(sqe, (void*)arguments->waiting, 0);
+	// If the operation is still in progress, cancel it:
+	if (arguments->waiting->completion) {
+		io_uring_prep_cancel(sqe, (void*)arguments->waiting->completion, 0);
 		io_uring_sqe_set_data(sqe, NULL);
+		IO_Event_Selector_URing_submit_sqe(sqe);
 		io_uring_submit_now(selector);
 	}
 	
-	IO_Event_Selector_URing_Waiting_cancel(arguments->selector, arguments->waiting);
+	IO_Event_Selector_URing_Waiting_cancel(arguments->waiting);
 	
 	return Qnil;
 }
@@ -704,10 +734,11 @@ io_write_submit(VALUE _argument)
 	
 	struct io_uring_sqe *sqe = io_get_sqe(selector);
 	
-	if (DEBUG) fprintf(stderr, "io_write_submit:io_uring_prep_write(fiber=%p, descriptor=%d, buffer=%p, length=%ld)\n", (void*)arguments->waiting, arguments->descriptor, arguments->buffer, arguments->length);
+	if (DEBUG) fprintf(stderr, "io_write_submit:io_uring_prep_write(waiting=%p, completion=%p, descriptor=%d, buffer=%p, length=%ld)\n", (void*)arguments->waiting, (void*)arguments->waiting->completion, arguments->descriptor, arguments->buffer, arguments->length);
 	
 	io_uring_prep_write(sqe, arguments->descriptor, arguments->buffer, arguments->length, io_seekable(arguments->descriptor));
 	io_uring_sqe_set_data(sqe, arguments->waiting->completion);
+	IO_Event_Selector_URing_submit_sqe(sqe);
 	io_uring_submit_pending(selector);
 	
 	IO_Event_Selector_fiber_transfer(selector->backend.loop, 0, NULL);
@@ -723,15 +754,17 @@ io_write_ensure(VALUE _argument)
 	
 	struct io_uring_sqe *sqe = io_get_sqe(selector);
 	
-	if (DEBUG) fprintf(stderr, "io_wait_rescue:io_uring_prep_cancel(%p)\n", (void*)arguments->waiting);
+	if (DEBUG) fprintf(stderr, "io_write_ensure:io_uring_prep_cancel(waiting=%p, completion=%p)\n", (void*)arguments->waiting, (void*)arguments->waiting->completion);
 	
-	if (!arguments->waiting->result) {
-		io_uring_prep_cancel(sqe, (void*)arguments->waiting, 0);
+	// If the operation is still in progress, cancel it:
+	if (arguments->waiting->completion) {
+		io_uring_prep_cancel(sqe, (void*)arguments->waiting->completion, 0);
 		io_uring_sqe_set_data(sqe, NULL);
+		IO_Event_Selector_URing_submit_sqe(sqe);
 		io_uring_submit_now(selector);
 	}
 	
-	IO_Event_Selector_URing_Waiting_cancel(arguments->selector, arguments->waiting);
+	IO_Event_Selector_URing_Waiting_cancel(arguments->waiting);
 	
 	return Qnil;
 }
@@ -829,6 +862,7 @@ VALUE IO_Event_Selector_URing_io_close(VALUE self, VALUE io) {
 		
 		io_uring_prep_close(sqe, descriptor);
 		io_uring_sqe_set_data(sqe, NULL);
+		IO_Event_Selector_URing_submit_sqe(sqe);
 		io_uring_submit_now(selector);
 	} else {
 		close(descriptor);
@@ -919,32 +953,38 @@ unsigned select_process_completions(struct IO_Event_Selector_URing *selector) {
 	unsigned head;
 	struct io_uring_cqe *cqe;
 	
+	if (DEBUG) fprintf(stderr, "select_process_completions...\n");
+	
 	io_uring_for_each_cqe(ring, head, cqe) {
+		if (DEBUG) fprintf(stderr, "select_process_completions: cqe res=%d user_data=%p\n", cqe->res, (void*)cqe->user_data);
+		
 		++completed;
 		
-		// If the operation was cancelled, or the operation has no user data (fiber):
+		// If the operation was cancelled, or the operation has no user data:
 		if (cqe->user_data == 0 || cqe->user_data == LIBURING_UDATA_TIMEOUT) {
 			io_uring_cq_advance(ring, 1);
 			continue;
 		}
 		
-		if (DEBUG) fprintf(stderr, "cqe res=%d user_data=%p\n", cqe->res, (void*)cqe->user_data);
-		
 		struct IO_Event_Selector_URing_Completion *completion = (void*)cqe->user_data;
 		struct IO_Event_Selector_URing_Waiting *waiting = completion->waiting;
 		
+		if (DEBUG) fprintf(stderr, "select_process_completions: completion=%p waiting=%p\n", (void*)completion, (void*)waiting);
+		
 		if (waiting) {
 			waiting->result = cqe->res;
 			waiting->flags = cqe->flags;
 		}
 		
-		io_uring_cq_advance(ring, 1);
-		
 		if (waiting && waiting->fiber) {
+			assert(waiting->result != -ECANCELED);
+			
 			IO_Event_Selector_fiber_transfer(waiting->fiber, 0, NULL);
 		}
 		
+		// This marks the waiting operation as "complete":
 		IO_Event_Selector_URing_Completion_release(selector, completion);
+		io_uring_cq_advance(ring, 1);
 	}
 	
 	if (DEBUG && completed > 0) fprintf(stderr, "select_process_completions(completed=%d)\n", completed);