Function __io_uring_flush_sq

src/queue.c:247–282

* Sync internal state with kernel ring state on the SQ side. Returns the * number of pending items in the SQ ring, for the shared ring. */

 * number of pending items in the SQ ring, for the shared ring.
 */
static unsigned __io_uring_flush_sq(struct io_uring *ring)
{
	struct io_uring_sq *sq = &ring->sq;
	unsigned tail = sq->sqe_tail;

	/*
	 * With IORING_SETUP_SQ_REWIND, the kernel ignores the SQ head / tail
	 * and submits entries from the beginning of the queue. Continue using
	 * the indices as before but reset the tail on submission. With the
	 * head kept to be zero, io_uring_get_sqe() / etc. will work without
	 * any extra changes.
	 */
	if (ring->flags & IORING_SETUP_SQ_REWIND) {
		sq->sqe_tail = 0;
		return tail;
	}

	if (sq->sqe_head != tail) {
		sq->sqe_head = tail;
		/*
		 * Ensure kernel sees the SQE updates before the tail update.
		 */
		if (!(ring->flags & IORING_SETUP_SQPOLL))
			*sq->ktail = tail;
		else
			io_uring_smp_store_release(sq->ktail, tail);
	}
	/*
	* This load needs to be atomic, since sq->khead is written concurrently
	* by the kernel, but it doesn't need to be load_acquire, since the
	* kernel doesn't store to the submission queue; it advances khead just
	* to indicate that it's finished reading the submission queue entries
	* so they're available for us to write to.
	*/
	return tail - IO_URING_READ_ONCE(*sq->khead);
}

/*
 * If we have kernel support for IORING_ENTER_EXT_ARG, then we can use that