iouring msg ring

实现源码: io_uring/msg_ring.c

op 为 IORING_OP_MSG_RING ，一共两个模式:

/*
 * IORING_OP_MSG_RING command types, stored in sqe->addr
 */
enum io_uring_msg_ring_flags {
	IORING_MSG_DATA,	/* pass sqe->len as 'res' and off as user_data */
	IORING_MSG_SEND_FD,	/* send a registered fd to another ring */
};

commit e6130eba8a84 (“io_uring: add support for passing fixed file descriptors”)

这里解释了两个使用场景:

Two common use cases for this are:

1) Server needs to be shutdown or restarted, pass file descriptors to another one

2) Backend is split, and one accepts connections, while others then get the fd passed and handle the actual connection.

哦，提供了 io_uring_prep_msg_ring 和 io_uring_prep_msg_ring_fd 来作为 内核中，不得不说，使用 io_uring_sqe::addr 来区分，还是有点傻的。

IOURINGINLINE void io_uring_prep_msg_ring(struct io_uring_sqe *sqe, int fd,
					  unsigned int len, __u64 data,
					  unsigned int flags)
	LIBURING_NOEXCEPT
{
	io_uring_prep_rw(IORING_OP_MSG_RING, sqe, fd, NULL, len, data);
	sqe->msg_ring_flags = flags;
}

IOURINGINLINE void io_uring_prep_msg_ring_fd(struct io_uring_sqe *sqe, int fd,
					     int source_fd, int target_fd,
					     __u64 data, unsigned int flags)
	LIBURING_NOEXCEPT
{
	io_uring_prep_rw(IORING_OP_MSG_RING, sqe, fd,
			 (void *) (uintptr_t) IORING_MSG_SEND_FD, 0, data);
	sqe->addr3 = source_fd;
	/* offset by 1 for allocation */
	if ((unsigned int) target_fd == IORING_FILE_INDEX_ALLOC)
		target_fd--;
	__io_uring_set_target_fixed_file(sqe, target_fd);
	sqe->msg_ring_flags = flags;
}

一些 lock 的优化

https://lore.kernel.org/all/cover.1670384893.git.asml.silence@gmail.com/

https://www.phoronix.com/news/Decouple-TASK_WORK-TWA_SIGNAL

commit 0617bb500bfa (“io_uring/msg_ring: improve handling of target CQE posting”)

这个解释了 io_msg_send_fd 中为什么去使用 remote install 的操作:

	if (io_msg_need_remote(target_ctx))
		return io_msg_fd_remote(req);
	return io_msg_install_complete(req, issue_flags);

没细看，但是优化就是说，让任务在

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