Behavior of io_uring_submit with unprepared SQEs

[DigoqueDigo]

The man page for io_uring_queue_submit says:

After the caller retrieves a submission queue entry (SQE) with io_uring_get_sqe(3) and prepares the SQE using one of the provided helpers, it can be submitted with io_uring_submit(3).

In this code, the next available submission queue entry is retrieved until it returns null, and finally a submit is performed. Considering that no SQE was actually prepared, how is it possible that io_uring_submit returns 2? This seems to indicate that 2 SQEs were prepared, when in fact that did not happen.

Perhaps this is expected behavior, but it seems illogical to me.

struct io_uring ring;

if (io_uring_queue_init(2, &ring, 0) < 0) {
    exit(1);
}

struct io_uring_sqe* sqe = io_uring_get_sqe(&ring);
std::cout << sqe << std::endl;

sqe = io_uring_get_sqe(&ring);
std::cout << sqe << std::endl;

sqe = io_uring_get_sqe(&ring);
std::cout << sqe << std::endl;

int submitted = io_uring_submit(&ring);
std::cout << submitted << std::endl;

exit(1);

$ ./foo
0x7f14e3b5f000
0x7f14e3b5f040
0
2

[krisman]

Diogo Marques ***@***.***> writes:

The man page for [`io_uring_queue_submit`](https://man7.org/linux/man-pages/man3/io_uring_submit.3.html) says: > After the caller retrieves a submission queue entry (SQE) with > [io_uring_get_sqe(3)](https://man7.org/linux/man-pages/man3/io_uring_get_sqe.3.html) > and prepares the SQE using one of the provided helpers, it can be > submitted with > [io_uring_submit(3)](https://man7.org/linux/man-pages/man3/io_uring_submit.3.html). In this code, the next available submission queue entry is retrieved until it returns `null`, and finally a submit is performed. Considering that no SQE was actually prepared, how is it possible that `io_uring_submit` returns `2`? This seems to indicate that 2 SQEs were prepared, when in fact that did not happen. Perhaps this is expected behavior, but it seems illogical to me.

It is expected behavior. io_urig_get_sqe gives you an SQE to fill, and advances the userspace tail, indicating this sqe is "pending submission". You are supposed to fill this sqe *before* calling io_uring_submit. If you don't, io_uring will gladly submit everything that is pending and you'll get an error CQE when it is processed, indicating you submitted an invalid request. io_uring_submit is basically telling the kernel you are done preparing the SQEs and the kernel can try to execute them. You are required to prepare them before calling io_uring_submit. It returns the number of SQEs that were submitted to the kernel, not what was already executed.

…

```c struct io_uring ring; if (io_uring_queue_init(2, &ring, 0) < 0) { exit(1); } struct io_uring_sqe* sqe = io_uring_get_sqe(&ring); std::cout << sqe << std::endl; sqe = io_uring_get_sqe(&ring); std::cout << sqe << std::endl; sqe = io_uring_get_sqe(&ring); std::cout << sqe << std::endl; int submitted = io_uring_submit(&ring); std::cout << submitted << std::endl; exit(1); ``` ``` $ ./foo 0x7f14e3b5f000 0x7f14e3b5f040 0 2

-- Gabriel Krisman Bertazi