Apparently, Linux already had an Asyn[c]-IO (AIO) API. I believe it is not fully asynchronous. So what was the issue with AIO?
If you're extra careful and match all its constraints, the "old" Linux AIO interface will behave asynchronously. However, if you "break" ANY of the (hidden) rules submission can suddenly (and silently) behave in a synchronous fashion (i.e. submission blocks non-deterministically and for inconveniently long periods of time). Some of the many "rules" are given in answers to asynchronous IO io_submit latency in Ubuntu Linux (the overall issues are also listed in section 1.0 of the io_uring document you linked).
how io_uring overcomes it?
- It is a radically different interface (see this answer on the "Is there really no asynchronous block I/O on Linux?") which is harder to get wrong.
- It has a workqueue/thread pool mechanism which it will punt requests to when it is aware that blocking will take place before the result of submission can be returned (and thus it is able to return control back to the caller). This allows it to retain asynchrony in more (hopefully all!) submission cases.
- It has an optional privileged mode (
IORING_SETUP_SQPOLL) where you don't even have to make syscalls to submit/retrieve results. If you're "just" manipulating memory contents it's going to be hard to be blocked on a call you never made!
How io_uring internally works?
There are two ring buffers where the first ring is used to queue submissions and when the work has been completed the "results" are announced via the second ring buffer (which contains completions). While it's hard to give you something more than a very high level view if you're uncomfortable with things like C structs/C function interfaces you may enjoy this video by Jens presenting io_uring nonetheless and find the explanations in https://www.scylladb.com/2020/05/05/how-io_uring-and-ebpf-will-revolutionize-programming-in-linux/ and https://mattermost.com/blog/iouring-and-go/ more accessible.
io_uring's advantages over Linux AIO don't stop at better asynchrony though! See the aforementioned link for "Is there really no asynchronous block I/O on Linux?" for a list of other benefits...
Thanks for the detailed answer.