Re: IORING_OP_POLL_ADD/IORING_OP_POLL_REMOVE questions

[Olivier Langlois <olivier@trillion01.com>]

On Tue, 2021-05-04 at 14:06 -0400, Olivier Langlois wrote:
> 
> 1. 195 is the cqe result for a successful IORING_OP_POLL_ADD
> submission. I only check the POLLIN|POLLOUT bits. What is the meaning
> of the other bits?

I did answer myself for question #1:

fs/io_uring.c:
static void io_poll_complete(struct io_kiocb *req, __poll_t mask, int
error)
{
        struct io_ring_ctx *ctx = req->ctx;

        io_poll_remove_double(req);
        req->poll.done = true;
        io_cqring_fill_event(req, error ? error : mangle_poll(mask));
        io_commit_cqring(ctx);
}

include/linux/poll.h:
#define __MAP(v, from, to) \
        (from < to ? (v & from) * (to/from) : (v & from) / (from/to))

static inline __u16 mangle_poll(__poll_t val)
{
        __u16 v = (__force __u16)val;
#define M(X) __MAP(v, (__force __u16)EPOLL##X, POLL##X)
        return M(IN) | M(OUT) | M(PRI) | M(ERR) | M(NVAL) |
                M(RDNORM) | M(RDBAND) | M(WRNORM) | M(WRBAND) |
                M(HUP) | M(RDHUP) | M(MSG);
#undef M
}

=195
=0xC3
=1100 0011
// from bits/poll.h:
=POLLIN|POLLPRI|POLLRDNORM|POLLRDBAND

The fd for which I get this result is a TCP socket on which the
WebSocket protocol over SSL is implemented.

Does anyone knows if WebSocket and/or TLS/SSL protocols are using TCP
out-of-band?

AFAIK (but I don't much), they don't...

Or maybe Linux poll implementation simply turns on these bits all the
time...

Bottomline, it is unrelated with io_uring but help for my 2 other
questions is still welcomed.

Greetings,
> 
> 2. I don't understand what I am looking at. Why am I receiving a
> completion notification for a POLL request that has just been
> cancelled? What is the logic behind silently discarding a
> IORING_OP_POLL_ADD sqe meant to replace an existing one?
> 
> 3. As I am writing this email, I have just noticed that it was possible
> to update an existing POLL entry with IORING_OP_POLL_REMOVE through
> io_uring_prep_poll_update(). Is this what I should do to eliminate my
> problems? What are the possible race conditions scenarios that I should
> be prepared to handle by using io_uring_prep_poll_update() (ie:
> completion of the poll entry to update while my process is inside
> io_uring_enter() to update it...)?
> 
> thx a lot for your guidance in my io_uring journey!
> Olivier
> 
>