Re: [PATCH] io_uring: allow async accept on O_NONBLOCK sockets

[Dylan Yudaken <dylany@fb.com>]

On Thu, 2022-03-24 at 08:46 -0600, Jens Axboe wrote:
> On 3/24/22 8:42 AM, Jens Axboe wrote:
> > On 3/24/22 8:34 AM, Dylan Yudaken wrote:
> > > Do not set REQ_F_NOWAIT if the socket is non blocking. When
> > > enabled this
> > > causes the accept to immediately post a CQE with EAGAIN, which
> > > means you
> > > cannot perform an accept SQE on a NONBLOCK socket asynchronously.
> > > 
> > > By removing the flag if there is no pending accept then poll is
> > > armed as
> > > usual and when a connection comes in the CQE is posted.
> > > 
> > > note: If multiple accepts are queued up, then when a single
> > > connection
> > > comes in they all complete, one with the connection, and the
> > > remaining
> > > with EAGAIN. This could be improved in the future but will
> > > require a lot
> > > of io_uring changes.
> > 
> > Not true - all you'd need to do is have behavior similar to
> > EPOLLEXCLUSIVE, which we already support for separate poll. Could
> > be
> > done for internal poll quite easily, and _probably_ makes sense to
> > do by
> > default for most cases in fact.
> 
> Quick wire-up below. Not tested at all, but really should basically
> as
> simple as this.
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 4d98cc820a5c..8dfacb476726 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -967,6 +967,7 @@ struct io_op_def {
>         /* set if opcode supports polled "wait" */
>         unsigned                pollin : 1;
>         unsigned                pollout : 1;
> +       unsigned                poll_exclusive : 1;
>         /* op supports buffer selection */
>         unsigned                buffer_select : 1;
>         /* do prep async if is going to be punted */
> @@ -1061,6 +1062,7 @@ static const struct io_op_def io_op_defs[] = {
>                 .needs_file             = 1,
>                 .unbound_nonreg_file    = 1,
>                 .pollin                 = 1,
> +               .poll_exclusive         = 1,
>         },
>         [IORING_OP_ASYNC_CANCEL] = {
>                 .audit_skip             = 1,
> @@ -6293,6 +6295,8 @@ static int io_arm_poll_handler(struct io_kiocb
> *req, unsigned issue_flags)
>         } else {
>                 mask |= POLLOUT | POLLWRNORM;
>         }
> +       if (def->poll_exclusive)
> +               mask |= EPOLLEXCLUSIVE;
>  
>         if (!(issue_flags & IO_URING_F_UNLOCKED) &&
>             !list_empty(&ctx->apoll_cache)) {
> 

Thanks!

Will take a look and update the test case to check it.