Re: how is register_(buffer|file) supposed to work?

[Glauber Costa <glauber@scylladb.com>]

On Wed, Feb 12, 2020 at 9:20 AM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 2/11/20 9:12 PM, Glauber Costa wrote:
> > Hi,
> >
> > I am trying to experiment with the interface for registering files and buffers.
> >
> > (almost) Every time I call io_uring_register with those opcodes, my
> > application hangs.
> >
> > It's easy to see the reason. I am blocking here:
> >
> >                 mutex_unlock(&ctx->uring_lock);
> >                 ret = wait_for_completion_interruptible(&ctx->completions[0]);
> >                 mutex_lock(&ctx->uring_lock);
> >
> > Am I right in my understanding that this is waiting for everything
> > that was submitted to complete? Some things in my ring may never
> > complete: for instance one may be polling for file descriptors that
> > may never really become ready.
> >
> > This sounds a bit too restrictive to me. Is this really the intended
> > use of the interface?
>
> For files, this was added in the current merge window:
>

Ok, so this is what I was missing:

> commit 05f3fb3c5397524feae2e73ee8e150a9090a7da2
> Author: Jens Axboe <axboe@kernel.dk>
> Date:   Mon Dec 9 11:22:50 2019 -0700
>
>     io_uring: avoid ring quiesce for fixed file set unregister and update
>
> which allows you to call IORING_REGISTER_FILES_UPDATE without having to
> quiesce the ring. File sets can be sparse, you can register with an fd
> of -1 and then later use FILES_UPDATE (or IORING_OP_FILES_UPDATE) to
> replace it with a real entry. You can also replace a real entry with a
> new one, or switch it to sparse again.

^^^ this

I thought I've seen EBADF errors when registering -1, but maybe it
wasn't -1, it was
just really an invalid fd.

For memory I guess I could register early on and draw from a poll.
That's a bit inconvenient
as ideally that poll would grow and shrink dynamically, but it works

I'll give it a try