[RFC 01/13] io_uring: add infra for uring_cmd completion in submitter-task

[Luis Chamberlain]

On Thu, Feb 17, 2022 at 08:50:59AM -0700, Jens Axboe wrote:
> On 2/17/22 8:39 AM, Kanchan Joshi wrote:
> > On Thu, Feb 17, 2022 at 7:43 AM Luis Chamberlain <mcgrof at kernel.org> wrote:
> >>
> >> On Mon, Dec 20, 2021 at 07:47:22PM +0530, Kanchan Joshi wrote:
> >>> Completion of a uring_cmd ioctl may involve referencing certain
> >>> ioctl-specific fields, requiring original submitter context.
> >>> Export an API that driver can use for this purpose.
> >>> The API facilitates reusing task-work infra of io_uring, while driver
> >>> gets to implement cmd-specific handling in a callback.
> >>>
> >>> Signed-off-by: Kanchan Joshi <joshi.k at samsung.com>
> >>> Signed-off-by: Anuj Gupta <anuj20.g at samsung.com>
> >>> ---
> >>>  fs/io_uring.c            | 16 ++++++++++++++++
> >>>  include/linux/io_uring.h |  8 ++++++++
> >>>  2 files changed, 24 insertions(+)
> >>>
> >>> diff --git a/fs/io_uring.c b/fs/io_uring.c
> >>> index e96ed3d0385e..246f1085404d 100644
> >>> --- a/fs/io_uring.c
> >>> +++ b/fs/io_uring.c
> >>> @@ -2450,6 +2450,22 @@ static void io_req_task_submit(struct io_kiocb *req, bool *locked)
> >>>               io_req_complete_failed(req, -EFAULT);
> >>>  }
> >>>
> >>> +static void io_uring_cmd_work(struct io_kiocb *req, bool *locked)
> >>> +{
> >>> +     req->uring_cmd.driver_cb(&req->uring_cmd);
> >>
> >> If the callback memory area is gone, boom.
> > 
> > Why will the memory area be gone?
> > Module removal is protected because try_module_get is done anyway when
> > the namespace was opened.
> 
> And the req isn't going away before it's completed.

Groovy, it would be nice to add a little /* comment */ to just remind
the reader?

> >>> +{
> >>> +     struct io_kiocb *req = container_of(ioucmd, struct io_kiocb, uring_cmd);
> >>> +
> >>> +     req->uring_cmd.driver_cb = driver_cb;
> >>> +     req->io_task_work.func = io_uring_cmd_work;
> >>> +     io_req_task_work_add(req, !!(req->ctx->flags & IORING_SETUP_SQPOLL));
> >>
> >> This can schedules, and so the callback may go fishing in the meantime.
> > 
> > io_req_task_work_add is safe to be called in atomic context. FWIW,
> > io_uring uses this for regular (i.e. direct block) io completion too.
> 
> Correct, it doesn't schedule and is safe from irq context as long as the
> task is pinned (which it is, via the req itself).

Great, a kdoc explaining the routine and that it can be called from
atomic context and the rationale would be very useful to users. And ..
so the callback *must* be safe in atomic context too or can it sleep?

  Luis