[PATCHSET v2] Add io_uring support for waitid

[Jens Axboe <axboe@kernel.dk>]

Hi,

This adds support for IORING_OP_WAITID, which is an async variant of
the waitid(2) syscall. Rather than have a parent need to block waiting
on a child task state change, it can now simply get an async notication
when the requested state change has occured.

Patches 1..4 are purely prep patches, and should not have functional
changes. They split out parts of do_wait() into __do_wait(), so that
the prepare-to-wait and sleep parts are contained within do_wait().

Patch 5 adds io_uring support.

I wrote a few basic tests for this, which can be found in the
'waitid' branch of liburing:

https://git.kernel.dk/cgit/liburing/log/?h=waitid

Also spun a custom kernel for someone to test it, and no issues reported
so far.

 include/linux/io_uring_types.h |   2 +
 include/uapi/linux/io_uring.h  |   2 +
 io_uring/Makefile              |   2 +-
 io_uring/cancel.c              |   5 +
 io_uring/io_uring.c            |   3 +
 io_uring/opdef.c               |   9 +
 io_uring/waitid.c              | 313 +++++++++++++++++++++++++++++++++
 io_uring/waitid.h              |  15 ++
 kernel/exit.c                  | 132 +++++++-------
 kernel/exit.h                  |  30 ++++
 10 files changed, 453 insertions(+), 60 deletions(-)

The code can also be found here:

https://git.kernel.dk/cgit/linux/log/?h=io_uring-waitid

Changes since v1:
- Rebase on io_uring-futex again, as that saw changes.
- Add compat handling of siginfo (Arnd)
- Add reviewed-by to patches 1+2
- Minor cleanups

-- 
Jens Axboe