io_uring support for linked timeouts

First of all, if you haven't already noticed, there's a new and shiny
io_uring mailing list. It's:

io-uring@vger.kernel.org

Subscribe if you are at all interested in io_uring development. It's
meant to cover both kernel and user side, and everything from questions,
bugs, development, etc.

Anyway, this is support for IORING_OP_LINK_TIMEOUT. Unlike the timeouts
we have now that work on purely the CQ ring, these timeouts are
specifically tied to a specific command. They are meant to be used to
auto-cancel a request, if it hasn't finished in X amount of time. The
way to use then is to setup your command as you usually would, but then
mark is IOSQE_IO_LINK and add an IORING_OP_LINK_TIMEOUT right after it.
That's how linked commands work to begin with. The main difference here
is that links are normally only started once the dependent request
completes, but for IORING_OP_LINK_TIMEOUT they are armed as soon as we
start the dependent request.

If the dependent request finishes before the linked timeout, the timeout
is canceled. If the timeout finishes before the dependent request, the
dependent request is attempted canceled.

IORING_OP_LINK_TIMEOUT is setup just like IORING_OP_TIMEOUT in terms
of passing in the timespec associated with it.

I added a bunch of test cases to liburing, currently residing in a
link-timeout branch. View them here:

https://git.kernel.dk/cgit/liburing/commit/?h=link-timeou...

Patches are against for-5.5/io_uring, and can currently also be found in
my for-5.5/io_uring-test branch.

 fs/io_uring.c                 | 203 +++++++++++++++++++++++++++++++---
 include/uapi/linux/io_uring.h |   1 +
 2 files changed, 187 insertions(+), 17 deletions(-)

-- 
Jens Axboe