Re: [PATCH 5/6] io_uring: add support for futex wake and wait

[Gabriel Krisman Bertazi <krisman@suse.de>]

Jens Axboe <axboe@kernel.dk> writes:

> Add support for FUTEX_WAKE/WAIT primitives.

This is great.  I was so sure io_uring had this support already for some
reason.  I might have dreamed it.

The semantics are tricky, though. You might want to CC peterZ and tglx
directly.

> IORING_OP_FUTEX_WAKE is mix of FUTEX_WAKE and FUTEX_WAKE_BITSET, as
> it does support passing in a bitset.

As far as I know, the _BITSET variant are not commonly used in the
current interface.  I haven't seen any code that really benefits from
it.

> Similary, IORING_OP_FUTEX_WAIT is a mix of FUTEX_WAIT and
> FUTEX_WAIT_BITSET.

But it is definitely safe to have a single one, basically with the
_BITSET semantics.

> FUTEX_WAKE is straight forward, as we can always just do those inline.
> FUTEX_WAIT will queue the futex with an appropriate callback, and
> that callback will in turn post a CQE when it has triggered.

Even with an asynchronous model, it might make sense to halt execution
of further queued operations until futex completes.  I think
IOSQE_IO_DRAIN is a barrier only against the submission part, so it
wouldn't hep.  Is there a way to ensure this ordering?

I know, it goes against the asynchronous nature of io_uring, but I think
it might be a valid use case. Say we extend FUTEX_WAIT with a way to
acquire the futex in kernel space.  Then, when the CQE returns, we know
the lock is acquired.  if we can queue dependencies on that (stronger
than the link semantics), we could queue operations to be executed once
the lock is taken. Makes sense?

> Cancelations are supported, both from the application point-of-view,
> but also to be able to cancel pending waits if the ring exits before
> all events have occurred.
>
> This is just the barebones wait/wake support. Features to be added
> later:

One item high on my wishlist would be the futexv semantics (wait on any
of a set of futexes).  It cannot be implemented by issuing several
FUTEX_WAIT.

-- 
Gabriel Krisman Bertazi