Re: Improving AIO cancellation

[Zach Brown]

> We don't know if the bio was succesfully cancelled until the bio is
> completed (and this doesn't change anything about how bio completion
> works) - if it was cancelled, the bi_end_io callback will get
> -ECANCELLED or something.

Yeah, I think that's the right design.

> This is very different from the existing AIO cancellation; KIF_CANCEL
> means "this kiocb _has been cancelled_". (sort of, I was just rereading
> that code and I'm convinced it's buggy).

I don't recall the details, but the last time I looked at it I was also
convinced that it was buggy.

> What we want is a new io_cancel_sane() syscall that doesn't return
> anything, and only marks the iocb as "cancellation pending". The
> completion would still get returned via io_getevents(), and userspace
> would find out if it was cancelled or not then.

Agreed.  I wonder if we can mash those semantics in to the existing call
somehow.  This *does* have access to the iocb, so our hands aren't as
tied as we are with io_setup()'s inability to request behavioural
changes.

Maybe we mark the submitted iocbs as ASYNC_BEST_EFFORT_CANCEL and then
io_cancel() on them can return success and completion will proceeed as
normal.  Maybe much more quickly with a cancelled status but maybe not.

- z
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