Re: [PATCH 6/6] fs: replace f_ops->get_poll_head with a static ->f_poll_head pointer

On Thu, Jun 28, 2018 at 02:11:17PM -0700, Linus Torvalds wrote:

Christoph, do you have a test program for IOCB_CMD_POLL and what it's
actually supposed to do?

https://pagure.io/libaio/c/9c6935e81854d1585bbfa48c35b185849d746864?branch=aio-poll

is the actual test in libaio.  In addition to that the seastar library
actually has a real life user.  But given that is c++ with all modern
bells and whistles you'll probably have an as hard time as me actually
understanding that.

Because I think that what it can do is simply to do the ->poll() calls
outside the iocb locks, and then just attach the poll table to the
kioctx afterwards.

We could do that on the submit side fairly easily.  The problem
is really the completion side, where I'd much avoid introducing a
spurious context switch.  Right now even with a NULL qproc we can't
guarantee any of that.  So we'll need to schedule out to a workqueue,
and then from that schedule the potential multiple NULL qproc calls,
which might actually block elsewhere even if __pollwait is never called.

This whole "poll must not block" is a complete red herring. It doesn't
come from any other requirements than BAD AIO GARBAGE CODE.

I comes from the fact to avoid a totally pointless context switch.
aio code itself works just fine called from a workqueue, we have
exatly that case when file system do non-trivial operations in their
end_io handler.