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

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Jun 28, 2018 at 10:30:27PM +0100, Al Viro wrote:

> I'm not saying that blocking on other things is a bug; some of such *are* bogus,
> but a lot aren't really broken.  What I said is that in a lot of cases we really
> have hard "no blocking other than in callback" (and on subsequent passes there's
> no callback at all).  Which is just about perfect for AIO purposes, so *IF* we
> go for "new method just for AIO, those who don't have it can take a hike", we might
> as well indicate that "can take a hike" in some way (be it opt-in or opt-out) and
> use straight unchanged ->poll(), with alternative callback.

PS: one way of doing that would be to steal a flag from pt->_key and have ->poll()
instances do an equivalent of
        if (flags & LOOKUP_RCU)
                return -ECHILD;
we have in a lot of ->d_revalidate() instances for "need to block" case.  Only
here they would've returned EPOLLNVAL.

Most of the ->poll() instances wouldn't care at all - they do not block unless
the callback does (and in this case it wouldn't have).  Normal poll(2)/select(2)
are completely unaffected.  And AIO would just have that bit set in its
poll_table_struct.

The rules for drivers change only in one respect - if your ->poll() is going to
need to block, check poll_requested_events(pt) & EPOLL_ATOMIC and return EPOLLNVAL
in such case.