Subject: Re: [take12 0/3] kevent: Generic event handling mechanism.
From: Nicholas Miell <nmiell@comcast.net>
To: "Randy.Dunlap" <rdunlap@xenotime.net>
Cc: Evgeniy Polyakov <johnpol@2ka.mipt.ru>,
       lkml <linux-kernel@vger.kernel.org>, David Miller <davem@davemloft.net>,
       Ulrich Drepper <drepper@redhat.com>, Andrew Morton <akpm@osdl.org>,
       netdev <netdev@vger.kernel.org>, Zach Brown <zach.brown@oracle.com>,
       Christoph Hellwig <hch@infradead.org>
In-Reply-To: <20060822143747.68acaf99.rdunlap@xenotime.net>
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Date: Tue, 22 Aug 2006 15:58:12 -0700
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On Tue, 2006-08-22 at 14:37 -0700, Randy.Dunlap wrote:
> On Tue, 22 Aug 2006 14:13:02 -0700 Nicholas Miell wrote:
> 
> > On Wed, 2006-08-23 at 00:16 +0400, Evgeniy Polyakov wrote:
> > > On Tue, Aug 22, 2006 at 12:57:38PM -0700, Nicholas Miell (nmiell@comcast.net) wrote:
> > > > On Tue, 2006-08-22 at 14:03 +0400, Evgeniy Polyakov wrote:
> > > > Of course, since you already know how all this stuff is supposed to
> > > > work, you could maybe write it down somewhere?
> > > 
> > > I will write documantation, but as you can see some interfaces are
> > > changed.
> > 
> > Thanks; rapidly changing interfaces need good documentation even more
> > than stable interfaces simply because reverse engineering the intended
> > API from a changing implementation becomes even more difficult.
> 
> OK, I don't quite get it.
> Can you be precise about what you would like?
> 
> a.  good documentation
> b.  a POSIX API
> c.  a Windows-compatible API
> d.  other?
> 
> and we won't make you use any of this code.

I want something that I can be confident won't be replaced again in two
years because nobody noticed problems with the old API design or they're
just feeling very NIH with their snazzy new feature.

Maybe then we won't end up with another in the { signal/sigaction,
waitpid/wait4, select/pselect, poll/ppol,  msgrcv, mq_receive,
io_getevents, aio_suspend/aio_return, epoll_wait, inotify read,
kevent_get_events } collection -- or do you like having a maze of
twisted interfaces, all subtly different and none supporting the
complete feature set?

Good documentation giving enough detail to judge the design and an API
that fits with the current POSIX API (at least, the parts that everybody
agrees don't suck) goes a long way toward assuaging my fears that this
won't just be another waste of effort, doomed to be replaced by the Next
Great Thing (We Really Mean It This Time!) in unified event loop API
design or whatever other interface somebody happens to be working on.

---

This is made extraordinarily difficult by the fact kernel people don't
even agree themselves on what APIs should look like anyway and Linus
won't take a stand on the issue -- people with influence are
simultaneously arguing things like:

- ioctls are bad because they aren't typesafe and you should use
syscalls instead because they are typesafe

- ioctls are good, because they're much easier to add than syscalls,
type safety can be supplied by the library wrapper, and syscalls are a
(relatively) scarce resource, harder to wire up in the first place, and
are more difficult to make optional or remove entirely if you decide
they were a stupid idea.

- multiplexors are bad because they're too complex or not typesafe

- multiplexors are good because they save syscall slots or ioctl numbers
and the library wrapper provides the typesafety anyway.

- instead of syscalls or ioctls, you should create a whole new
filesystem that has a bunch of magic files that you read from and write
to in order to talk to the kernel

- filesystem interfaces are bad, because they're take more effort to
write than a syscall or a ioctl and nobody seems to know how to maintain
and evolve a filesystem-based ABI or make them easy to use outside of a
fragile shell script (see: sysfs)

- that everything in those custom filesystems should ASCII strings and
nobody needs an actual grammar describing how to parse them, we can just
break userspace whenever we feel like it

- that everything in those custom filesystems should be C structs, and
screw the shell scripts

- new filesystem metadata should be exposed by:
	- xattrs
	- ioctls
	- new syscalls
		or
	- named streams/forks/not-xattrs
  and three out of four of these suggestions are completely wrong for
  some critical reason

- meanwhile, the networking folks are doing everything via AF_NETLINK
sockets instead of syscalls or ioctl or whatever, I guess because the
network stack is what's most familiar to them

- and there's the usual arguments about typedefs verses bare struct
names, #defines verses enums, returning 0 on success vs. 0 on failure,
and lots of other piddly stupid stuff that somebody just needs to say
"this is how it's done and no arguing" about.

Honestly, somebody with enough clout to make it stick needs to write out
a spec describing what new kernel interfaces should look like and how
they should fit in with existing interfaces.

It'd probably make Evgeniy's life easier if you could just point at the
interface guidelines and say "you did this wrong" instead of random
people telling him to change his design and random other people telling
him to change it back.

-- 
Nicholas Miell <nmiell@comcast.net>

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