[Devel] Re: [RFC][PATCH] rename 'struct pid'

[Dave Hansen]

On Tue, 2007-04-10 at 22:52 -0600, Eric W. Biederman wrote:
> Dave Hansen <hansendc at us.ibm.com> writes:
> >> A pid (pid_t or
> >> struct pid) isn't just an identfier it is a handle to processes.
> >> struct pid just does so more directly because it is inside the kernel.
> >
> > Let's face it, "pid" has a meaning.  It's a number.  It's what you
> > kill(1).  The meaning has been there for a long, long time.  'struct
> > pid' is a completely different concept, and it's certainly more than
> > "just a number".
> 
> Yes.  "pid" has a meaning.  The meaning is old and well established.
> That meaning is not just a number, just like a file descriptor is not
> just a number.

That's a great example.  Userspace fds are to 'struct file' as pids are
to 'struct pid', right?

I actually think 'struct file' is a pretty good name.  Think of what
do_sys_open() might look like if we called 'struct file' 'struct fd'
instead and 'fdp' instead of 'filp'.

We end up with lines like:

	fd_install(fd, fdp);

Which makes it confusing which fd we're dealing with or what the 'fd_'
in the name refers to, the 'fd' or the 'fdp'.  It makes quite a bit of
sense to have 'fd' and 'struct file' named quite distinctly.

> > So, please consider that there are actual kernel developers hacking on
> > this stuff who are having problem with it.  The function of 'struct pid'
> > is great, it's a wonderful concept.  It just needs a slightly different
> > name in order to more easily relate that concept to those that are
> > trying to use it.
> >
> > If anyone can think of some more incremental ways to do this, or has
> > other suggestions on how to make it more clear, I'm all ears.
> 
> So what I have seen are examples down in the guts of the pid hash
> table that are problematic.  And a few complaints about pid_nr.
> 
> However the conversions I have done and I have looked at have just
> been a drop in replacement for pid_t except for reference counting
> issues.  That to me at least is rather convincing.

I think this is more indicative of the great design of 'struct pid' in
concept.  It truly is a drop-in replacement for how things were used in
the past.  The concept is *great*.

> So I'm not convinced there is a fundamental problem.  Just a bit of a
> problem in the guts of things where everything seems to have the
> same name.  I'm not at all certain how a different prefix would
> sort that out.

I agree that there's no fundamental problem with the structure.  Its
function is quite ideal.  The issue for me comes in the ability for
people to update, enhance and review what is going on.  There's no
fundamental barrier here, as Suka demonstrated getting some of his
pidspace code to work.  It just crossed my pain threshold as I was
trying to debug some of it.

Once we get pidspaces fully working, the hacking in the guts will
certainly be reduced.  But, there are always bugs, and this is a common
enough code area that people are bound to touch it as time goes on.  I
just want to make that as easy as possible.

> My feeling is that changing this just caters to people who are not
> going to be able to understand what is going on no matter what the
> structure is named, and is going to make it harder for me to update
> the code when I find the time to do it.

I'm completely sure that you'll grasp the entire concept, no matter to
what we change the names.  The mess that you've unraveled so far in
there makes has given me supreme confidence in this. :)

My worry is the ramp-up time for people who want to understand it enough
hack it or just audit the code, but who won't grasp it on quite the same
level that you have.  

-- Dave

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