request for review: backport of sx and rwlocks from 7.0 to 6-stable

[Alfred Perlstein]

* Attilio Rao <attilio at freebsd.org> [070901 18:01] wrote:
> 2007/8/31, Alfred Perlstein <alfred at freebsd.org>:
> > Hi guys,
> >
> > Some work here at work was approved for sharing with community so
> > I'm posting it here in hope of a review.
> >
> > We run some pretty good stress testing on our code, so I think it's
> > pretty solid.
> >
> > My only concern is that I've tried my best to preserve kernel source
> > API, but not binary compat though a few simple #defines.
> >
> > I can make binary compat, in albeit a somewhat confusing manner, but
> > that will require some rototilling and weird renaming of calls to
> > the sleepq and turnstile code.  In short, I'd rather not, but I will
> > if you think it's something that should be done.
> >
> > There's also a few placeholders for lock profiling which I will
> > very likely be backporting shortly as well.
> >
> > Patch is attached.
> >
> > Comments/questions?
> 
> Hello Alfred,
> I started looking at the patch and I have 2 things to say:
> - why you backported the allocating patch with UMA in sleepqueues? it
> is ortogonhal to this problem and it is not necessary due in this case

Well, it has performance implications so I took it. 

> I think
> - Instead than using the stub __aligned() for struct thread, you
> should use what we alredy do for 7.0 as dealing with uma allocation
> functions and a separate stub for thread0. you can workaround the
> missing of uma functions with a simple macro.
> 
> I will try to give a line-by-line revision ASAP.

I don't really agree here.   I think that since we rely on
the object to be aligned such that otherwise bad and hard to track things
happen, putting the alignment declaration in the variable rather
than structure declaration will lead to an easy to avoid problem
that is nearly impossible to track down.

Ie. if someone makes thread1, then everyone with non default
16 byte aligned platforms will break unless they are smart
enough to see thread0 and understand.

I guess the question is, why NOT force the alignment at the
structure declaration other than asthetics?

-- 
- Alfred Perlstein