rwlocks, correctness over speed.

[Alfred Perlstein]

In summary, I am proposing (temporarily) making read-recursion
on rwlocks not supported in order to avoid livelock due to writer
starvation.

More details:

We currently have a problem with our implementation of rwlocks.

I think this is a key issue for 7.x as what we decide to support
will have rammifications for many years to come.

We do not support writer priority or "fair share" usage of rwlocks
between readers and writers.

We have several choices to rectify this.

1. Disallow recursion on rwlocks (witness can be used to enforce this),
this simplifies rwlocks such that we can avoid deadlock when a single
reader is trying to recurse while a writer is pending.

2. Track ownership of rwlocks, this can be implemented with a "rwlock
stack" in the per-thread control block (struct thread).  Using this
ownership information we can determine if someone is recursing and
allow them to continue recursing despite a pending write request.

I think the most simple solution currently is to drop support for
recursive reads on rwlocks until we have the facility in place
to properly support starvation avoidance.

Why is this important?

Simply put, developers that quickly "fix" some portion of code,
whether that be a driver or part of the kernel proper who use read
recursion will open the system to writer starvation and hence the
system will destabilize, particulary for high load situations.

I would like to get this in before 7.0-RELEASE becasue otherwise
we're forced to implement something like the above mentioned solution
#2, which will degrade performance for most use cases of rwlocks.

Comments?

-- 
- Alfred Perlstein