rwlock patch to prefer writers and improve spinning.

[Jeff Roberson]

On Tue, 29 Jan 2008, Attilio Rao wrote:

> 2008/1/29, Jeff Roberson <jroberson at chesapeake.net>:
>> http://people.freebsd.org/~jeff/rwlock.diff
>>
>> Attilio and I have come up with a rwlock patch that prevents writer
>> starvation by blocking readers whenever there are writers waiting on the
>> turnstile.
>>
>> To avoid deadlocks caused by recursive readers a new per-thread count of
>> outstanding rw read locks is maintained.  When a thread is known to own
>> read locks it will bypass the pending writers checks.  This could lead to
>> writer starvation in pathological cases but will not deadlock.
>>
>> In addition, I have added code to optimistically spin in the write lock
>> path when there are readers.  The spin is limited by two counters.  One
>> controls the number of spins while waiting for the lock state to change
>> and another controls the number of lock state changes we'll observe before
>> we give up.
>>
>> To add these two features new flags were required.  I removed the
>> recursion flag and instead there is an extra branch in the inlined code to
>> check the recursion count before the atomic.  Another option would be to
>> further increase the alignment of struct thread, however, I don't really
>> think that is necessary.
>>
>> This has been thoroughly tested with nokia's stack but it had to be
>> forward ported to current.  Any rwlock users are encouraged to test before
>> I commit.
>>
>> Feedback is welcome.
>
> Jeff,
> it is not missing a kern/subr_witness.c part where some checks are axed out?

Oh you are right I forgot to include some turnstile pieces as well.  I 
will provide them soon.

>
> Attilio
>
>
> -- 
> Peace can only be achieved by understanding - A. Einstein
>