rwlocks, correctness over speed.

[Skip Ford]

Attilio Rao wrote:
> 2007/11/24, Robert Watson <rwatson at freebsd.org>:
> > While I'm no great fan of recursion, the reality is that many of our kernel
> > subsystems are not yet ready to disallow recursion on locks.  Take a look at
> > the cases where we explicitly enable recursive acquisition for mutexes--in
> > practice, most network stack mutexes are recursive due to the recursive
> > calling in the network stack.  While someday I'd like to think we'll be able
> > to eliminate some of that, but it won't be soon since it requires significant
> > reworking of very complicated code.  The current model in which recursion is
> > explicitly enabled only where still required seems to work pretty well for the
> > existing code, although it's hard to say yet in the code I've looked at
> > whether read recursion would be required--the situations I have in mind would
> > require purely write recursion.  There's one case in the UNIX domain socket
> > code where we do a locked test and conditional lock/unlock with an rwlock for
> > exclusive locking because recursion isn't currently supported, and that's not
> > a usage I'd like to encourage more of.
> 
> Oh, I just didn't notice this -- rwlock are only present in 7.0 and in
> 7.0 they support recursion in exclusive mode, so I'm not sure what do
> you mean with 'recursion isn't currently supported'.

locking(9) and rwlock(9) both say it isn't supported.

-- 
Skip