rwlocks, correctness over speed.

[Attilio Rao]

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'.

Thanks,
Attilio


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