cvs commit: src/sys/contrib/ipfilter/netinet ip_auth.c

[Darren Reed]

On Mon, Dec 27, 2004 at 02:48:46PM -0600, Alan Cox wrote:
> Darren,
> 
> Based upon a quick look at the above URL, I would say that the problem
> is that you are acquiring an sx lock after a mutex is held.

Yes, that is what I thought the problem was too.

> That is not allowed.

Well, it should be.

> More generally, an sx lock is termed a "sleepable" lock
> and a mutex is not; all desired sleepable locks must be acquired
> before any non-sleepable locks are acquired.  Witness enforces this
> prohibition, but perhaps causes confusion by calling it a reversal.

What can I say that I haven't already ?
This is a nonsense rule and I'm quite happy that it isn't enforced at
all.  The assumption by someone that all of these scenarios lead to
deadlocks is false.

I imagine it is part of why things like the IFNET_*LOCK macros use mtx
rather than sx.  On systems where the interface list is of non-trivial
size, I imagine this would have a measurable impact.

Perhaps the most stupid realisation from this has been that IPFilter
code gets called with a UDP mutex of some sort held.  Like WTF ?

Darren