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Date: Thu, 6 Nov 2003 13:33:25 -0500 (EST)
From: Robert Watson <rwatson@freebsd.org>
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To: Sam Leffler <sam@errno.com>
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cc: sam@freebsd.org
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cc: "M. Warner Losh" <imp@bsdimp.com>
Subject: Re: PERFORCE change 41566 for review
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On Thu, 6 Nov 2003, Sam Leffler wrote:

> On Thursday 06 November 2003 12:26 am, M. Warner Losh wrote:
> > In message: <200311060513.hA65Da5J091575@repoman.freebsd.org>
> >
> >             Sam Leffler <sam@freebsd.org> writes:
> > : 	diff reduction against head
> > : -		SIS_UNLOCK(sc);
> > :  		(*ifp->if_input)(ifp, m);
> > : -		SIS_LOCK(sc);
> >
> > does this mean that we can call if_input with our locks held?
> 
> sis's lock is setup MTX_RECURSE in which case re-entry is handled.  You
> only need to release the lock when you may be re-entered and it's not
> setup as recursive.  I think recursive locks are bad but so long as the
> driver is setup to use them I'm not releasing them when calling up. 

However, this could potentially lead to lock order reversal between the
network stack locks and the interface lock.  We know the stack will grab
lots of locks in the input routine with direct dispatch.  We also know the
stack will invoke the interface output routine while holding stack
locks...  I think it makes the most sense to place interface locks after
network stack locks in the lock order, which would preclude holding the
interface lock and then entering the stack. 

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert@fledge.watson.org      Network Associates Laboratories