Date: Tue, 28 Dec 2004 03:17:07 +0000 From: Darren Reed <darrenr@hub.freebsd.org> To: Alan Cox <alc@cs.rice.edu> Cc: "M. Warner Losh" <imp@bsdimp.com> Subject: Re: cvs commit: src/sys/contrib/ipfilter/netinet ip_auth.c Message-ID: <20041228031707.GA38011@hub.freebsd.org> In-Reply-To: <20041227204846.GC15591@noel.cs.rice.edu> References: <20041226165927.GA18879@hub.freebsd.org> <Pine.BSF.4.53.0412261707210.68472@e0-0.zab2.int.zabbadoz.net> <20041226182537.GB20920@hub.freebsd.org> <20041226.222435.52824948.imp@bsdimp.com> <20041227054931.GC20920@hub.freebsd.org> <20041227204846.GC15591@noel.cs.rice.edu>
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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
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