From owner-freebsd-net@FreeBSD.ORG Sun Apr 20 09:27:16 2008 Return-Path: Delivered-To: freebsd-net@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 79BC2106564A; Sun, 20 Apr 2008 09:27:16 +0000 (UTC) (envelope-from rwatson@FreeBSD.org) Received: from cyrus.watson.org (cyrus.watson.org [209.31.154.42]) by mx1.freebsd.org (Postfix) with ESMTP id 4D3FB8FC12; Sun, 20 Apr 2008 09:27:16 +0000 (UTC) (envelope-from rwatson@FreeBSD.org) Received: from fledge.watson.org (fledge.watson.org [209.31.154.41]) by cyrus.watson.org (Postfix) with ESMTP id ED43A46B03; Sun, 20 Apr 2008 05:27:15 -0400 (EDT) Date: Sun, 20 Apr 2008 10:27:15 +0100 (BST) From: Robert Watson X-X-Sender: robert@fledge.watson.org To: Brooks Davis In-Reply-To: <20080418152827.GA20382@lor.one-eyed-alien.net> Message-ID: <20080420102047.R67663@fledge.watson.org> References: <490341.95478.qm@web33501.mail.mud.yahoo.com> <20080418152827.GA20382@lor.one-eyed-alien.net> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Cc: freebsd-net@freebsd.org, vijay singh Subject: Re: Regarding if_alloc() X-BeenThere: freebsd-net@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 20 Apr 2008 09:27:16 -0000 On Fri, 18 Apr 2008, Brooks Davis wrote: > On Thu, Apr 17, 2008 at 06:35:23PM -0700, vijay singh wrote: >> Hi all. How do we avoid a race in populating the ifindex_table? Id this is >> a TODO, as it seems from the code below, would it be acceptable if I wrote >> a patch and reused the ifnet_lock [IFNET_WLOCK, IFNET_WUNLOCK]? > > Locking if_index management with ifnet_lock should be ok. Ideally we should > probably be using ALLOC_UNR(9) to manage if_indexes instead of this rather > expensive loop. > > Be aware, that if_index generation is least of the issues in this area. The > if_grow() call is much riskier since it changes the value of the global > ifnet pointer which I'm not sure we can afford to lock. It would be worth > experimenting with rmlocks to see what the impact if of locking would be. > > I'm serious tempted to kill if_grow in favor of some sort of if_index_max > tunable. I've seen a number of reports of panics that may well be traceable to races in if_index handling, and have looked a bit at possible fixes. Quite a few uses of if_index are inherently racy, as they rely on stability of the index value, which of course can't be guaranteed with removable interfaces. I think a reasonable interim fix would be to protect all use of the byindex arrays using the ifnet lock, but remember that the read methods, not just the write methods, need protection, and as such should move from being macros in if_var.h to functions in if.c. if_grow is probably OK if this is done right, but it will need to be set up to drop the lock, grow, re-aquire, and re-validate assumptions (i.e., repeat the search for a free index and loop if it fails to find one). Once the read methods are using the lock also, we should seriously consider converting it to an rwlock. We can probably also un-publicize at least one of the byindex lookup routines (the dev lookup, which is needed only in if.c). This would prevent races on modifying and evaluating the index array, but not on disappearing cdevs and ifnets, which are a separate problem, and one that probably is exercised significanty less. The reports I've seen appear to have only to do with pulling the array out from other consumers while in use. Robert N M Watson Computer Laboratory University of Cambridge > > -- Brooks > >> if_alloc(u_char type) >> { >> struct ifnet *ifp; >> >> ifp = malloc(sizeof(struct ifnet), M_IFNET, M_WAITOK|M_ZERO); >> >> /* >> * Try to find an empty slot below if_index. If we fail, take >> * the next slot. >> * >> * XXX: should be locked! >> */ >> for (ifp->if_index = 1; ifp->if_index <= if_index; ifp->if_index++) { >> if (ifnet_byindex(ifp->if_index) == NULL) >> break; >> } >> >> >> >> >> >> ____________________________________________________________________________________ >> Be a better friend, newshound, and >> know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ >> _______________________________________________ >> freebsd-net@freebsd.org mailing list >> http://lists.freebsd.org/mailman/listinfo/freebsd-net >> To unsubscribe, send any mail to "freebsd-net-unsubscribe@freebsd.org" >> >