From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Hemminger Subject: Re: [Xen-devel] Re: [PATCH 3/4] [Net] Support Xen accelerated network plugin modules Date: Tue, 22 May 2007 08:05:12 -0700 Message-ID: <20070522080512.6c9528a1@freepuppy> References: <1179837868.28562.22.camel@moonstone.uk.level5networks.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Keir Fraser , muli@il.ibm.com, netdev@vger.kernel.org, xen-devel@lists.xensource.com, herbert@gondor.apana.org.au To: Kieran Mansley Return-path: Received: from smtp.osdl.org ([207.189.120.12]:42747 "EHLO smtp.osdl.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756790AbXEVPGM (ORCPT ); Tue, 22 May 2007 11:06:12 -0400 In-Reply-To: <1179837868.28562.22.camel@moonstone.uk.level5networks.com> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Tue, 22 May 2007 13:44:28 +0100 Kieran Mansley wrote: > On Tue, 2007-05-22 at 08:48 +0100, Keir Fraser wrote: > > > > > > On 22/5/07 08:28, "Kieran Mansley" wrote: > > > > > On Tue, 2007-05-22 at 08:15 +0100, Kieran Mansley wrote: > > >> RCU on its own wouldn't > > >> prevent the accelerated plugin being unloaded while netfront was using > > >> one of the hooks. > > > > > > Hmm, actually I think it could be used to do that. I'll take a look. > > > > Eagerly zap the function pointers, then wait one RCU period so every CPU > > goes through a quiescent point before unloading the module? > > > > -- Keir > > Am I right in thinking that if one of the functions that was protected > by RCU was to block, that would be a bad thing? Clearly the data path > hooks can't/don't block, but I'm not sure it's so obvious for things > like probing a new device. > > Kieran > The same thing is already done to handle network protocols already. RCU is used for the object handle (including function pointers). You need to use: * put rcu structure in accelerator list member and initialize it to the callback * on addition increase refcount on deletion * call list_del_rcu() on removal * in rcu callback you do last step like drop module refcount and free. -- Stephen Hemminger