From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756213Ab1BJALF (ORCPT ); Wed, 9 Feb 2011 19:11:05 -0500 Received: from ovro.ovro.caltech.edu ([192.100.16.2]:54756 "EHLO ovro.ovro.caltech.edu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755911Ab1BJALE (ORCPT ); Wed, 9 Feb 2011 19:11:04 -0500 Date: Wed, 9 Feb 2011 16:10:55 -0800 From: "Ira W. Snyder" To: Dmitry Torokhov Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] misc: add CARMA DATA-FPGA Access Driver Message-ID: <20110210001055.GB5303@ovro.caltech.edu> References: <1297208267-27087-1-git-send-email-iws@ovro.caltech.edu> <1297208267-27087-2-git-send-email-iws@ovro.caltech.edu> <20110209083325.GA7256@core.coreip.homeip.net> <20110209173532.GB21766@ovro.caltech.edu> <20110209182740.GC23867@core.coreip.homeip.net> <20110209233544.GA5303@ovro.caltech.edu> <20110209234231.GA5707@core.coreip.homeip.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110209234231.GA5707@core.coreip.homeip.net> User-Agent: Mutt/1.5.20 (2009-06-14) X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.2.6 (ovro.ovro.caltech.edu); Wed, 09 Feb 2011 16:10:56 -0800 (PST) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 09, 2011 at 03:42:31PM -0800, Dmitry Torokhov wrote: > On Wed, Feb 09, 2011 at 03:35:45PM -0800, Ira W. Snyder wrote: > > On Wed, Feb 09, 2011 at 10:27:40AM -0800, Dmitry Torokhov wrote: > > > > [ snip stuff I've already fixed in the next version ] > > > > > > > > > > The requirement is that the device stay open during reconfiguration. > > > > This provides for that. Readers just block for as long as the device is > > > > not producing data. > > > > > > OK, you still need to make sure you do not touch free/used buffer while > > > device is disabled. Also, you need to kick readers if you unbind the > > > driver, so maybe a new flag priv->exists should be introduced and > > > checked. > > > > > > > I don't understand what you mean by "kick readers if you unbind the > > driver". The kernel automatically increases the refcount on a module > > when a process is using the module. This shows up in the "Used by" > > column of lsmod's output. > > > > The kernel will not let you rmmod a module with a non-zero refcount. You > > cannot get into the situation where you have rmmod'ed the module and a > > reader is still blocking in read()/poll(). > > However you can still unbind the driver from the device by writing into > driver's sysfs 'unbind' attribute. > > See drivers/base/bus.c::driver_unbind(). > I was completely unaware of that "feature". I hunch that many drivers are incapable of dealing with an unbind while they are still open. Matter of fact, I don't see how this can EVER be safe. The driver core automatically calls the data_of_remove() routine while there are still blocked readers. This kfree()s the private data structure, which contains the suggested priv->exists flag. What happens if the memory allocator re-allocates that memory to a different driver before the reader process is woken up to check the priv->exists flag? The only way to solve this is to count the number of open()s and close()s, and block the unbind until all users have close()d the device. Thanks, Ira