From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-iw0-f179.google.com (mail-iw0-f179.google.com [209.85.214.179]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority" (verified OK)) by ozlabs.org (Postfix) with ESMTPS id E0C22B7088 for ; Thu, 10 Feb 2011 11:39:44 +1100 (EST) Received: by iwb12 with SMTP id 12so789574iwb.38 for ; Wed, 09 Feb 2011 16:39:41 -0800 (PST) Date: Wed, 9 Feb 2011 16:39:35 -0800 From: Dmitry Torokhov To: "Ira W. Snyder" Subject: Re: [PATCH 1/2] misc: add CARMA DATA-FPGA Access Driver Message-ID: <20110210003935.GA6034@core.coreip.homeip.net> 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> <20110210001055.GB5303@ovro.caltech.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20110210001055.GB5303@ovro.caltech.edu> Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Wed, Feb 09, 2011 at 04:10:55PM -0800, Ira W. Snyder wrote: > 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. Hmm, maybe older drivers... Anythig hotpluggable (USB, PCI, etc) should be in a better shape because they expect to be yanked at any time. > > 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. > Yes, you can kick readers and wait, or you can refcount that private structure and have readers grab a reference when they open your device and drop it in their fops->release() method. Your remove() should also drop reference instead of doing kfree() outright. Thanks. -- Dmitry