From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1764941AbYEOUmT (ORCPT ); Thu, 15 May 2008 16:42:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755635AbYEOUls (ORCPT ); Thu, 15 May 2008 16:41:48 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:57844 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754891AbYEOUlr (ORCPT ); Thu, 15 May 2008 16:41:47 -0400 Date: Thu, 15 May 2008 13:40:40 -0700 From: Greg KH To: Miklos Szeredi Cc: torvalds@linux-foundation.org, ajones@riverbed.com, a.p.zijlstra@chello.nl, mszeredi@suse.cz, akpm@linux-foundation.org, kay.sievers@vrfy.org, trond.myklebust@fys.uio.no, linux-kernel@vger.kernel.org Subject: Re: [BUG] mm: bdi: export BDI attributes in sysfs Message-ID: <20080515204040.GA19403@kroah.com> References: <20080513190435.GG23649@ajones-laptop.nbttech.com> <20080514144030.GD6458@ajones-laptop.nbttech.com> <20080515203748.GC7491@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080515203748.GC7491@kroah.com> User-Agent: Mutt/1.5.16 (2007-06-09) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 15, 2008 at 01:37:48PM -0700, Greg KH wrote: > On Thu, May 15, 2008 at 09:27:50PM +0200, Miklos Szeredi wrote: > > > On Thu, 15 May 2008, Miklos Szeredi wrote: > > > > > > > > This is not meant as a final solution, I'm sure Greg or Kay can help > > > > find a better solution. > > > > > > Yeah, don't do this: > > > > > > > +static struct backing_dev_info *dev_get_bdi(struct device *dev) > > > > +{ > > > > + mutex_lock(&bdi_dev_mutex); > > > > + mutex_unlock(&bdi_dev_mutex); > > > > + > > > > + return dev_get_drvdata(dev); > > > > +} > > > > > > This kind of serialization can often hide bugs, and in some cases even > > > make them go away (if the caller of the function means that the device is > > > pinned and the tear-down cannot happen, for example), but it's really > > > really bad form. > > > > Yeah, I know. > > > > > In order to use locking in a repeatable manner that is easy to think > > > about, you really need to *keep* the lock until you've stopped using the > > > data (or have dereferenced it into a stable form - eg maybe accessing the > > > pointer itself needs locking, but some individual data read _off_ the > > > pointer does not). > > > > > > So the above kind of "get and release the lock" does obviously serialize > > > wrt others that hold the lock, but it's still wrong. > > > > > > > static ssize_t read_ahead_kb_store(struct device *dev, > > > > struct device_attribute *attr, > > > > const char *buf, size_t count) > > > > { > > > > - struct backing_dev_info *bdi = dev_get_drvdata(dev); > > > > + struct backing_dev_info *bdi = dev_get_bdi(dev); > > > > char *end; > > > > unsigned long read_ahead_kb; > > > > ssize_t ret = -EINVAL; > > > > > > You should just get the lock in the routines that acually use this thing. > > > > > > Or, if the "struct backing_dev_info *" pointer itself is stable, and it > > > really is just the access from "struct device" that needs protection, then > > > at the very least it should have been > > > > Actually nothing should need protection. The only problem AFAICS is > > that the device_create()/dev_set_drvdata() interface is racy: somebody > > can come in after the device has been created but before drvdata has > > been set, and then we are in trouble. > > Then that needs to be fixed in the code that registered the device > itself. The driver core knows nothing about this at all. Is this > something in the block layer? > > > I'm quite sure this is not the only place in the kernel where this > > would be an issue, that's why I expect the sysfs guys to have some > > sort of alternative solution, that doesn't necessarily involve adding > > a new mutex. > > It should be fixed in the bus/subsystem that is creating the device, the > pointer must be set up before device_register() is called (or > device_add()). Oh nevermind, Linus is right. We need to just add another parameter to device_create() for this field. For now I can make up a device_create_drvdata() like Linus suggested. Give me a few minutes... thanks, greg k-h